LAST MIN IGCSE Physics Paper 4 Revision | Quick Recap

Added: 2026-05-14

[00:00:00]Hello guys and welcome back to one more IQCSA physics paper for revision video.

[00:00:04]In this video, we're going to cover a lot of important concepts uh including these topics that is going to help you review for um your exams the night before. I hope this video does help you and if it does, please like and subscribe and let's begin. So, uh always make sure that you know all the formulas and the units. If you know the formulas and the units, you can get a lot of marks. So we're going to start with with chapter one motion forces and energy. Uh we can quickly review that speed is distance divided by time. Remember distance is meter, time is second. So it's going to be m/s. Velocity is displacement over time. This is also m/s. The only difference between two of them that this is scalar while velocity is vector. That means velocity also has direction. If there's a question in which you see that the object is moving at 20 m/s but it suddenly turns right the speed is same but the direction changes. So does the velocity change?

[00:01:04]Yes, it does. Velocity has changed because direction has changed. However, speed is same. Okay. Acceleration is v minus u / t. Acceleration basically means the change in velocity. How does velocity changes with time? The unit is m/s squared. If it's for example -2 m/s this represents deceleration that means speed is reducing by 2 m/s in 1 second.

[00:01:30]A gradient of a distance time graph represents speed. For speed times graph uh the gradient will give you acceleration.

[00:01:38]For a graph you should only know for speed uh speed time graph not distance time graph. It represents the distance traveled. I have made a detailed video about all the physics graphs that you need to know. If you want to check it out, the link is in the description.

[00:01:52]Okay. Now, we have equations of motion.

[00:01:54]V= U + A. V²= U square + 2 A S. This is these both are for velocity. And then we have um S. The S represents displacement.

[00:02:04]So it could either be u + v / 2 * t or s = u t +/ a² where u is initial velocity, v is final velocity, s is displacement and t is uh time, a is acceleration.

[00:02:19]Okay, what if we're talking about falling objects? That means an object is falling down under the force of gravity.

[00:02:26]Let's say an object is falling from a cliff. So its initial velocity is going to be zero. We're going to include this value in equation one. This one. So we can say initial velocity is zero. So we'll write zero. And then a will be replaced with g multiplied by time. Why g? Because um the value of acceleration is 9.8 m/s squared. Uh the last equation if you want to find the distance traveled what we can write is s = to/ g t² because uh u t will become zero initial velocity is zero. What if um the object is thrown upwards g will be negative v = minus gt or s = -/ g t².

[00:03:17]Okay. uh for mass and weight remember weight equals to mass time gravity. The difference between weight and mass is weight is a vector quantity. Mass is a scalar quantity. Mass is constant while weight changes with the uh with the values of G. So we have a question uh the G on planet X is 1x 6 of that of Earth. If mass is 20 kg, what is the ratio of weight on Earth to planet X? So let's find out the weight on Earth. It's going to be 196. What about for moon?

[00:03:48]It's going to be 32.7.

[00:03:52]So to find the ratio, it's going to be 196 to 32.7 6 to 1. Without calculation, we can know weight is directly proportional to G when mass is constant. So this would be 1 to 6.

[00:04:09]Okay. Uh then remember for density density equals to mass divided by volume. The unit for that would be kilogram per me cube. Now um sometimes they won't give you the volume directly but they can give you something like this. So what is the density? Density is going to be mass which is 1,000 divided by volume. And they have given you side of the cube. Remember volume of um cube is s uh cube. So this is going to be 50 * 50 * 50. This one would be in g per cm cube.

[00:04:50]Now for moments remember moment is force multiplied by perpendicular distance.

[00:04:55]This is an example of moment. The seesaw has a clockwise moment or an anticlockwise moment on the other direction. The unit would be Newton meter. Remember when an object is in equilibrium that means that this seesaw is standing like this. It's not tilted towards clockwise direction or anticlockwise direction. It's horizontal. This means an object is in equilibrium. Values of resultant moment and resultant force is zero. Okay. If you look at this question um when we have a moment like this, this is an equilibrium. We can say clockwise moment equals to anti-lockwise moment. So what is the moment in clockwise direction? It's force time distance. This is going to be 30 * 8 / 100. To convert from centime to meter, this equals to the moment on the opposite direction. If you notice both of these forces are contributing to the moment in anticlockwise direction. Then uh it's going to be 20 * 6 + 2 8 because this is the distance from the pivot plus c * 2x 100. Then you can equate to calculate the value of c which is uh the third force. Momentum on the other hand is something different. Momentum is the quantity of motion. It means a moving object has some momentum.

[00:06:17]An object which is stationary has zero momentum. So momentum equals to mass multiplied by velocity. This is the formula. So the unit is going to be kilogram m/ second. It could also be newton second. Now there's another formula for change in momentum. This is force time which is impulse.

[00:06:37]This equation basically comes from F= to M A and F = M V - U / T. Remember this v minus u / v comes from the equation of acceleration. We can expand this. This is mv minus m u. So remember mass times velocity is momentum. So you can see force equals to change in momentum initial momentum minus final momentum divided by time. Now if you bring this over there you get impulse and this equals to change in momentum mv minus mu. How do seat belts and airbags reduce injury? Now uh if you have seat belts your change in momentum will be constant. So we'll not look at this. But what do seat belts do is they increase the time for that momentum to occur. To move from uh from your seat to the steering wheel the time is increased. So when time increases the force reduces and the force faced by the person reduces and therefore there is less injury to the passengers.

[00:07:38]Another concept is called as conservation of momentum. This means momentum before collision equals to momentum after collision. So here we have um two objects object A and object B. They are traveling with initial velocity. So M A ua this is momentum before for A plus momentum before for B m U. Now they are both traveling together towards right. In momentum you have to take care of direction as well. So here we're considering to direction towards right as positive. You can combine both of these masses m a b because they're traveling together multiplied by this is going to be multiplied by multiplied by the final velocity. This is the velocity that both of them are traveling together towards right. Okay. Moving on to forces. The si unit for force is newton. Force can bring about a lot of changes to an object. It can increase the size, decrease the size, it can change the direction. It can also be used to stretch elastic substances for example springs or elastic bands. Um if you look at this graph u we have on the x axis we have the extension which is the the stretch length minus the initial length and on yaxis we have the force. F is directly proportional to x and it has k.

[00:08:56]If you want to find k from this graph remember k is going to be the gradient.

[00:09:00]Um this region where it's a straight line is called as elastic region and the region beyond that is called plastic region. Hulk's law is only obeyed in the elastic region. After that in a plastic region is not obeyed anymore. The relationship is not linear. The point until which it is obeyed is called as limit of proportionality. After that the line becomes curved. Maybe it broke or it's permanently deformed. Now we can look at energy, work and power. They are related to each other. Remember work done equals to force times distance.

[00:09:31]Work done is always written in jewels.

[00:09:33]It can also be written in in kilogjles or mega. So make sure that you convert them. Uh force is newton and distance is meter. They also have other units. So convert them appropriately. So work done equals to when you apply certain force how much is the distance that is moved by an object. Now in some cases for example if you're pushing the ball you are applying force to the wall but the distance is zero. The the wall does not move. So in that case work done is going to be zero. Work done is also called as energy transferred. For example, if you look at look at a ball which is falling through a height. U work is being done to bring the ball down and energy is transferred from gravitational potential energy into kinetic energy. Now rate of work done is called as power jewles over second is known as watt. You can also write this as energy transferred per unit time. Okay. We have kinetic energy and gravitational potential energy. The equation is half mv² for kinetic and for potential is mgh. Sometimes both of them can be equated. That means again for a falling object as an object is falling down gravitational potential energy is being converted into kinetic energy.

[00:10:50]Initially GP is max and kinetic energy before falling is zero. and then right before hitting the ground kinetic energy is max and gravitational potential energy becomes zero. So this is also the principle of conservation of energy because no energy is lost is only converted from one form to another. So we can say they're both equal in value this equation we can write it as half mv² equals to mgh.

[00:11:20]Now both mass is same because we talking about the mass of the ball. The mass can be cancelled and we can write this as v ² = to 2 gh. So if you want to find the velocity of the falling object but you don't have the mass, you can use something like this. You don't have to memorize this equation. Just know that you you will equate both of these energies. So efficiency is the ratio between useful energy or total energy.

[00:11:44]Multiply this by 100 because the unit is in percentage. Efficiency can also be written as useful power over total power. Basically how much um energy we are giving versus how much energy is being actually used to do that uh to do the process that you want to do. That is efficiency. If it's 100% efficient useful in energy and total energy will have same value. Okay. Now pressure equals to force applied per unit area.

[00:12:16]What is the unit of pressure? It's pascals. We have another definition for pressure which is height ro. Again the unit is a pascal but this is specifically for liquid pressure. So for example if you have an object which is submerged in a liquid could be water. So you will write g as 9.8.

[00:12:37]And what about density? Density of water or oil or whatever the liquid that is here. So for example, if it's water, it's going to be 1,00 kilogram me cube.

[00:12:48]And then you want to know the height. So height is the distance from the top. It it's not it's not the distance from the bottom up. Okay. If an object is deeper in, for example, here this is this is facing more pressure because the height is greater. Uh the surface area of the jar or the width of the jar has no effect on the pressure. It only depends on height not the surface area. So this one will experience less pressure compared to this because the this distance is greater. So terminal velocity is basically the velocity at which a falling object experiences zero acceleration because the resultant forces have been balanced. So you can see initially this was an object which is falling down and the only force that experience is weight. It's falling down due to its weight and it has constant acceleration. Then you have resultant force acting down. After some time there's also an opposing force which is drag. So uh if you see the resultant force will reduce because it's going to be w minus drag. Resultant force reduces. Resultant force is directly proportional to acceleration. So because the force reduced acceleration will reduce as well. What happens after some time? The opposing forces become equal.

[00:14:05]Weight equals to drag. So if you um to find the resultant force you have to subtract both of them right because they're in opposite direction. If you subtract them they're going to be zero.

[00:14:16]Since resultant force is zero acceleration is zero and the object is now moving at terminal velocity.

[00:14:22]Terminal velocity depends on the mass and surface area. If the mass is greater terminal velocity will be greater.

[00:14:28]Surface area however reduces terminal velocity. For example if you have a big beach ball compared to a small tennis ball. Okay. Now we can quickly review thermal physics. So first this equation equals to Q = to MC delta theta. This equation focuses on specific heat capacity. C here represents specific heat capacity. It means what is the amount of energy needed to increase the temperature of 1 kg substance by 1°C.

[00:14:55]For example, if you have a block of block aluminum block that has 1 kilogram, how much Q should be there?

[00:15:03]How much energy in jewels you need to give to increase the temperature by 1°C for so this is the formula where Q is the energy absorbed okay energy absorbed when you're increasing the temperature but it can also be for reducing the temperature by 1°C at that point it's going to be energy released the unit is jewels m is the mass of substance in kilog is specific heat capacity the unit is jew per kilog and delta theta is change in temperature which is final temperature minus initial temperature. The unit is either in degrees CC or in Kelvin. And then we have two more equations. Q= to MLF and Q= to MLV. LF is the latent heat of fusion. This is the hidden heat. This is basically the amount of heat required to change a substance from a solid to liquid and it melting point without an increase in temperature. Q is again in jewels. Mass is in kilog and LF is going or latent heat of fusion is going to be jew per kilogram. On the other hand, LB is the latent heat of vaporization. This is the amount of heat required to change a substance from a liquid to a gas at it boiling point without change in temperature. Both of these processes involve energy changes but there's no change in temperature. Okay. So now we can look at these points. Internal energy of a substance is kinetic energy and potential energy. When the temperature rises, for example, from 20 to 30°C, only the kinetic energy is increasing. The temperature is increasing, the particles are vibrating faster. So, there's an increase in kinetic energy. However, when there's a change in state over here and here, when for example, ice is melting or the liquid is boiling, uh the temperature is not rising, but there is a the particles are moving away from each other as they're melting. The molecular forces are broken down are being broken, the intermolecular forces are being broken and potential energy is increasing.

[00:17:04]All right? So, we can say that our internal energy depends on four factors.

[00:17:08]Okay? First, it depends on temperature.

[00:17:10]If the temperature is higher, it's going to have more kinetic energy. Second, it also depends on the state of matter. Um, if for example, gas particles have the highest internal energy. Now, what about the mass of substance? If the mass is greater, more mass means more particles.

[00:17:27]So, more more internal energy. And then finally, specific heat capacity.

[00:17:32]Internal energy depends on specific heat capacity because it it says how much energy is needed to change the temperature.

[00:17:40]So if a substance has very high heat capacity, it's going to have high internal energy as well. Now to change from Celsius to Kelvin at 273.

[00:17:50]Uh one more rule, a important rule is boy's law. This means that when temperature is constant, pressure and volume are inversely proportional. For example, if you want to find V_sub_1, it's going to be P2 * V_sub_2 / P1. Now we'll start with magnetism. Okay. So one of the most important concept is in magnetism is induced emf. What is induced emf? Basically it's a way in which we can create electricity using using magnets. Okay. So here we have two magnets and they have um like all magnets have a magnetic field between them which goes from the north pole to the south pole. Then there is a conductor in between them. Now um whenever you cut you take this uh conductor and move it up or down and basically you are cutting through these field lines you are creating an induced emf then uh you will see that the galvanometer is show a reading because because an emf or voltage is induced in the wire. So basically um emf is induced at a rate at which conductor cuts the magnetic field lines or if it cuts a lot of magnetic field lines the induced emf will be more. Therefore if you want to increase the induced emf you have to increase the speed of motion of magnet or coil. So you can either have a conductor you can even have a coil. For example in solenoids you have a coil or if you move your magnets very quickly that is also going to increase the emf.

[00:19:21]It also depends the strength of magnet not the coil because if the magnet is stronger it's going to have more field lines per unit area and that means more lines are going to be cut. Now if you see an example of AC generator in an AC generator alternating emf is induced for IGCS you need to know the diagram uh the labeling and the working of an AC generator. Basically in simple terms, an AC generator has a coil of wire and this coil of wire rotates in a magnetic field. As the coil rotates, it cuts through the magnetic field and an emf is induced. So in AC generators, electricity is produced by mechanical energy.

[00:20:03]Mechanical energy or the energy during because of movement is converted into electrical energy. Now um in DC motors however the opposite happens. So in DC motors you have a permanent magnet that has electromagnetic field around it from north to south pole and you also have um a conductor that has current flowing through it. Whenever there's um a conductor that has some current flowing through it for example if the current is going down it also has a magnetic field around it something like this. And if you don't want to know the direction of the magnetic field, thumb points towards the direction of current that is thumb. And uh where your fingers roll which is going to be here that shows the direction of the field. In DC motors we have a permanent magnet which has its own magnetic field and then there's also coil or basically an armature which carries current. When you turn on the current and current flows through the conductor which is already in a in a magnetic field, the magnetic field lines of the of the wire are going to cut through the magnetic field lines of the mag of the strong magnet of the permanent magnet and that is going to cause a force on the wire. So so this wire is going to move up or down. The electrical energy in the motor is converted into motion or mechanical energy. Okay. If right hand rule is always used for generators or whenever there is an induced emf. If you have two of these quantities, you can determine the third quantity. So your thumb represents the motion the motion of the conductor where the conductor is being moved. Um your first finger represents the direction of magnetic field always from north pole to south pole. And uh the second finger represents the induced current. This is conventional current.

[00:21:52]So it shows a direction from the positive to negative. On the other hand, Fleming's left hand rule is also known as motor rule. This is used when motion is created. Okay. The thumb represents a thrust. This is basically the motion or the force experienced by the coil or conductor. The first finger represents the direction of magnetic field from north to south pole. And the second finger represents the current the flow of current from positive to negative.

[00:22:19]Now the application of lim's left hand rule is in is in DC motors and it's also used in charge for example if you have a charged particle which is traveling through a magnetic field plus represents that the magnetic field is going into the paper dots represents that magnetic field is going out of the paper and then you have a charged particle could be a proton could be an electron and an electron that's going in. So you can apply Fleming's left hand rule to identify where this electron is going to move. Now another very important concept is for transformer. Transformer is an electrical device in which is used to change the value of alternating voltage in the in from your source to either a greater value or to a smaller value. For example, if your um if your input voltage was 1,000 volt, you can increase it to 2,000 volt. So that is called a step up transformer. Or you can reduce it to maybe 5 volts. That is a step down transformer. So this is what a transformer looks like. It has a primary coil and a secondary coil and a soft iron core in between it. So whenever the current is switched on in the primary or off or the current is changed a voltage is induced in the neighboring coil in the second one. Basically the most important concept is that field lines are cut. V_sub_1 or V in the primary coil divided by voltage in the secondary coil equals to N_sub_1. The number of turns of coils in the primary to the number of turns of coils in the secondary. An easy way to identify whether this is a step up or step down a transformer is to look at the number of coils. If the secondary uh coil has a greater number of turns of coil, that means it's a step up. If the secondary one has less number of coils, it's a step down transformer. One more equation is that power in the primary or power in the input equals to power out. So remember power equals to IV. So I primary multiplied by wtage primary equals to I of secondary and wtage of secondary. You can also write this as IP / I equals to V S / VP. You can even equate this equation with this equation.

[00:24:33]So here is V secondary over V primary NS NP. Um next why do we need a transformer? So basically we have a powerhouse. A powerhouse is used to provide electricity to people's home. At homes we don't need very high voltage.

[00:24:49]We need low voltage. But the problem is if we have low voltage the current would be high.

[00:24:55]And if the current is high a lot of power is lost in cables. If you look at this current, if you look at this definition, power lost equals to I² R.

[00:25:03]Therefore, in cables, we use very high voltage and low current. So, what are some advantages of using high voltage in cables? Less power is lost in cables.

[00:25:12]And secondly, because the current is low, we can use wires that have a small cross-sectional area. So, this is going to be cheaper and much easier. First equation is Q= to it. This basically means current equals to charge per unit time. So, this is how you can define current. Electric current is basically the flow of charge. How much charge flows per unit time. Now uh this is one of the most important equation and this is going to show up a lot. Also make sure that you know the units. Q charge is for kum. I is for current which is ampere and time represents seconds.

[00:25:44]Sometimes you need to convert from one form to another. For example hour is 2 minutes or seconds something like that.

[00:25:49]The other equation is V equals to IR.

[00:25:51]This is accurate as long as the temperature is constant. Whenever there's a heating effect or something like that, it can affect the relationship. Another point to notice is this that resistance is directly proportional to length of a metallic conductor. For example, if you have a wire, if it has a long wire, it's going to have more resistance. On the other hand, area is inversely proportional to resistance. If the area of the wire is bigger, it's going to have smaller resistance because the electrons can flow easily. So, that reduces the resistance. Now, we have two more equations related to voltage or emf. EMF is basically the work done by a source that mean by the battery or cell to move a unit charge around the complete circuit. So as the equation says it's work done per unit charge. On the other hand, potential difference is only across a component. So this is also work done per unit charge but it's only for the component. So um you can measure emf and potential difference by a voltmeter by putting it across the component for potential difference and across the power source for the emf.

[00:26:56]Remember the volt the unit for both of these is volts.

[00:27:02]Next these are some power equations. So um now we have some power equations. So the first one is P = to IV. Power equals to current multiplied by voltage. The unit for power is W. Um another equation is power equals to energy transfer / time.

[00:27:22]This means how much energy is transferred to do something in unit time. Okay. So we can write energy equals we can also write it as energy equals to power multiplied by time. And so this can be written as power equals to IV. So we can write I Vt. And this is this equation. Sometimes you can also comi com combine two equations P = to IV and V = to IIR. Sometimes what if you want to find the power?

[00:27:51]You have the current but you don't have the voltage but you don't have the voltage and you do have the resistance. So in that case you can combine this. So uh some people like to combine the combine this equation in one equation or other people just like to do it in two steps. So if you want to do in two steps you will find the voltage by multiplying the current and resistance and then you will put this equation in the first one to find your power. Another way is to combine both of them. It's going to be P = to I and voltage is I I * R. So you will write I * I * R. This equals to I 2 R. This is when you do not have your voltage. If you don't have um your current, you can also do that. So remember I = to V / R. So it's going to be V * V / R, which equals to V ^ 2 by R. I don't um I think the easier way is instead of just memorizing a lot of equations, it's easy to just know these two basic equations and then you can and then you can find your power. Next important point is in series and parallel circuits. What happens to voltage, current and resistance? Voltage is always added in series and current is always same because it just flows around the circuit. In parallel, however, it's the opposite. Voltage is same but current is added. For resistance in series, you have to add your resistances and more. R1 + R2 + R3 if you have more.

[00:29:23]For um for parallel the formula is different. It's going to be 1 / RT = to 1 / R1 + 1 / R2 + 1 / R3 and so on depending on how many resistors you have. If you have only two resistors, you can use this formula. RT = to R1 * R2 / R1 + R2. In that case, you don't have to find the reciprocal that um that you find here. Some people just find R 1 / RT as for example 16. That is not the resistance. RT is going to be 1 / 16 if you want to find the resistance. So make sure that you find the reciprocal. So here I have combined few electrical symbol but make sure that you go back and review all of the symbols. You can be asked to identify them. Four resistances. These are um this is a fixed resistor. This is a fuse. It's not a resistor but it's another electrical component. Basically you can change the resistance using this component. We have two more types of resistors which are LDR and thermister. LDR is light dependent resistor and this is the symbol for that. When light increases, resistance decreases. So they're inversely proportional. Thermister on the other hand has this symbol. If it's negative coefficient, resistance and temperature are inversely proportional.

[00:30:35]Okay. Now we have a potential divider. A potential divider is basically a circuit in which your input wtage can be divided into smaller wtages. Why is that important? Maybe because you have another component here which needs a smaller voltage. So you can have different components that require different voltages and you can divide your input wtage into smaller voltages.

[00:30:56]Okay, remember when the uh current is constant, wtage is directly proportional to resistance. This equation is a specific equation for a potential divider. If you have any of three three of these, you can find the fourth quantity. R1 is the resistance of the first resistor. V_sub1 is the voltage of the corresponding voltmeter and on the other hand R2 and V_sub2 are there too.

[00:31:17]So uh you can easily find any of your unknown values. Let's discuss about waves. There are two types of waves.

[00:31:23]Either you can have a transverse wave or you can have a longitudinal wave. In a transverse wave the uh the direction of vibration is perpendicular to the direction of propagation or to the movement of of the wave. Include all the electromagnetic radiation including light. Also the waves in water the systemic S waves.

[00:31:45]In longitudinal waves the direction of vibration is parallel to the direction of wave propagation. So some examples of longitudinal waves are sound waves and cysmic primary waves. So this is an example of a transverse wave. You have a crest that goes up and you have a trough that goes down. But in longitudinal waves you have regions that are particles are very close. This region is called as compression. And then you have regions where particles are far from each other. That is called the region of rare faction. And then again you have a compression. Distance between two consecutive compressions or rare fraction is is the wavelength. Okay.

[00:32:26]Let's look at this wave in detail. Peak is a crest and so is this. Uh on the other hand, this one is the dip is called a trough. Amplitude is a region from undisturbed position to the maximum height. That's amplitude. Amplitude can also be from here to the minimum. A wavelength which has this symbol is the distance between two consecutive points on a wave. So it could be from the starting of the crest to the next crest.

[00:32:55]Okay. It can also be from um from the top from a peak to peak. That's also wavelength. It can also be from trough to the from peak of the trough to another trough. So one crest and one trough makes up a complete wave and this is uh the and the wavelength is measured in meters. Now we have two more equations. First is V = to F lambda. V represents the wave speed. It is frequency multiplied by wavelength. So frequency is basically number of complete waves. Frequency is basically how many waves are produced in 1 second.

[00:33:31]Its unit is in hertz. Okay. Time period on the other hand is the time taken to complete one cycle of wave. Okay. Time period is measured in seconds. Now if you have a wave and you're asked to draw a wave that is louder than the original wave. So all you have to do is increase the amplitude. If you see that this light purple represents the original wave, we just increase the amplitude to create a wave that has a louder sound.

[00:33:59]On the other hand, if you're asked to draw a a wave which has higher pitch, you have to increase the frequency. The amplitude does not change. Waves can undergo reflection, defraction and refraction. So when reflection occurs, the speed of wave, the frequency and the wavelength of the wave remain same. The only change that happens is direction.

[00:34:20]So something like this and direction is has reversed. On the other hand, draction is when the wave passes through a hole. Again the speed, frequency and wavelength remain same. Only the shape of the wave changes and then finally you have refraction. When the wave travels from one medium to another. So speed changes, wavelength changes. However, the frequency remains same. As the wave moves from rarer to denser medium, it becomes closer to normal. Why? Because the speed has reduced and the wave becomes closer to the normal. However, if it goes on the other direction from denser to rarer, for example, from glass to air, uh it moves away from normal. So looking at where the wave is moving, you can identify which medium is denser and which medium is rarer. Cell's law is used to find a refractive index.

[00:35:12]What is a refractive index? Refractive index or n measures how much light is bent when it moves from one medium into another. So the formula for refractive index is speed in air divided by speed in medium. For example, this diagram shows that the light is moving from air to another medium. This bend can be measured by the refractive index. Every material has its own refractive index.

[00:35:38]Then you can also find N by sin I the incident angle over the refractive angle sin I or sin R and N can also be found by the critical angle. Critical angle is the incident angle in when a light is moving in a denser material and it and it doesn't move into the rear material instead it goes through the surface. The angle of refraction is 90° that angle is critical angle. So the formula is n= 1 / sin c.

[00:36:08]Anything uh if the angle of incidence becomes greater than the critical angle we have total internal reflection. So the light does not move outside. It does not refract it does not refract rather all of the light stays inside the medium and goes on. One application of this is an optical fibers. So lenses can be of two type. You can either have converging lens or diverging lens. In converging lens the light rays are going to converge at one point. However, in diverging they are going to diverge. So there are some rules. So if your object is for example beyond 2F somewhere here for example the image will be real but the size will be smaller. It's going to be inverted and it's going to be closer to the lens compared to the object. If your image is between F and 2F the image is going to be real bigger inverted but further away. If it is on to f over here, the image is going to be real same size inverted but at equal distance as that of the object. If the image is between lens and the focal length length that means if the object is here and then you you draw the diagram you will see that these lines never meet. So you extend these line backwards and it's going to meet here. So the image is going to be um virtual bigger upright.

[00:37:28]is not inverted and it's going to be behind the lens. That means here this is the image. So you'll keep your eye here to look at the image. One example of that is for magnifying glass.

[00:37:40]Now let's move on to nuclear physics. So um some important concepts in nuclear physics are these which are explained here. During radioactive decay alpha, beta or gamma are released. they have their own properties uh their own charges and so on. So this um table summarizes that alpha particle is basically a heavy particle. This is the symbol and it has the same proton and neutron number as helium nucleus. Two protons and two neutrons. It is a positively charged particle because it has more number of protons. So it's going to be plus two. The penetration is low but ionization is high. Okay.

[00:38:21]Remember penetration and ionization are opposites. This is low and high. But for gamma it's the opposite. We'll go through it one by one. Alpha particles um have very high ionization and very low penetration. Basically because they have a large mass and they have a charge of plus2 and it interacts with the particle and it rapidly loses its energy. Therefore it because it ionizes it pen it has very low penetration. What about beta? Beta is basically an a high energy electron. These are negatively charged and this is what beta looks like and it has only one electron.

[00:38:58]It has no protons, no neutrons. The penetration is medium and so is ionization blocked by materials like a thin aluminium sheet. And finally we have gamma. Gamma is not a particle.

[00:39:10]Gamma is basically a radiation. Okay, it's an electromagnetic radiation. This is the symbol of that. is charge is zero. It has very high penetration but very low p penetration. And if you want to block it, you need to um block the source by thick lead or concrete because it has very high penetration.

[00:39:31]Okay. Now let's look at nuclear equations for radioactive decay. Let's say you have a uranium atom which has the mass of 238 and the proton number of 92 and it under goes alpha decay. So it um you can say something like this helium 42 or you can or an alpha particle.

[00:39:54]So you have four and two here. Therefore you're going to subtract this by 4 and this by 2. Now this is balanced. 234 + 4 is 238 and 90 + 2 is 92. Now let's say you have um iodine with the mass of 131 and protons and 53 protons. It under goes um beta decay. So beta particle is released. Remember beta particle is just um an electron, right?

[00:40:24]So it's zero and minus1 um xenon. You don't have to learn the names of new element. All you have to know is give give the new numbers something like that. You you can be asked to identify the new particle. Okay. You can see that the mass number did not decrease at all.

[00:40:40]It's zero. So you can write this as 131.

[00:40:43]But um there's an increase but you have negative 1 here. So you will write 53 + 1. You will write 54. Why? because 54 + minus1 is going to be 53. So this is a beta decay. Now let's look at a gamma decay. Remember gamma is not a particle.

[00:41:04]Gamma is a ray. So again we have uranium 238 and 92.

[00:41:11]Um gamma ray has no mass or charge. So it's 0 0. For this equation it also released an alpha particle. So four and two for helium. So what are going to be the new numbers for mass and um the proton number? This is going to be 90 because 90 + 2 is going to be 92 and this is going to be 234.

[00:41:33]This is how it's balanced.

[00:41:36]Now another concept in nuclear physics is halflife.

[00:41:41]Radioactive atoms have unstable nuclei which constantly decay into atoms of different element and they emit alpha beta particles. So half life is the time taken for half of the nuclei to decay.

[00:41:53]It can be uh as small as a few milliseconds and it could be as long as hundreds of thousands of years. Now um the unit for half life is beareral example uh let's say this was 200 200 and the time taken for it to reduce by half which is 100 is your half life.

[00:42:13]Half of nuclei have decayed. So at 100 you will find the corresponding time.

[00:42:19]Let's say this is 50 years. So that is the half life. Now if you find 50 and if you find the corresponding time that is also going to be 50 years.

[00:42:31]So this is the half life for that particular element. Now for space physics I am focusing on the equations okay or the formulas. Orbital speed is 2 pi orbital radius divided by time period. Basically, if you have this is the orbit and to find out the speed of a planet to move around this orbit, you have to divide 2 pi of the orbital radius from the center to the the time period. Time period is basically the time taken for um for a moon or a satellite or something to move around the complete orbit. that is orbital speed in terms of formulas is going to be v = to 2 pi r / time period.

[00:43:21]You may also have to use this equation speed equals to distance divided by time. For example, if asked to find the time taken for sunlight to reach earth.

[00:43:29]So you have to find the distance between the sun and the earth. It can also be the orbital radius and then you have to uh the speed is going to be speed of light and then you can easily find the time.

[00:43:42]Now um what one light here equals to 9.5 * 10^ 15 m. So for example in your question if they say that this uh this object or this planet travels for 15 light years. So you have to multiply 15 with this number. Now what is Hubble's constant? Hubble's constant equals to recession velocity divided by distance traveled. Recession velocity is the speed at which a galaxy moves away from the earth and the distance is between the distance of the earth and the galaxy. The current estimate of Hubble's constant as per today is going to is 2.2 * 10^ -18 per second. Now if you find the reciprocal of Hubble's constant this gives you the age of universe. This also tells us um that this also proves that universe or earth expanded from a single point and it's contin continuously expanding away from us. If you look at this uh graph here when you have distance on the x-axis and on the y-axis you have recession velocity you can find the gradient. Gradient is always y /x. So recession velocity divided by distance is going to give you the gradient of Hubble's constant H