Class 12 Physics in 30 Days|Day 27|Complete Crash Course|Board 2025|Most Important Topics + PYQs#pyq

Added: 2026-05-09

[00:00:11]In the chapter interesting chapter wave optics Previous chapter reflection refraction complete previous chapter entire topics reflection section refraction section we are moving to wave optics in the wave optics again you'll be talking about reflection refraction How this chapter starts?

[00:01:02]Interesting.

[00:01:22]I repeat second in the boomie in the earth 7.5 times rotate [snorts] complete so light previous chapter light always travel in Straight line refractive index.

[00:01:52]Refractive index upon ideally proportional to the velocity of particular medium.

[00:02:08]velocity and then other than Isaac Newton famous he he gave so many ideas laws of course first second third law one of the most fascinating time Newton air Newton rarer and compared to other medium.

[00:03:30]complete opposite Christian hygiens.

[00:03:52]opposite of the hygien. This principle water First, sorry.

[00:04:57]Concentric circle.

[00:05:02]forward forward.

[00:05:17]Okay. Forward.

[00:05:25]I repeat, particle In the stone number of points will be there points.

[00:06:00]So many points will be there.

[00:06:05]N number of them.

[00:06:07]Few points we have selected.

[00:06:15][snorts] for tangent.

[00:06:49]We get second wave. Here we get second wave secondary number of points number of points.

[00:07:27]We get the third wave.

[00:07:31]We get the third wave.

[00:07:36]Simple simple definition principle Right? According to hygien's principle, each point of the wave, right? Each point of the wave is the source of the secondary disturbance.

[00:08:17]Each point, right? Each point of the wave the first I'll say what is this wave each point is the source source to create another wave. I repeat the source to create another wave.

[00:08:35]tangent to the secondary wave.

[00:08:38]Each source of a secondary disturbance, right? And the wave eminent from these points spread out in all the directions.

[00:08:57]Each points acts as a source.

[00:09:04]Each point act as a source.

[00:09:15]You get the next wavecept.

[00:09:25]See the diagram above.

[00:09:30]for [snorts] distance.

[00:09:59]It is plain wave right? It is plain wave spirch.

[00:10:42]Right. It moves forward. I repeat, it moves forward.

[00:10:54]Up and down.

[00:10:57]Up and down. [snorts] I repeat motion up and down but it moves forward in the wave in the wave always 90° I repeat in the wave as well as wave always 90° wave wave always 90° wave and wave Always perpendicular.

[00:11:38]Always perpendicular.

[00:11:45]Up and down.

[00:11:49]It moves like this forward.

[00:11:52]Up and down. Moves forward on the wave.

[00:11:59]90° it starts with hygien need to prove reflection concept refraction concept reflection refraction topic Imagination.

[00:12:34]Imagine I repeat five balls.

[00:12:54]I repeat certain height.

[00:13:21]foreign.

[00:13:50]for the second for in the First drop First point in the refraction and then reflection, Right. So I'm going to talk refraction now.

[00:15:35]Consider this is the surface of the water.

[00:15:54]Wave wave perpendicular to each other.

[00:16:03]perpendicular.

[00:16:20]The wave forward in the point in the first.

[00:16:47]I don't know how your teacher taught this.

[00:17:04]In the point last in the first right last point. First, first join. Okay. Right.

[00:17:47]I'm going to join the point in the tangent join.

[00:17:55]joint. Okay.

[00:18:00]Wave refraction medium denser medium. [snorts] Correct.

[00:18:26]We know that very well.

[00:18:45]Okay, just recolct last.

[00:19:08]Okay.

[00:19:21]It will be like this 90° refracted. I repeated incident angle refracted angle in the red primary secondary.

[00:19:51]primary and then secondary wave primary wave 90°.

[00:20:07]If this is 90° 90us 90us I 90°.

[00:20:24]Correct.

[00:20:26]90° 90° 90° 90US R 90 in the distance.

[00:21:11]Okay.

[00:21:18]Large distance, right?

[00:21:24]small distance.

[00:21:47]He proved this, right? proof refraction.

[00:21:55]He said the concept equations we are going to follow N1 sin I = N_sub_2 sin R N1 sin I N_sub_2 sin R.

[00:22:26]You need I and R.

[00:22:29]I opposite hypotenus large distance v_sub_1 t v_sub_2 tn Different velocity, difference in velocity, V1, V2 because there is a change in medium.

[00:23:17]by V2 refraction, right? Refraction refraction.

[00:23:39]So keep in mind segment concept like I've merged everything. So reflection the reflection I'll draw here medium denser medium normal normal.

[00:24:25]It is because reflection refraction.

[00:24:57]last I'm going to vertical distance vertical distance I'm going to mention refracted sorry reflected ray I R V right blue color and then yellow 90° 90us I 90US I 90US R in the blue sorry in the red in the 90 minutes.

[00:26:51]In the distance in the distance. Same distance.

[00:26:58]Same distance.

[00:27:06]Same distance velocity.

[00:27:12]Same v1al.

[00:27:30]Same speed.

[00:27:34]same distance to whichever it is.

[00:27:48]I repeat diagram even it can be a reflection part they are giving half half moon AC transformer AC generator torque rectangular loop Okay.

[00:28:25]Refraction. So from this he proved that Final they concluded.

[00:28:51]Okay.

[00:28:58]Next concept going to talk about coherent and incoherent plain surface.

[00:29:09]already.

[00:29:12]Plain mirror distance.

[00:29:27]This is not here in this topic actually.

[00:29:32]Focal length. Focal length for plane mirror is infinity. Right.

[00:29:44]So, so focal length for plane mirror is zero. Focal length of plane mirror is zero.

[00:29:59]Right? Next we are moving to coherent.

[00:30:02]Okay.

[00:30:06]Concept coherent and incoherent addition of waves.

[00:30:18]Addition of waves. We will talk about constructive and then destructive. Okay.

[00:30:24]Constructive and then destructive. I'll explain first. What is that?

[00:30:28]Constructive and then destructive. Okay. destructive.

[00:30:39]It's just the addition.

[00:30:46]Your resultant will be somewhat bigger.

[00:30:49]Correct.

[00:30:54]When you add this becomes nothingitive added to bigger one positive negative 0 negative positive again zero suppose it is not zero because magnitude is completely different but in the amplitude you won't get the result somewhat little destructive construct destructive.

[00:31:28]Okay.

[00:31:30]Coherent and incoherent coherent wavegen frequency phase difference.

[00:31:52]difference.

[00:31:58]Okay. No phase difference, no difference in wavelength, no difference in frequency is considered to be coherent wave. coherently next which mean I'm just explaining what is that you have incoherent incoherent we already said coherent it is exactly opposite of that incoherent Different wavegent wave incoherent wave they won't be same and then right I I'll I'll give some other thing also Imagine you are at the midpoint which mean the bicting point or equatorial point.

[00:33:15]You know very well.

[00:33:31]Suppose difference person. First person.

[00:34:12]He reached the that spark first.

[00:34:24]Vertical in the distance difference I repeat difference in both distance path difference. I repeat, path difference.

[00:35:07]Correct. Path difference.

[00:35:21]Okay. opposite.co.

[00:35:49]Okay.

[00:35:53]S1 P dash S2 P ddash distance difference we are going to point S1 P S_UB_2 P they both are distance right cos omega t 11th standard waves simple harmonic motion displacement velocity acceleration Simple harmonic motion.

[00:36:57]You can also mention sign.

[00:37:01]We'll go with the yome bing equal distance y1 plus y2 * a cos omega right 2 * of a cos omega t total displacement And at the same time already 11th stand resultant of two vectors resultant of two vectors A B 2 A cos theta correct cos theta Now talking about inphase concept I talk about inphase in A will be equal to your B and then angle difference 0° A 2 A square A square root which is your resultant which is the more intensity.

[00:38:47]I can also say this in different manner.

[00:38:50]R square already intensity intensity directly proportional to square of its amplitude. square of its amplitude.

[00:39:20]square of its amplitude.

[00:39:29]I and then four square again it is same which is I suppose 90° which isal 2 I out of phase condition when it is 90° theta= 90° theta= 0 I'll share the notes also don't worry I'll send once in the chapter already few we have shared intensity is directly proportional to square of it amplitude Okay.

[00:40:31]S2 Pdash S1 Pdash S2 and then S1 Pdash distance. So conditions N lambda. Okay. N lambda constructive condition after they observed certain experiment and the experiment just they gave the value constructive and the experiment it is not in our syllabus it is beyond so just they mentioned only the formula s2 pdash s2 sorry s1 pdash S1 right?

[00:41:25]Yes.

[00:41:29]Okay. Which is equal to n + 1 by 2 lambda.

[00:41:39]I repeat the condition [snorts] for y similar. Okay.

[00:41:51]I'll write okay.

[00:41:55]D fraction I'll be giving some extra derivations but it is not necessary to write in your board exam not necessary for you see what you have to remember constructive conditions equation is not it is not same y1 a cos omega t y2 is Not same a it can be something omega t - 4 pi omega t - 4 a cos omega t omega tus 4 a cosme Omega y1 + y2 again it comes 2 a cos omega t 2 a cos omega t you get the result construct constructive I repeat constructive similar to in >> [snorts] >> Suppose y1 right y1 a cosmeus a cos omega t minus a cos omega Y1 + Y2 this becomes zero destruct a cosme Omega t + I'll explain. Don't worry.

[00:44:22]Y2 A cos omega t - 4 pi 4 wavelength 2 pi another wavelength 2 s1 and then s2 waves. Five wavelength difference.

[00:45:16]2.

[00:45:19]It can be 6. It can be 8. It can be 10 pi. If it is even, I repeat, I repeat, if it is even, A in the omega tus 4 I'm going to change a little okay a cos minus theta they both are sameus theta 4 minus US in the equation the previous step sorry the 4us omega t a cos - theta aus theta a cos 4 - omega t 2 4 2 pi 2 + 2us omega a cos 2<unk> + theta I repeat a cos 2 + theta correct a cos theta angular Displacement is nothing but angular velocity into time.

[00:47:13]You get constructive which is 2 omega lambda constructive destructive 4 and the last again.

[00:48:07]minus right distract. I repeat lambda n + 1 by lambda 1 by 2 lambda we don't know exactly We don't know that. So I'll move to the previous slide.

[00:49:07]I'll go with this page.

[00:49:10]Y2 we don't know exactly a cos omega t plus can be odd can be even it can be anything we go with that y1 + y2 up a cos omega t plus a cosme Omega t + 5.

[00:49:43]Correct.

[00:49:44]Omega t + mathematical express a cos b 2 * of cos a + b upon 2 and then cos a minus b upon 2 either A omega t omega t + 2 2 * of cos a omega t + b omega t + 5 by 2. Okay.

[00:50:43]And then cos a minus b up minus - by >> [snorts] >> Amplitude make a note independent on time. So we will cancel we can neglect because it is time independent independent a 2 cos 2 omega t +<unk> by 2 and then cos - 5 by 2 in the two cancel because it is a division part I can neglect neglecting only because of time because of time because amplitude so you'll go with this cost in A 2 A and then in the cos - 5 by 2 - 5 by 2. Okay. If you consider your magnitude that becomes plus that becomes plus. Okay. So very very very very important concept I have clearly mentioned like 90° which is nothing but your intensity for coherent and then for incoherent. Okay.

[00:53:16]So just to go through this uh next uh topic we will discuss in the next class.

[00:53:22]Uh thank you so much for joining the session. Thank you.