Red Shift Blue Shift: The Doppler Effect and Light

Added: 2026-05-08

[00:00:00]Let's talk about red shift, blue shift, the Doppler effect, and light. This is part two of the applications of the Doppler effect video. The previous video deals with more of the applications in the medical field, so go check that out if you haven't yet. But, let's speak about the impact of the Doppler effect and how we see that in terms of light.

[00:00:17]If certain celestial bodies like stars are moving towards or away from Earth, we can sometimes observe a change in frequency. And that change in frequency is associated with a color change, either blue or red, hence blue shift or red shift.

[00:00:33]So, remember things like stars give off light, and when they move, that movement, as we know with the Doppler effect, look at this diagram over here.

[00:00:41]That movement would result in a change in the wavelength depending on whether the object is moving towards you or away from you. And that change in that wavelength result in a frequency change, and we know that we can directly link different frequencies with different colors on the visible spectrum. Let me remind you.

[00:00:57]Here is the visible light spectrum, and you may be more familiar with this, ROYGBIV. So, red, orange, yellow, green, blue, indigo, violet. On this particular picture, they've just reversed the order. So, the reds are down here, and then orange, yellow, green, blue, indigo, violet. It's just a reverse.

[00:01:14]But, what I want you to pay attention to, and this is very important that you memorize and know this, and that is that red has the longest wavelength, longest.

[00:01:24]And you can see that over here. Red long wavelength. And remember, because of this formula that we learned in grade 10, we also know that V is speed in this case when we're talking about electromagnetic radiation like light.

[00:01:39]Now, we're speaking about visible light, which is a form of electromagnetic radiation as well as X-rays, UV rays, um infrared, you know, all those things.

[00:01:48]It's all electromagnetic radiation, and all of those types of electromagnetic radiation travel at the same speed, which we refer to as C, which is the speed of light, which is 3 * 10 to the 8 m/s. So, the important thing to note is that it that is constant in a vacuum, so C is a constant. What that means is that these two variables are what we call inversely proportional.

[00:02:11]Another way to write that is that my frequency is inversely proportional to wavelength. So, when we say one over, that means inversely proportional. If C is constant, that means that the bigger the wavelength is if wavelength is very big, it means that frequency is very small. That is what inversely proportional means. They do the opposite things. Then, on the other end of the spectrum, the violet, bluey, purpley end of the spectrum, we've got a short little wavelength. You can see tiny little wavelength, but higher frequency.

[00:02:40]So, different colors are associated with different frequencies. The lower your frequency is, the more orangey red the color is. The higher the frequency is, the more purpley blue the color is. So, let's speak about what a shift is. If a star or a celestial body, here it says galaxy, if it is approaching us on Earth, so imagine that this person is standing on Earth. Here, we're staring with our eyes, and this light is coming towards us. If an object comes towards you, okay? Just remember what we spoke about with the Doppler effect in general. If you have a moving car with its siren on and a person standing over here, and the car is moving towards the listener, then all the little wavelengths are going to be bunched up together because the car is moving towards the listener or the observer. So, it's the same thing here.

[00:03:24]It's just with light. The star is moving towards Earth, towards us. The wavelength decreases. Look at this It's got a small wavelength. Small wavelength. You can see this, small bunched up waves, compressed, small wavelength. Therefore, frequency is higher. And what did we say about a higher frequency if we consult our ROYGBIV? Higher frequency corresponds to the blue end of the spectrum, which means blue shift. If we look at it from a different perspective, so pretend that Earth is now over here. So, let's switch up the perspective. Here is Earth.

[00:03:58]We are standing on Earth, and we're watching, and what's happening is this star is moving away from us. So, again, it's like a little person is over here, and then a truck making a lot of noise is moving this way. The sound waves, this is what what's going to happen to the sound waves at the back. It's going to get bigger and bigger and bigger, further apart like that because it's moving away. And then, it's moving in that direction, so it'll be bunched up in this direction, but it'll be moving away. So, it's increased wavelength in this direction. So, remember, we're standing here. It's the same thing here.

[00:04:25]We're on Earth over here. The star is moving away from us, so that means that the wavelength is going to be bigger, frequency is going to decrease. And like we said, what corresponds with a lower frequency? Lower frequency, the red end of the spectrum, which is why we call it a red shift. So, here is basically a little bit of a summary of what a blue shift is and what a red shift is.

[00:04:48]And what's pretty cool to conclude about this is that the greater the shift, the greater the speed of the star, and that makes sense. And astronomers, they basically come to the conclusion that they're seeing a lot of red shifts happening. So, literal red light emanating off of these celestial bodies, and that means that they can conclude that most stars are moving away from Earth, not towards. So, don't panic.

[00:05:09]Away from Earth. And that means that the universe is expanding. The distance between all the galaxies is increasing.

[00:05:16]Sometimes they speak about shifts in spectral lines, and I know that that throws people off because they'll see spectral lines, and they don't understand what that means. They're very confused. What you need to understand is that hot objects, so if we're talking about the sun, or in our example, we were talking about the stars, they produce what we call a continuous emission spectrum. And that's basically like the rainbow of colors here, and we touch on this spectra in a lot more detail in the photoelectric effect, which also forms part of your curriculum for physical sciences. So, please go watch those videos if you haven't yet.

[00:05:49]But, a star, the sun, or whatever, they produce a continuous spectrum. But, you need to remember that stars are they're basically made up of or they consist of different types of gases. And what happens is that these gases, the cold outer layer of these gases absorb light of certain frequencies, and what that results in is what we call an absorption spectrum.

[00:06:12]And an absorption spectrum basically looks like our continuous spectrum. Do you see this is a continuous spectrum?

[00:06:17]So, you see a rainbow. The absorption spectrum looks the same. You can see the rainbow, but there's these black lines that are basically cut out of the spectrum. That these lines over here, they represent different frequencies of light that have been absorbed. And different stars consist of different gases, and different types of gases absorb different types of frequencies.

[00:06:38]So, maybe hydrogen will absorb this pattern, whereas a different gas might absorb a different frequency of light, so these lines may look different in different places.

[00:06:47]Now, what we can do is we can compare these spectral lines from these absorption spectra, and we can determine whether there's been a red shift or a blue shift. So, what scientists do is the following. They look at the spectrum produced by the star if the star was not moving at all. So, basically, they study the gases on the star on Earth, and basically they This is considered to be the control. It's when the star is at rest. They see this particular pattern here. Then, they look at what happens when the star is moving. They're either going to see one of two things, and this is what I need you to understand. Can you look at the diagrams and pinpoint certain distinct lines that match in the diagram? So, what I mean is call this one A. Okay? Can you see that that solid black line? And then, to the right of it, you've got these two little skinny lines here. So, let's call that B. Look at that on this diagram. This is A. This is B. Do you see that it's the same lines, but it's shifted? And here, this would be A, and this would be B. And then, over here, to the right of A, we see a skinnier line, and then another skinnier line. Let's call that C and D.

[00:07:50]To the right of A, we see a skinnier line and another skinnier line. You can see that they match up, but there's been a shift. So, what they do is they take the control, and they compare it to what is happening to the actual star found not on Earth cuz this is their control that they analyze on Earth in a lab on planet Earth, so it's their control. And then, they take a look at a sample from the actual star that is moving. And they can see here. Look at A in the top diagram relative to A in the bottom diagram. Do you see that in the top diagram, A is further to the left. B is further to the left. C is further to the left. D is further to the left. So, it has been shifted, and what is What's happening to the left? The red end of the spectrum is towards the left. So, this is called a red shift because all the spectral lines are shifted towards the red end of the spectrum. And when they see this pattern, so when they compare their control to their experiment, and they see that kind of shift, they're like, "Oh, it's a red shift." Which means the star is basically moving away from Earth. But, let's say they got a star a sample from the star, and then they've um concluded the following. Look at A over here, and look at A over here. It's shifted that way. B has shifted that way. C has shifted that way. D has shifted that way. All towards the blue end of the spectrum. So, that would be considered a blue shift. So, here's one example of how they've asked this in a past paper before. So, you can read the question, pause, and see if you would be able to do the question.

[00:09:16]So, it basically says, "An observation of the spectrum of a distant star shows that it's moving away from the Earth.

[00:09:21]Explain in terms of the frequencies of the spectral lines." Now, you know what spectral lines is. "How's it possible to conclude that the star is moving away from Earth?" So, the answer is that the spectral lines of the star should be shifted towards the lower frequency end, which is the red end of the spectrum.

[00:09:38]Here is another past paper question based on the exact same thing. Take a look at this one over here. So, pause and see if you can try it. So, we've got, "Study of spectral lines obtained from various stars provide valuable information. Two diagrams below represent different spectral lines of an element. Diagram one represents the spectrum of the elements in a lab on Earth." So, remember, that's my control, and in this case my star is at rest.

[00:10:01]Then, diagram two represents the spectrum of the same element from a distant star. So, this is my star that is potentially moving, okay? Now, as you can see, let's label this A, B, and C.

[00:10:12]Take a look at what happens.

[00:10:13]A, B, and C. You can see that they've all moved that way. They've all shifted.

[00:10:19]In this case, it's left, but importantly, they've shifted they've shifted towards the blue end of the spectrum. So, is the star moving towards or away from Earth? So, you'll have to look at the spectral lines to decide that. Explain your answer by referring to the shifts in the spectral lines. So, there is my answer. The star is moving towards Earth. It is approaching Earth.

[00:10:38]It's moving towards Earth. Why? Because we can see that the spectral lines are shifted towards the blue end, the blue shift of the spectrum, which would mean, as we know, that that means that the frequency would be higher and the wavelength would be smaller. I would just add that in as part of my explanation, um because that is basically what ends up telling us that blue is linked to increased frequency, decreased wavelength, which tells us, "Oh, if the wavelength is smaller and smaller, it means that the star is moving towards us."

[00:11:06]I hope that you found this helpful. I can't wait to see you in another video very soon. Bye, everyone.