Today, we tackle a high-yield topic in physics, specifically about light and optics. We’re covering the electromagnetic spectrum, refraction, and understanding wavelength and frequency as it relates to light.
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
High-Yield Topic in Physics: Light and Optics
[04:45] Question 7
Yellow light has a wavelength of 570 nm. Which of the following has a higher frequency than yellow li ght?
(A) Orange light
(B) Infrared light
(D) Radio waves
The whole electromagnetic spectrum comes up on the MCAT all the time. All you really need to know for this is the order. For instance, where is the orange light compared to the infrared or x-rays?
You don’t need to memorize any concrete numbers around it except for visible light that falls between 400 and 700 nm. And this is literally the only number you need to memorize. Everything else is relative positioning.'Higher frequency means shorter wavelength.'Click To Tweet
The correct answer here is C. Note that the MCAT is not trying to trick you. It’s just rewarding careful reading. So make sure you read every word and understand it.
One trick Clara has found to be fool-proof when it comes to the electromagnetic spectrum is to remember that higher frequency corresponds to higher energy. It’s easier to figure out which types of light are higher energy. A lot of times, higher frequency or higher energy light are more damaging.
For instance, the very high-frequency end of the electromagnetic spectrum is the gamma radiation, which is super damaging. It can cause all of the problems to the DNA and to human physiology in general.
X-rays are relatively up there as well. If not administered carefully, x-rays can be very harmful to the DNA. Ultraviolet rays damage the cells too. These are all on the higher frequency end of the spectrum.
[Related episode: Physics Series 3: Light Waves and the MCAT]
[11:22] Question 11
Which changes are experienced by visible light as it moves from Medium 1 (n-1.16) to Medium 2 (n=1.68)?
(A) Wavelength remains constant while frequency decreases.
(B) Wavelength decreases while frequency remains constant.
(C) Wavelength decreases while frequency increases.
(D) Wavelength increases while frequency decreases.
The correct answer here is B. Frequency remains constant. The identity of a type of radiation is not going to change just because it moves into a different medium.
This is just something you have to memorize. One easy way to remember that something has to remain constant is to think about the velocity of a wave = wavelength x frequency.
When you move into a different medium, the velocity of the wave changes. In this case, it moves into a medium with a higher index of refraction so it’s going to slow down.
And if velocity slows down, something else will have to decrease. But the other thing can’t correspondingly increase. Beyond that, just remember that frequency is not going to ever change.
[14:50] Question 14
Of the situations below, which is LEAST likely to lead to total internal reflection?
(A) Visible light moves from diamond (n=2.4) to air (n=1.0) at an angle of incidence of 81°.
(B) Visible light travels from diamond (n=2.4} to air (n=1.0) at an angle of incidence of 46°.
(C) Visible light travels from water (n=1.34) to air (n=1.0} at an angle of incidence of 81°.
(D) Visible light travels from water (n=1.34) to air (n=1.0} at an angle of incidence of 46°.
The correct answer here is D. Even though it’s easier to picture in our heads, it’s tricky to picture out real-life optics.
In real life, we actually have light rays coming from all different directions and angles. Hence, we need to know more about total internal reflection to make sure we get this one right.
Total internal reflection is where a light ray goes through a medium, hits an interface between that medium and another medium, and it doesn’t leave. So it’s stuck in the original medium.
This happens when there’s a big difference between the index of refraction of the first medium and the second one. The bigger the difference, the more the light is going to bend when it hits that border. And if it bends enough, it can actually bend all the way back to the medium itself.
So for A and B, there is a really big difference so that is likely to undergo total internal reflection. So both A and B can be eliminated.
In optics, when there is a large angle of incidence, it’s almost parallel to the border between the diamond and the air or the water and the air. It only needs to bend a little bit when it hits that surface to actually be pointed back in towards the diamond or towards the water.
Whereas the 46° has a smaller angle of incidence so it’s coming in about halfway between the normal and the border. So it will essentially have to bend all the way around to bend back into the medium.
In short, total internal reflection is much more likely when you have a large angle of incidence. And since we’re asked with a LEAST question, we pick the one with the smaller angle.
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