Physics Series 3: Light Waves and the MCAT

Session 91

Why is the sky blue? That’s just one of the questions that may come up on the MCAT when talking about physics and light waves. Check out the questions.

Also, check out MedEd Media and you’ll find The Premed Years Podcast, Specialty Stories, OldPreMeds Podcast, and Ask Dr. Gray: Premed Q&A.

By the way, The Premed Playbook: Guide to the MCAT is going to be available very soon. Written with Blueprint MCAT (formerly Next Step Test Prep), we will soon be putting it up on Amazon and other stores as soon as possible. Go to MCATbook.com to sign up and be notified. Also check out our other books The Premed Playbook: Guide to the Medical School Interview and another one coming up in August is The Premed Playbook: Guide to the Medical School Personal Statement.

This episode is a continuation of our series from two weeks ago and this time we talk all about light!

[02:20] Light Frequencies

Question 7: Yellow light has a wavelength of 570 nanometers. Which of the following has a higher frequency than yellow light?

(A) Orange light
(B) Infrared light
(C) X-rays
(D) Radio waves

Bryan’s Insights:

For MCAT, you need to know the electromagnetic spectrum with radio waves on one end with low frequency and low energy. Microwaves are like that with relatively low energy. Infrared is low energy, low frequency. ROYGBIV goes through the visible spectrum. Then it goes up to x-rays which are very high frequency, high energy all the way up to gamma rays that have very high energy and very high frequency.

So you should recognize these terms in where they fall generally on the electromagnetic spectrum. In this case, x-rays would have a higher frequency than the yellow light for sure.

[04:50] Why is the Sky Blue (Of Oceans and Polar Bears)

Question 9: Which of the following colors of light will bend the least when moving from a vacuum to a glass prism?

(A) Blue Light
(B) Red Light
(C) Green Light
(D) Yellow Light

Bryan’s Insights:

This is another thing you have to know. If you’re a visual learner, you can have a classic picture in your head of the prism and light going through the prism and getting bent. This might help.

So it’s like one of the classic questions, why is the sky blue? The answer is because blue light scatters the most or bends the most when going through from one medium to the next. When the sunlight is just cruising along the earth’s atmosphere, the blue gets bent or scattered the most.

Hence, blue or violet light bends the most. Then if you follow blue as on the one end of the extreme and it scatters the most or bends the most, then red would bend the least.

Now, why does the sky change its color when the sun sets? Bryan explains that you’re now looking right next to the sun. It’s the same phenomenon as the blue light getting scattered but it’s just a difference of where you’re looking. During daytime, you essentially get the blue photons getting scattered down to earth. When you’re looking right at the sun, the blue light coming out of the sun is getting scattered away from your eyeballs, so what’s left is the red.

As to why is the ocean blue, it’s really not. Rather, it just reflects the sky. Now, what is the color of the polar bear’s fur? It’s not white actually, but clear. Bryan assumes it’s just bending the entire spectrum because it’s clear, not scattering one wavelength preferentially over another. Or if blue light is getting bent more, because the polar bear is fluffy and the hairs point out in random directions, you’re ultimately getting all of the various wavelengths of light coming out of the fur. So all the wavelengths mush back together into white when it hits your eye.

[09:25] Index of Refraction

Question 11: Which changes our experience by visible light as it moves from medium 1 with N (index of refraction) of 1.16 to medium 2 which has an index of refraction of 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.

Bryan’s Insights:

We perceive color as a function of frequency in the same way we perceive pitch as a function of frequency. It’s often written as wavelength in the charts. If you imagine speed of light in a vacuum, frequency and wavelength are inversely related. So one can swap out for the other. But when you move from an optically less dense medium at 1.16 to a more dense medium of 1.68, the light actually shows down. The media so optically dense is that you can jog faster than the light.

You actually have to walk into the MCAT knowing this. The thing to remember is that frequency is constant. When you go from one medium to the next, any wave is constant. Frequency is a property of the source. Hence, this only leaves us with answer choice B.