Wave Confusion

[plzNOCarribbean]

Hey everyone, I am having some trouble w/ some concepts and I think I may have a few things mixed up. If someone could please clarify and explain my gaps in knowledge....I would REALLY appreciate it, especially bc this is a very high yield topic.

Ok, so I thought that the energy of a wave was based on the amplitude of the wave, or how much it was displaced from its origin. For SHM and oscillating objects, or waves on a string tied to a tree at one end, it stated that the more energy is put into displacing the wave, the higher the amplitude and more E assoc. w the wave? is this correct????

This would mean that, the E of a wave does not depend on FREQUENCY. However, when I came across E.M waves after reading optics, it stated that E.M, the energy of the wave depends on the frequency of the wave???? WTF?? After looking at the spectrum, I saw the UV light in the visible spectrum had the highest frequency==> so it has the highest energy???

Is this because the E of a photon = h f , and because the photons are particles of light, then if the photon has more E due to a higher frequency, then the light must also????? Am i correct here?? How can the Energy of a wave be associated w the amplitude for one kind of wave, and frequency for another??

Lastly, (sorry I know this is long!) wave speed depends on the MEDIUM + the type of wave not the f, wavelength, or amplitude. Then how come, when I got to optics, it stated the E.M, upon striking a prism, well bend at diff wavelength. I thought all E.M radiation traveled at the speed of light. It said that diff color light travels at diff speeds due to the diff in energy of their photons, and thus, because of this, the index of refraction for blue light is diff from that of red light, which is why the two have diff angles of refraction upon hitting a prism????? ^is that right??? EM radiation travels at diff speeds and its photons have diff frequencies?? is the reason EM travels at diff speeds bc its photons have diff frequencies .....v= wavelength x frequency?



Thank you! Thank you! Thank you! ...to anyone who can clear the specifics of this up for me. it is really giving me trouble and I have missed several questions on practice tests because of it so your help is much appreciated!

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[Isoprop]

Since no one has answered yet, I'll give it a shot.

Energy depends on the amplitude for mechanical waves (a wave that requires a medium). For light/EM waves, energy does not depend on amplitude but on frequency. So for waves, there are 2 different energy equations. One for EM waves and one for all other waves. You don't have to memorize them for the MCAT. Just know the principles we talked about here.

The speed of light changes depending on the medium you are in. Actually, this is true for all waves. So when a wave (sound, light, whatever) changes its medium from air to water, it will change speeds.

Also, the frequency is often the "character" of the wave. What I mean is this: when a wave changes medium, the frequency always stays constant.

Here's a little brain teaser. Let's say you are underwater and you shine a red laser beam from above the surface. The wavelength of light changes (because the speed changes) from air to water. Yet you see the same color regardless. Why?

B/c the only wavelength that matters is the one right before the photon hits the your photoreceptors in your eye. So that "red" photon might have spent some of its time being blue, green, or whatever in other media. But you don't observe this because it will always change to being red right before it hits your photoreceptors. Neat, huh?

 

[crystall]

Since no one has answered yet, I'll give it a shot.

Energy depends on the amplitude for mechanical waves (a wave that requires a medium). For light/EM waves, energy does not depend on amplitude but on frequency. So for waves, there are 2 different energy equations. One for EM waves and one for all other waves. You don't have to memorize them for the MCAT. Just know the principles we talked about here.

The speed of light changes depending on the medium you are in. Actually, this is true for all waves. So when a wave (sound, light, whatever) changes its medium from air to water, it will change speeds.

Also, the frequency is often the "character" of the wave. What I mean is this: when a wave changes medium, the frequency always stays constant.

Here's a little brain teaser. Let's say you are underwater and you shine a red laser beam from above the surface. The wavelength of light changes (because the speed changes) from air to water. Yet you see the same color regardless. Why?

B/c the only wavelength that matters is the one right before the photon hits the your photoreceptors in your eye. So that "red" photon might have spent some of its time being blue, green, or whatever in other media. But you don't observe this because it will always change to being red right before it hits your photoreceptors. Neat, huh?

Hi Isoprop,

Thanks for the reply. I like it.

Just one question, why do you think the light that hits the retina is red? I thought the refractive index of the eyes might be similar to that of water.

 

[Isoprop]

That's true, but the point is this. If you are observing the light beam from source, every photon that you perceive will be red even when you know that the light from the the beam in the air is a different wavelength.

It's sort of a perception vs reality paradox. We perceive all red light based on frequency even though wavelengths at the source may be completely different.