wave traveling in a medium

[cloak25]

---

Members don't see this ad.

Lamda and frequency are the respective wavelength and frequency of an electromagnetic wave traveling in a medium. Which of the following statements are true of the wave traveling in a medium having index of refraction n?

I. Its speed equals c/n
II. Its wavelength equals lamda/n
III. Its frequency equals fn

A. I only
B. I and II
C. II and III
D. I, II, and III

Statement I is obviously right. Can someone explain why II is right? Thanks.
The correct answer is B

 

[Giantra Volta]

Not 100% sure if this is correct, but here is my best guess...

So we know that v = c/n , and that c = f *(lambda). So when the light travels through a medium with an index of refraction of "n", the overall formula for its new (slower) velocity would be v = (f *(lambda) ) /n.

If we're talking about visible light (the ultimate logic would hold true for any type of light), the frequency of the light is what determines its color. If we pointed a monochromatic red laser pen at a thick glass window (with "n" index of refraction), an observer on the other side of the window would still see red light (although it'd be dimmer --> the amplitude of the wave would be smaller). So, in other words, the frequency of the light would remain the same; not affected by the higher "n" of the glass when compared to that of air.

So the only property of the light wave that is decreased, which accounts for the decrease in speed (from c --> v) is the wavelength of the light. Specifically, the wavelength (lambda) is divided by "n".

So if we multiply our new wavelength, (lambda)/n , by the unchanged frequency, f , we get the velocity that we would expect, v = (f * lambda)/n

I hope this makes sense. It's how I would've interpreted the question at least, lol.