## Speed of light

$c=\lambda v$

tonyhu
Posts: 57
Joined: Tue Sep 24, 2019 12:17 am

### Speed of light

How can problems require us to use constant c=speed of light, when speed of light differs across mediums?

Andrew Liang 1I
Posts: 105
Joined: Fri Aug 30, 2019 12:18 am

### Re: Speed of light

In high school I learned something about the refraction index of a medium. The equation is n = c/v. n is the index, c is the speed of light constant, and v is the velocity of light in the medium. To answer your question, i think the question will have to do something with the index.

Sahil Jog 1F
Posts: 126
Joined: Thu Jul 11, 2019 12:16 am

### Re: Speed of light

Light does differ through mediums, but for any of these problems, they are assuming that this light travels through a vacuum. Also, light traveling through mediums should not affect its speed to a substantial amount.

sarahwu3a
Posts: 50
Joined: Sat Aug 17, 2019 12:17 am

### Re: Speed of light

If a question asks about speed of light not in a vacuum you should be able to calculate that speed using the index of that material.

Shrayes Raman
Posts: 129
Joined: Sat Jul 20, 2019 12:15 am

### Re: Speed of light

When the photo-electric experiment was done, it was tested under a vacuum. Therefore we are assuming that the speed of light is in vaccum and therefore constant.

Megan Jung 3A
Posts: 50
Joined: Thu Jul 11, 2019 12:17 am

### Re: Speed of light

The speed of light, or 3.00 x 10^8 m/s, is a constant under a vacuum. Therefore, for the sake of calculations in problems, it is assumed that they take place under the condition of a vacuum so that we can use the equation c=v lambda in calculations.

Sally Qiu 2E
Posts: 105
Joined: Fri Aug 30, 2019 12:18 am

### Re: Speed of light

we use the constant c when we are assuming the situation is in a vacuum. however, it is true that light travels different under different mediums.

Julia Mazzucato 4D
Posts: 64
Joined: Fri Aug 30, 2019 12:17 am

### Re: Speed of light

In this course we should always be able to assume that it's under a vaccuum. If there are any problems not under a vaccuum, you should be given the appropriate velocity so you're still able to calculate what you need. The relationship between wavelength and frequency is the same with a non-c velocity.

So the formula would be velocity (given) = wavelength x frequency. Just swap c for the velocity of the wave you're given. This holds true throughout physics.

sbottomley3a
Posts: 50
Joined: Wed Sep 18, 2019 12:19 am

### Re: Speed of light

If the question specifically states that it's not in a vacuum, then we can calculate the speed based on the index of the material that it's travelling through. However I think that we'll always be asked about light in a vacuum for the scope of this class? We should ask the TAs for clarification.

Michelle N - 2C
Posts: 117
Joined: Wed Sep 18, 2019 12:19 am

### Re: Speed of light

Maybe the problems would emphasize if it's not under a vacuum? Maybe clarify with TAs, UAs, and Professor Lavelle. :)