Page 1 of 1

### Speed of light

Posted: Thu Oct 10, 2019 9:24 pm
How can problems require us to use constant c=speed of light, when speed of light differs across mediums?

### Re: Speed of light

Posted: Thu Oct 10, 2019 9:35 pm
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.

### Re: Speed of light

Posted: Thu Oct 10, 2019 9:35 pm
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 3:54 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 8:35 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 9:36 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 9:41 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 10:28 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 10:35 am
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.

### Re: Speed of light

Posted: Fri Oct 11, 2019 11:05 am
Maybe the problems would emphasize if it's not under a vacuum? Maybe clarify with TAs, UAs, and Professor Lavelle. :)