OFFTOPIC: physics question

Peter Fairbrother peter at
Mon May 13 11:47:03 PDT 2019

On 13/05/2019 02:59, \0xDynamite wrote:
> Sorry for this little diversion, but it has occurred to me that
> physics has a bit of a logical contradiction and I think highly of the
> group's rational faculties here to help me sort this out.
> If light travels at a. different speed for different colors in order
> to account for the rainbow of a prism, how fast is the. speed of light
> then?  Is there real physics to optics?  How can light know what
> direction to bend after it leaves the lens?

The speed of light in glass is slightly less than the speed of light in 
air. This causes light to be refracted (roughly speaking, change the 
direction of its path) when it enters glass at an angle.

The mathematics of this is called Snell's law, sin(a)/sin(b) = v(a)/v(b) 
where sin(a) is the angle of incidence, sin(b is the exit angle, v(a) is 
the velocity of light in air and v(b) is the velocity of light in glass.

However, light of different frequencies travels at different speeds in 
glass. Thus blue light bends more than red light, and if the light 
travels through a triangular prism the familiar spectrum is formed.

Peter Fairbrother

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