Why does the change in speed of light causes refraction?

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valentin68's profile pic

valentin68 | College Teacher | (Level 3) Associate Educator

Posted on

See the figure below. Suppose you have an incoming ray of light (in medium 1) in the position a) in the figure. The wave front (the locus of all points in wave that oscillate in phase) is segment`AB` . Being angles with perpendicular sides

`/_i = /_BAC`

Now the wave has progressed and is in the position b) in the figure. Only half of the wave front has remained in medium 1 (segment `A_1B_1` ) the other half of the wave front is already in medium 2 (segment `A_1B_1'` ). Because the speed of light is different in the two mediums the length of the segments `B_1C_1` and `B_1C_1'` will be different.

Finally all the wave front is in the medium 2. For the length of the segments `BC` and `A_2C_2` we can write

`(BC)/(A_2C_2) = v_1/v_2`

Since the wave front has remained unchanged `AB =A_2B_2`

and `sin(BAC)/sin(A_2B_2C_2) =(BC)/(A_2C_2) = v_1/v_2 `

Again because they are angles with perpendicular sides

`/_r =/_(A_2B_2C_2)`


`sin(i)/sin(r) =v_1/v_2 `

This is why the change in speed of light causes refraction. The path of the wavefront (`BC` and `A_2C_2` )  is different in the two mediums.

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iamtheginger's profile pic

iamtheginger | Student | (Level 3) eNoter

Posted on


refraction = changing the speed of light.

refraction describes the effect that different mediums have on how quickly light can travel through them. When light passes through an acrylic block, the beam bends as a result of the block being of higher density and therefore slowing the light down.

zerokloppy's profile pic

zerokloppy | Student, College Freshman | (Level 1) Honors

Posted on

Look up "Huygen's Principle".

It's not something that would normally be taught at high school level though, I don't think.

I think you may find this page to be a helpful visual aid if you are curious about it anyway.