# Index of refraction

## Main Question or Discussion Point

If a light travels from A to B and there's a plane of glass in between the two points, will the total time it takes to make the journey be different than if there was no glass? If light slows down in glass, then I guess the answer would be yes, but I've read that the index of refraction is more subtle than that.

## Answers and Replies

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actual time taken between two static space points will be a product of the speed and distance traveled. The refraction of light by a dielectric material will effect both speed and distance. The light will slow down in higher-index materials, but it will also bend towards the normal which decrease the distance it travels through the layer. With the speed slower but the distance shorter, both effects could cancel out and you could end up with the same travel time. I think it will depend on the specifics of the material's index refraction, thickness, angle of incidence, etc. to determine which effect dominates.

The light will slow down in higher-index materials..
Is that a fact? I've heard contradicting points on this. Someone quite knowledgeable about physics told me that the glass just affects the phase of the light in a way that makes it appear as if the light slowed down in glass.

Is that a fact? I've heard contradicting points on this. Someone quite knowledgeable about physics told me that the glass just affects the phase of the light in a way that makes it appear as if the light slowed down in glass.
Yes that's a fact. Actually refractive index is just comparison ratio between materials where n of vacuum is taken 1 as reference. Light doesn't just pass by from a medium, it interacts with it. Material having higher refractive index interacts with light more than lower index one. Each interaction is a delay thus affects the average velocity.

Light travels at different velocities in different mediums. Even more there are commercial IC's that offers features such as digitally controlling the speed of light in a chip. Refraction index is related to molecule's geometrical structure and dipole alignments.