# Refraction of light when traveling from air to glass

• songoku
songoku
Homework Statement
Explain in terms of speed of light why ray entering glass from air along the normal is not refracted
Relevant Equations
n = c/v
I don't know how to explain it in terms of speed. I know the speed will decrease but if the ray entering the glass at certain angle, let say 10 degrees, the speed will also decrease so what is the relation of speed to the fact that the light will bend or not when entering the glass?

And the question states "is not refracted". In my opinion, the light is still refracted because there is change in speed. Or refraction means the light must change direction?

Thanks

songoku said:
Or refraction means the light must change direction?
That's what it means. This might guide your thinking.

songoku
kuruman said:
That's what it means. This might guide your thinking.
I have read the link but sorry I still don't understand the question. I can answer it by using Snell's law but I don't understand how to relate it to speed.

The speed will decrease but how the decrease in speed can be used to explain whether the light will bend or not?

And also why we say there is no refraction when the incidence angle is zero, even though the speed changes?

Thanks

It seems to me that you are confusing a fact with a definition.

The fact is that the speed of light in a dielectric medium is smaller than its value in vacuum.
The definition is (according to Wikipedia) "##~\dots~## the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium."

Refraction is not limited to light waves. Sound waves also refract at an interface separating two media where it propagates at different speeds. When the direction of propagation is perpendicular to the interface between two isotropic media, there is no redirection of the wave when it passes from one to the other, hence no refraction.
songoku said:
I can answer it by using Snell's law but I don't understand how to relate it to speed.
To answer the question, it should be sufficient to use Snell's law which is a consequence of the fact that light propagates at different speeds in different media.

Think of it this way: If light, at normal incidence, were to change direction when passing from one medium to another, what would that direction be?
Answer: It can only be forward, i.e. perpendicular to the interface because there is nothing to distinguish "up" from "down" from "left" from "right". There are infinitely many planes that contain the incident ray and the normal to the interface and on which the refracted ray must be. When the incidence is at an angle, there is a preferred plane defined by the incident ray and the normal to the interface on which the refracted ray in the second medium must also lie.

songoku
The etymology of the word is from the latin frangere: "to break up". I looked it up..... so Newton's prism comes to mind.` Interesting.

It is perhaps easier to use Fermat's Principle (of least time) to intuit the primary ray path for light.

Last edited:
songoku
The way that I think about relating refraction with a change in speed is through Huygen's idea of wavelets.

You can think about a longer wave front as a bunch of little circular wavelets all strung out in a row perpendicular to the direction of propagation. Any disturbance in the medium propagates out in a circle from each point of disturbance. A wave front is such a disturbance. The wavefront a moment from now will be at the far edge of where its wavelets have had time to reach.

If you carefully reason about how a wave propagates when it crosses from a high speed medium to a low speed medium at an angle, you can see that the wavelets on the one side, where the wave has already crossed are expanding more slowly than those on the other side. The result is that the wave front changes direction.

A wave always propagates at right angles to the wave front.

One can also reason about extremizing transit time and how this leads to constructive interference. Edit: scooped by @hutchphd on the transit time thing.

songoku
I think I get it. Thank you very much for all the help and explanation kuruman, hutchphd, jbriggs444

hutchphd and kuruman

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