# Refraction through prism

We know that the light gets refracted when it enter a medium low/high refractive index.

But why light is not refracted when it comes out from the vertex of a prism.

• I think you will need to clarify your question in order to get a useful answer. What do you mean when it comes out from the vertex of the prism? A diagram would be helpful. – Chris Mueller Jan 19 '14 at 16:37
• I think he's referring to light leaving the point of a prism. – mcFreid Jan 19 '14 at 16:58
• Very poorly worded: are you asking about diffraction or refraction? The vertex is a line, so for all practical purposes, no light passes through it. Any light you do see there gets out through imperfections or chips. – Carl Witthoft Jan 19 '14 at 17:21

Light is refracted on the way in and on the way out. The refractive index varies with the wavelength of the light, red being refracted less than blue.

If the "in" and "out" faces of the prism were parallel then the difference in refraction effectively cancels out - look through a window and light directions are not changed. But in a triangular prism the "cancelling " doesn't happen. You can see this if you draw a ray diagram, consult an elementary level book on optics, or just look at the above diagram.

• Sir, you said that Light is refracted on the way in and on the way out. I have done an experiment using pins and a prism and I found that when light is passed through the vertices there was no refraction. Kindly tell me why there is no refraction when light is passed through the vertices. – Singh Mar 21 '14 at 3:33
• I'm not sure I understand what you are doing, or why. However you would need a very narrow beam to shine through a vertex, and I think you will find that the vertex is actually round off, and almost flat, so the light will hit it near the normal, hence very little refraction or dispersion. – Dr Chuck Mar 21 '14 at 8:18

Light gets refracted at ANY interface between two transparent mediums, following the Snell-Descartes law:

$n_1\sin(i_1)=n_2\sin(i_2)$

That's it. The equation also accounts for the fact that there is no change of direction at normal incidence ($i_1=0 \implies i_2=0$).

Most prisms, certainly one you can hold in your hand, are large with respect to the wavelength of light, so diffraction is a insignificant effect. Prisms produce the color spectrum from sunlight, for example, due to refraction, not due to diffraction.

• True but not clear it is related to the question asked. – Carl Witthoft Jan 19 '14 at 17:22