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Name: Prabhat
Status: student
Grade: 6-8
Country: India
Date: Summer 2012


Question:
In a prism, white light disperses at the first surface, but not inside a glass block. Why?

Replies:
Hi Prabhat,

The light is still dispersing while in the glass, but it is not very obvious to us because the dispersion is very small. The dispersion begins at the point of where the two refractive materials meet the coincident light at the incidence angle and continues.

Let us think of sunlight in one material, air, The light moves through air and strikes the second material, glass, which has a different index of refraction at an angle, the incidence angle... the light begins to separate. The light continues to separate while it moves through the glass then it exits into air at another angle while it is still being dispersed. Inside the prism we may not see the tiny dispersion in such a short distance because our eyes simply are not able to discriminate the colors.

Only when we allow the dispersed light to shine its wavelengths for several meters may we determine that the angles of dispersion has begun, it persisted and the separated colors may be easily seen as a rainbow.

Wishing rainbows for all! PEHughes, Ph.D. Milford, NH


The change in refractive index at the interface is where you see the direction change -- once it is in a given medium, it simply moves straight through it (air, water, glass, etc.). Once it reaches another interface (with a change in RI), the direction will change a section time.

Hope this helps, Burr Zimmerman


Dear Prabhat,

Light travels at different speeds through different materials. In other words, it's fastest in a vacuum, slower in air, slower still in glass, even slower in a diamond. When light enters the glass, it slows down, (not a whole lot because light travels really, really, fast as you know). The light ray is bent slightly, this is called refraction. Evidence of this can be seen by shining a ray of light through a glass block on a white sheet of paper. You can see the bend. If your glass block is big enough, you can see the light ray come out again and bend back toward the way it first went in; it speeds back up because it is again in air. Diamonds sparkle because of refraction. Diamonds are cut to take advantage of this refraction.

The light does not bend anymore in the block because the glass does not change. Light travels at a constant speed through whatever sort of material it is in. The speed of the light for most transparent materials has been figured out. Transparent materials have what's called an index of refraction. This is a number that can be used in a cool physics equation called Snell's Law to determine how much the light will bend. You might learn more about this is you take physics in the future.

Have fun with science! Mrs. Martha Croll


Light refracts when going straight does not result in the shortest travel time. Inside the glass block, the speed of light is the same along all possible paths, so going straight is the quickest way.

Note, by the way, that light is not actually /trying/ to minimize its travel time. It is not trying to do anything; it is light - no brains, no motivation. What is actually happening is interference among all the possible paths that light could have taken. It turns out that paths near the shortest-time path are always the ones that add together, while all the other paths cancel with each other, but you need calculus (or a much more thorough explanation) to see how that works. If you want to understand this without calculus, Feynman explains it beautifully in the book "QED".

Tim Mooney


I think you are making an incorrect assumption, specifically that white light does not disperse inside a glass prism. Also you are apparently using the terms “glass block” and “glass prism” interchangeably. That is incorrect.

The mechanism for the behavior of a prism is that the speed of light inside a prism decreases with the path length traveled by the light beam. The change depends on wavelength of the light, and the optical properties of the materials of construction. You can find explanations of varying complexity if you search the term “how prisms work”.

Vince Calder


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