What causes diffraction of light in a prism?

Question

How does light change velocity inside the prims and why is it dispersed at different frequency than the input frequency?

Speeed of light is not constant?Why?

Answer 1

Well, "diffraction" is the bending, spreading and interference of waves when they pass by an obstruction or through a gap, while "refraction", is the change in direction of a wave due to a change in its velocity. This is most commonly seen when a wave passes from one medium to another, such as the case of a prism.

For prisms, light moves from one medium (e.g. air) to another denser medium (the glass of the prism), where it is slowed down and as a result either bent (refracted) or reflected. The angle that the beam of light makes with the interface, the frquency of the light, and as well as the refractive indices of the two media determine whether it is reflected or refracted, and by how much. White light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light. Upon exiting the prism into air, the different freqencies being bent slightly different therefore displays all the colors of the light like a rainbow.

Answer 2

You are talking about REfraction, not DIFfraction.

The frequency is the same. What changes is the wavelength. This is because the actual wavelength is the sum of the contributions of all light at the given frequency, and in the prism there are contributions from the electrons of the atoms which make up the glass. They are being jiggled by the incident radiation, and radiate in response at the same frequency.

If yoy have two colors, the electrons will jiggle at a linear combination of the frequencies, and so will re-radiate in two colors. The response--the part of the electron motion--for each frequency will be different, so that the wavelengths of different colors will be changed by different amounts.

Now the refraction--the bending of the path of a light ray--occurs because the light is a set of wavefronts. When you change the wavelength, in this case shortening it, the speed of the wave decreases (the frequency stays the same, speed=wavelength*frequency). So the part of the wave inside the prism just drags along, and the wavefront seems to bend more parallel to the surface of the prism. This happens to different degrees for different colors--hence the prismatic effect.

Answer 3

Diffraction is caused (in accordance with Snell's Law) when light travels from one medium to another that has a diffrent index of refraction. But that isn't what causes light to be broken up into a 'spectrum'.

If the exact value of the index of refraction for a material is not a constant but, rather, a function of the wavelength of the light passing through it, then prismatic distortion (also called chromatic aberration) occurs. This dependence of the index of refraction on the wavelength is called 'dispersion'.

In spectrum analyzers dispersion is a good thing. In cameras it is a bad thing and is responsible for the 'redish' or 'bluish' outlines around objects that the really cheap, throwaway cameras frequently leave on their pictures.


Doug

Answer 4

"Benjamin N" is correct in every respect.

I would also like to add that white light is separated out by wavelengths in to a rainbow due to the difference of the indices of refraction in the prism. The light bends as it enters the prism, but the amount each color in white light bends is, every-so-slightly, different.

Snell's Law describes the angle to the normal which the light rays takes as it enters/exits a different medium,
n_1 * sin (theta_1) = n_2 * sin (theta_2)
Where n_1, and n_2 are the indices of refraction for the two mediums.
When the light enters a more optically dense medium (a higher index of refraction), light is bent away from the normal, and when light enters a less optically dense medium, light is bent towards the normal.

Glass is much more optically dense that air so this causes the light to bend.
But the index of refraction is not a constant number for all wavelengths of light, this means that "red" light will bend more than "violet" light....causing the white light beam to be broken up and spread out.


Light slows down as it enters a more optically dense medium. The amount it slows down can be found using the index of refraction....since the index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium,
n = c / v
So for an index of refraction value of (for example) 1.33, light travels 1.33 times faster in a vacuum than in the medium.

Answer 5

The different frequencies of light travel at slightly different speeds through the glass prism, causing them to spread out into a rainbow.

Answer 6

white light consist of seven light : red , violent, blue , red , green etc they have difference wavelengh and this causes differaction .

Answer 7

density of the matter of the prism

Answer 8

try