Why does dispersion take place in a prism

Question Why does dispersion take place when light is passed through prism and not through glass slab? Asked by: Kavita Answer A light ray is refracted bent when it passes from one medium to another at an angle and its speed changes. At the interface, it is bent in one direction if the material it enters is denser when light slows down and in the OTHER direction if the material is less dense when light speeds up. Because different wavelengths colors of light travel through a medium at different speeds, the amount of bending is different for different wavelengths.

Violet is bent the most and red the least because violet light has a shorter wavelength, and short wavelengths travel more slowly through a medium than longer ones do. Because white light is made up of ALL visible wavelengths, its colors can be separated dispersed by this difference in behavior. When light passes through glass, it encounters TWO interfaces--one entering and the other leaving. It slows down at the first interface and speeds back up at the second.

If the two interface surfaces are parallel to each other, as in a 'slab' of glass, all of the bending and dispersion that takes place at the first interfaces is exactly reversed at the second, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ray, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined.

If the second interface is NOT parallel to the first, as in a prism, the effects of the first interface are NOT reversed and the colors separated at that interface continue along different paths upon leaving the glass. Answered by: Paul Walorski, B. Physics, Part-time Physics Instructor Well, in fact, dispersion does occur when light is passed through a glass slab - it is just harder to observe that way.

Is it a combination of two prisms As we know, that dispersion is the splitting up of white light or a composite light into its constituent colours. The dispersion occurs in prism but not in glass slab because of the geometric design. In a slab, the opposite sides are parallel to each other whereas in the case of the prism, the sides are not parallel to each other.

The absorbed energy causes the electrons in the atom to vibrate. If the frequency of the light wave does not match the resonance frequency of the vibrating electrons, then the light will be reemitted by the atom at the same frequency at which it impinged upon it. The light wave then travels through the interatomic vacuum towards the next atom of the material. Once it impinges upon the next atom, the process of absorption and re-emission is repeated.

The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance.

Thus, while a light wave travels through a vacuum at a speed of c 3. The index of refraction value n provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void.

The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction. A light wave would be slowed down to a greater extent when passing through such a material. What was not mentioned earlier in this unit is that the index of refraction values are dependent upon the frequency of light.

For visible light, the n value does not show a large variation with frequency, but nonetheless it shows a variation. For instance for some types of glass, the n value for frequencies of violet light is 1. The absorption and re-emission process causes the higher frequency lower wavelength violet light to travel slower through crown glass than the lower frequency higher wavelength red light. It is this difference in n value for the varying frequencies and wavelengths that causes the dispersion of light by a triangular prism.

Violet light, being slowed down to a greater extent by the absorption and re-emission process, refracts more than red light. Upon entry of white light at the first boundary of a triangular prism, there will be a slight separation of the white light into the component colors of the spectrum. The amount of overall refraction caused by the passage of a light ray through a prism is often expressed in terms of the angle of deviation.

The angle of deviation is the angle made between the incident ray of light entering the first face of the prism and the refracted ray that emerges from the second face of the prism.