Dispersion of Light

Q:
How does dispersion of light take place? please explain in detail. Thanks!
- Anonymous
Singapore
A:
In Physics, ’dispersion’ is the property by which light is spread out according to its color as it passes through an object. For example, when you shine a white light into a , all of the different colors of light are bent different amounts, so they spread out and make a rainbow. This works because of the fact that different colors of light actually have different wavelengths. (The wavelength is the distance between the peaks in the wave.) Light interacts with the molecular structure of the prism differently depending on its wavelength. So different wavelengths of light are bent different amounts.

For a really detailed description of how this works (with equations and so-forth), you should take a look at a college electrodynamics book.

-Tamara

(published on 10/22/2007)

Follow-Up #1: Prisms versus slabs

Q:
Why dispersion takes place only in prism and not in a glass slab.
- anu (age 25)
new delhi, India
A:
When a beam of light travelling in one material enters a new material, and when it is perpendicular to the common surface, it goes straight into the new material without bending.  When the beam enters at an angle it is bent. The amount of bending is related to the relative indices of refraction. Now it turns out that the index of refraction of most materials is dependent on the wavelength of light.  Hence different wavelengths are bent by different amounts; hence dispersion.

In a slab of glass the effects on the beam due to the entrance and exit angles cancel out.   In a prism these effects do not cancel out due to the different entrance and exit angles.