Resource Lesson
Dispersion
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As you can see in the diagram to the right each wavelength in the electromagnetic radiation actually has a unique
index of refraction
in any given specific medium. The value of the index is an indication of how closely the frequency of the electromagnetic radiation matches the resonance frequency of the electrons in the medium. The closer these values coincide, the greater the value of
n
and the greater the
interaction between the photons with the electrons in the medium
.
College Physics
, Wilson and Buffa, page 693
These distinctive values for the refractive index,
n
, cause white light to be dispersed, or break into its component frequencies, as it travels through an optically dense medium.
This effect is best noticed when light enters a medium obliquely, as in the triangular prism shown below. Since the red light interacts least with the electrons in the glass, it emerges first and is least bent. While the violet photons interact the most strongly with the electrons in the glass, emerge last, and are bent, or deviated from their original path, at the largest angle. This produces the familiar spectrum of light you see when sunlight passes through crystal chandeliers.
Physlet Animations
dispersion through a glass prism
dispersion through a glass slab
Continuing with the information learned in the lesson on
Snell's Law
we will now examine the angles resulting when polychromatic light passes through a transparent substance.
Refer to the following information for the next three questions.
Polychromatic magenta light enters a 30º-60º-90º triangular piece of flint glass perpendicularly as shown below.
Will the beams "bend" as they enter the prism or do they continue along a straight path?
Within the flint glass, the blue (460 nm) and red (660 nm) component wavelengths have different indices of refraction: blue n = 1.665; red n = 1.615. What is the angle between the component colors when they emerge from the prism?
Why would crown glass not produce as nice a spectrum?
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