What this implies is that white light is spread out according to wavelength in a rainbow. Dispersion is defined as the spreading of white light into its full spectrum of wavelengths. More technically, dispersion occurs whenever there is a process that changes the direction of light in a manner that depends on wavelength. Dispersion, as a general phenomenon, can occur for any type of wave and always involves wavelength-dependent processes.
Figure 2. Even though rainbows are associated with seven colors, the rainbow is a continuous distribution of colors according to wavelengths. Refraction is responsible for dispersion in rainbows and many other situations. The angle of refraction depends on the index of refraction, as we saw in The Law of Refraction. We know that the index of refraction n depends on the medium.
But for a given medium, n also depends on wavelength. See Table 1. Note that, for a given medium, n increases as wavelength decreases and is greatest for violet light.
Thus violet light is bent more than red light, as shown for a prism in Figure 3b, and the light is dispersed into the same sequence of wavelengths as seen in Figure 1 and Figure 2. Figure 3. Since the index of refraction varies with wavelength, the angles of refraction vary with wavelength.
A sequence of red to violet is produced, because the index of refraction increases steadily with decreasing wavelength. Any type of wave can exhibit dispersion. Sound waves, all types of electromagnetic waves, and water waves can be dispersed according to wavelength. Dispersion occurs whenever the speed of propagation depends on wavelength, thus separating and spreading out various wavelengths.
Dispersion may require special circumstances and can result in spectacular displays such as in the production of a rainbow. This is also true for sound, since all frequencies ordinarily travel at the same speed.
If you listen to sound through a long tube, such as a vacuum cleaner hose, you can easily hear it is dispersed by interaction with the tube. Dispersion, in fact, can reveal a great deal about what the wave has encountered that disperses its wavelengths. The dispersion of electromagnetic radiation from outer space, for example, has revealed much about what exists between the stars—the so-called empty space.
Figure 4. Part of the light falling on this water drop enters and is reflected from the back of the drop. This light is refracted and dispersed both as it enters and as it leaves the drop. Now the question remains, why do different wavelength photons change angle differently at the edge of a new medium?
I am going to use lattice structure here, but for air and water it is hard to talk about a lattice structure, so in this case what I mean is the molecular structure. First of all, lets clarify that the change in angle not only happens at the edge of a medium, but:. So, shooting back the question as a boomerang, why do photons travel parallel in white light inside a single medium, with no density and lattice structure changes :. So, everytime the lattice structure changes in any way like density or broken or the structure itself, or different covalent bonds , the photons will diffract change angle , and the different wavelength photons will change angle in different amounts.
Now you see that the answer to your question, why do the photons change angle in different amounts still remains, and the answer is the lattice structure. To be more precise, in air, or water, there is no lattice like a solid , but the structure of the molecules, and densities, can still be different. Still, water can act as a prism in air, and you get the same rainbow effect.
Because the edge of the water and air has a different molecular structure. Now the question is why do different wavelength photons bend differently at the edge of different structural media?
So shooting back the question again as a boomerang, why does a group of certain same wavelength photons bend parallel the same way at the edge of a new medium?
Why does the wavelength decide the angle? First of all, there is no two photons with the same exact wavelength, as the wavelength is continuous.
But, with our eyes, we cannot tell the difference at a certain level, so let's say there are two photons that seem to have the same wavelength for our viewing purposes. Why do these photons bend at the same angle at the edge?
Why do you see a static image of the colors at the prism, and why doesn't the absolute angles of the different color photons randomly change? And why doesn't the relative angle randomly change? Let me elaborate on the QM answer. When a photon interacts with an atom, three things can happen:. Now in the case of the prism, it is elastic scattering.
This is the only way, that the energy of the photons is kept, phase is kept, and relative energy is kept, and relative phase is kept. Now with elastic scattering, as the photons in the white light, combined of all the different wavelengths from the Sun traveling parallel reach the edge of the glass prism from the air, what happens is:. When the air is full of water, like after a rainstorm, the water droplets act like a prism and can make a rainbow.
Rainbows are circular in shape because the prisms raindrops that created them are spherical. U niversity of W isconsin —Madison. What is Heat? What is Light? What is Magnetism? What is Motion? What is Sound? Donate Volunteer.
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