Wavelength dispersion (nm/mm) is non-linear varying as the diffraction angle and the distance to focus at each wavelength. In practice, wavelength dispersion is linear enough to present near constant resolution especially for low resolution instruments.
- Diffraction of sound waves and of light waves will be discussed in a later unit of The Physics Classroom Tutorial. Reflection, refraction and diffraction are all boundary behaviors of waves associated with the bending of the path of a wave. The bending of the path is an observable behavior when the medium is a two- or three-dimensional medium.
- Dispersion is defined to be the spreading of white light into its full spectrum of 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.
- Dispersion of light occurs when white light is separated into its different constituent colors because of refraction and Snell's law. White light only appears white because it is composed of every color on the visible spectrum. Although they are very close, the index of refraction for each color is unique in non-vacuous materials.
As we consider more phenomena associated with light, one of our primary concerns will be the direction that light is traveling. We already know that light, like any wave, travels in a direction perpendicular to its planes of constant phase:
Figure 3.6.1 – Light Waves Travel in Several Directions at Once
So in our wave view of light, we say that the light wave is traveling in many directions at once, but now we are going to change our perspective to that of an observer and a source. When we do that, we narrow down all the possible directions of the light wave motion to a single line, which we call a light ray. This is a directed line that originates at the source of light, and ends at the observer of the light:
Figure 3.6.2 – Source and Observer Define a Ray
When most people encounter the idea of a light ray for the first time, what they think of is a thinly-confined laser beam. This is not what is meant here! The ray has no physical meaning in terms of the confinement of light – we just use it as a simple geometrical device to link a source to an observer. Always keep in mind that the actual physical manifestation of the light is a wave that is usually traveling in many directions at once! Our use of rays will become so ubiquitous that this will be easy to forget.
When the distance between the drops is similar to the wavelength of visible light, the light that shines through the cloud droplets is diffracted and dispersed in the manner shown below. On a clear night, for example, the light you see coming from the moon is coning straight from the moon. However if a thin cloud layer is found between the observer and the moon, the diffraction and dispersion of the moonlight actually casts a light larger than the original light source. This 'crown' of light around the sun or moon is called the corona.
Difference Between Dispersion And Diffraction Of Light
When the cloud droplets are very uniform in size, the diffracted light can cause the corona to be separated into its component colors, with blue light to the inside of the red light.These colors may repeat themselves, surrounding the moon with a series of colored rings, becoming fainter as each subsequent ring is locatedfurther from the moon.
Also, a combination of refraction, reflection and diffraction can combine to produce other optical effects such as glories and the 'Heiligenschein' effect -- which is a bright area around the head of an observer's shadow on a surface containing spherical water droplets. Glories are the rings of illuminated light seen most commonly from plane's shadows as they fly over clouds of liquid water. In both phenomena, the light ultimately is bent close to 180° right back to the observer.
Dispersion Scattering Interference And Diffraction Of Light
As a beam of light encounters a water droplet, it is refracted as it enters the droplet. Portions of the light are then internally reflected off the backside of the droplet. Before the light exits the droplet completely, it diffracts along the droplet's outer surface for just an instant as a surface wave before refracting as it leaves the droplet.
Dispersion Of Light Experiment