Diffraction vs Dispersion:Comparative Analysis


In optics, the study of diffraction and dispersion helps us understand the behavior of light, such as the bending and spreading of light when it passes through a medium.

The light wave has the ability to bend and then spread out itself; this behavior of light can be distinguished as diffraction and dispersion. Both terms sound similar, but there are certain natures that differ them from one another. In this post, we give a brief comparison of diffraction vs dispersion of light and their behavior.

Comparison of Diffraction vs Dispersion

The table given below helps you to understand the differences and also similarities between diffraction vs dispersion.

DiffractionDispersion
DefinitionWhen the light strikes the edge of the hurdle, it bends at the corner, creating the geometrical fringes.The propagation of light waves from one medium to another medium of different refractive indices splits into different wavelengths.
Dependency on wavelengthDiffraction depends on the wavelength of the incident light, as if the light wave of longer wavelength is incident on the slit, it diffracts more.The spreading up of light waves depends on their wavelength. The light with a shorter wavelength disperse first, and then, in increasing order, they spread out of the medium.
The velocity of the lightThe velocity of the light before and after diffraction of light remains constant, i.e., diffraction does not affect the velocity.There will be a change in the velocity of the incident light as the light is travelling between the two mediums of different refractive indices.
MonochromaticityThe diffraction can be obtained using both monochromatic and polychromatic light. We get diffraction fringes of seven colors in the polychromatic, i.e., using white light and fringes of dark and bright by using the monochromatic light.Dispersion can be achieved only by using white light or polychromatic light. We do not see the dispersion of waves in monochromatic light.
Dependency on the angle of deviationDiffraction is independent of the angle of deviation of the light if the monochromatic light is used for the diffraction process. If we are using white light for the diffraction process, it depends on the angle of deviation.Dispersion entirely depends on the angle of minimum deviation.
Refractive indices of the mediumSince diffraction depends on the wavelength and frequency of the incident light, which depends on the refractive index of the medium, thus diffraction also depends on the refractive indices of the medium of propagation.The Refractive index of the propagating medium is the leading cause of the dispersion. The dispersion of the light takes place when the light travels from a denser medium to a rarer medium or vice versa of different refractive indices.
ExamplesA diffraction grating is the best example of the diffraction of white light. Holograms and CD which reflects the rainbow colors are due to diffractionWhen white light travels through the prism, the white light disperses into seven colors of a different wavelength. The formation of a rainbow is also an example of dispersion.
Comparison of diffraction vs dispersion
diffraction vs dispersion
Image to illustrate comparison of diffraction vs dispersion

Some interesting points to remember

  • Not only light the sound waves can also undergo diffraction as well as dispersion. The diffraction of sound is already discussed in the previous article. The dispersion of sound waves is called acoustic dispersion, in which sounds of different frequencies are separated when a sound wave is passed through a certain medium.
  • In some cases, a diffracted light can be dispersed, i.e., when a narrow slit diffracts white light on a grating, the diffracted light is then dispersed into its prominent colors. For example, on the CD, we see the diffraction pattern that disperses into all the prominent colors of the rainbow.
  • The reason for dispersion is refraction. When the white light is refracted by a glass prism, we obtain the dispersion of colors of various wavelengths. For example, when the rainbow is formed, we see the seven colors, which are separated accordingly by their wavelength; the white light is first refracted, then dispersed into the colors of the rainbow.

From the study of diffraction vs dispersion, we know that dispersion is related to both diffraction and refraction, depending on the medium of propagation and the nature of the incident light.

Frequently asked questions                                                         

What is meant by angle of deviation?

When a wave passes from one medium to another, the wave may bend, making a certain angle θ. The bent wave that emerges out of the medium is called refracted or diffracted wave.

The incident wave deviated its direction when it propagated between two mediums or slits. The angle between the incident wave and the deviated wave is called the angle of deviation. The angle of deviation on the wavelength of the emerging wave. The angle of deviation can also be given by taking the difference between the angle of the incident wave and the angle of the emerging wave.

If we are talking about white light, then diffraction and dispersion are related.

When white light is passed through a narrow slit, and it is made to an incident on the grating, then we can observe the diffraction pattern on the grating. The diffraction pattern obtained by the grating is the dispersed colours of different wavelengths of light. Thus diffraction is related to dispersion splitting the white light into individual wavelengths.

Does the frequency of wave change during dispersion?

The dispersion is a property of the wave which depends on the speed as well as wavelength. Since wavelength is inversely related to frequency, the dispersion also depends on the frequency.

The shape of the wave pulse changes as it travels from one medium to another medium of dissimilar refractive indices, this change in the shape leads to the change in the velocity of the wave phase, which depends on the frequency. Thus any medium acts as a dispersive medium; there will be a regular change in the frequency.

The dispersion of light can be seen in a prism, not on a glass slab; why?

Even though the glass slab has the ability to refract the incident light, it does not have the ability to disperse the incident light is due to its geometrical structure.

A glass slab is designed with two parallel interfaces, which refracts the incident light and then emerges out of the slab parallel to the incident light; also, they are so closely spaced that splitting up of light would be difficult. However, in the case of a prism, the refractive interfaces are not parallel, and there is a sufficient gap to separate the colors into the desired wavelength.

Keerthi Murthi

I am Keerthi K Murthy, I have completed post graduation in Physics, with the specialization in the field of solid state physics. I have always consider physics as a fundamental subject which is connected to our daily life. Being a science student I enjoy exploring new things in physics. As a writer my goal is to reach the readers with the simplified manner through my articles. Reach me – keerthikmurthy24@gmail.com

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