Video: Definition of Dispersion of White Light

Which of the following is the term used to refer to the separation of white light into its component wavelengths due to the variation of the refractive index with the wavelength of an object that the white light passes through? [A] Aberration [B] Diffraction [C] Dispersion [D] Distortion [E] Deviation

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Video Transcript

Which of the following is the term used to refer to the separation of white light into its component wavelengths due to the variation of the refractive index with the wavelength of an object that the white light passes through? (A) Aberration, (B) diffraction, (C) dispersion, (D) distortion, (E) deviation.

Okay, so in this exercise, we’re looking for a word that defines or describes this process here. We’re told that what’s taking place is white light is incident on some object. And then, because the index of refraction of that object varies with wavelength, it refracts the different colors differently, so they spread out like this. And we want to identify a term from among these five that describes this process.

We can start from the top and work our way down, first aberration. This term describes something about an optical system that causes it to form images in an imperfect way. It could be, for example, that a lens, say, has an aberration. And this makes it refract light improperly, leading to a low quality image or even no image at all. So this term aberration does have to do with light, but not in the way we’re describing it here. So we’ll cross this option off our list.

Next, we get to diffraction. This is a property of light that describes how it spreads out when passing through a narrow opening or around a barrier. For example, if we send coherent light waves through a small opening like this, then diffraction makes those waves spread out through the opening like this. So this term does describe the spreading out of waves of light, but it’s not because of a variation of refractive index in a material. So this term isn’t quite the right one either.

Next, we get to dispersion, and this term agrees completely with what’s written in our question statement. When an object like this prism over here separates out white light according to its component wavelengths, in this case colors, because the refractive index of that prism varies with wavelength, then that is known as dispersion. So option (C) looks like it’ll be our answer. But just to make sure, let’s check options (D) and (E). Option (D) suggests distortion as the word for describing this process. But, in a physics context, this word typically means something different.

Say that we have a wave of light and we want to amplify the wave. If we do that and the amplified wave comes out looking like this, we say there’s been amplitude distortion or simply distortion of this wave. So this term distortion is not a match for describing white light being separated into its component wavelengths. And lastly, let’s think of this term deviation. Now, deviation actually is related to the way a prism interacts with light. It describes how light is refracted at this face here and then at this face here of the prism in such a way that its final trajectory, say in the case of our beam of red light, is deviated by some amount from the ray’s original direction.

However, this term does not describe how the refractive index of an object can vary with wavelength. And, therefore, though it describes how a particular wavelength of light is deviated as it passes through some object, it doesn’t explain the separation of white light into its component wavelengths. So we won’t choose option (E) either. And this confirms to us that option (C), dispersion, is the correct choice. This term refers to the separation of white light into its component wavelengths due to the variation of the refractive index with the wavelength of an object the white light passes through.