Dispersion of Light
The scientist Newton was the first to notice that sunlight, or white light, is a combination of different colours of light. He found that when white light falls on a prism, it disperses into seven different colours to form a band of light.
Experiment: Experiments show that white light is composed of seven colours.
When white light passes through a fine slit S onto a glass prism P, a band of seven lights of different colours is seen on a screen placed on the other side of the prism. This band of light is called a spectrum.
The ribbon colours are violet, indigo, blue, green, yellow, orange, and red from bottom to bottom. For ease of memorisation, the first letter of each colour in the spectrum starting with violet is ‘VIBGYOR’.
Definition: The phenomenon of splitting multi-coloured (or mixed) light into different colours is called dispersion of light.
Looking at the spectrum, it can be seen that different colours of light have different deviations. Violet light has the highest deviation, and red light has the lowest deviation. That is why it is often said that the refractivity of different colours of light rays is different. Yellow light is called the mean ray because yellow light is between the red and violet light. Again, the amount of this deviation also depends on the refractive index of the prism material. The higher the refractive index of the prism, the greater the amount of deviation. So, it can be said that the refractive index of a medium is different for different colours of light. The refractive index of the medium is lowest for red light and highest for violet light.
The order of refraction of a medium for red, orange, yellow, green, blue, indigo, and violet light is,
µr < µo < µy < µg < µb < µi < µv
Dispersion of Light
All colours of light travel at the same speed in a vacuum or through air. But in any other medium, different colours of light have different velocities. According to the wave theory of light, the refractive index of a medium is equal to the ratio of the ‘velocity of light in vacuum’ to the ‘velocity of light in that medium’. Red light travels faster than violet light in glass. That is why the refractive index of the same medium is different for different colours of light. The refractive index of a medium for red light is lower than the refractive index of that medium for other colours of light. As we know, the lower the refractive index value of the medium, the lower the deviation of the ray. So, when white light is refracted, the red light in it bends the least and the violet light bends the most. The rest of the colour rays diverge between red and violet. So, to summarise, if the speed of light of different colours is different in a medium, then mixed light dispersion occurs in that medium, and therefore that medium is called a dispersive medium.
White light does not disperse in a vacuum because light of different colours travels at the same speed. But white light is dispersed in glass. So, glass is a dispersive medium, but void is not a dispersive medium. Incidentally, light of different colours travels at almost the same speed through air. So, it can be said that air is not a dispersive medium either.
Related Questions
- What is spectrum?
- What causes the dispersion of light?
- What is the dispersive medium?
- What do you mean by dispersion of light?
Sensation of Colour
Colour perception depends on the wavelength of light. White light emits many wavelengths of light. If the wavelength is within a certain range, the radiation is able to induce vision in our eyes. Only radiations with wavelengths between 4000 Å and 8000 Å are visible to our eyes. The same spectrum is called visible range or visible spectrum. The angstrom, Å unit (1 Å = 10-8 cm) is the wavelength of light in the seven fundamental colours of the visible spectrum (wavelength values are not very precise). As it turns out, red light has the longest wavelength and violet has the shortest wavelength.
Angular Dispersion and Dispersive Power
Let a thin prism have refractive angle A and refractive index µ. Hence, the amount of deviation when the light ray is refracted in the prism,
δ = (u – 1) A
Suppose a white light beam is incident on the refracting surface of a prism. As a result, the rays emitted from the prism are dispersed into rays of seven colours, from red to violet. Obviously, different colours of light have different deviations. The deviation of yellow rays in the middle of the spectrum,
δ = (u – 1) A ……..(1)
Here, µ = refraction of the prism or refraction between the prisms for yellow rays.
Similarly, if the refractive indices of the prism for red and violet light are µr and µv, respectively, and the deviation values are δr and δv, respectively,
δr = (µr – 1) A ………(2)
δv = (µv – 1) A ………(3)
Therefore, the difference or difference in deviation of violet-red light is,
δv – δr = (µv – µr) A ………(4)
(δv – δr) is called the angular dispersion with respect to these two colours.
Angular dispersion depends on the nature of the medium of the prism and the value of the refractive angle of the prism. The unit of angular dispersion is the degree, or radian.
Definition: The difference between two different colours of light due to refraction is called the angular dispersion of the incident light relative to the two colours.
ω is called the dispersive power of the refractive medium.
Dispersive power depends only on the nature of the medium. Dispersive power is a dimensionless quantity. It has no units.
Definition: The ratio between the deviations of the violet and red-light rays and the deviation of the middle (i.e., yellow) rays is called the dispersive power of the refractive medium.
From equation (6), it is found that,
and angular dispersion,
δv – δr = ω × δ = Dispersive power × Deviation of the intermediate beam
Impure and Pure Spectrum
In general, the seven colours in the spectrum formed by prisms are not distinctly and discretely present. That is, one colour is mixed with another colour. The spectrum thus produced is called the impure spectrum. In reality, it is not possible to separate a single ray of light. So, no matter how fine a hole the light passes through, it consists of multiple rays. Those multiple rays falling on the prism form separate spectra. As a result, different colours of one spectrum fall on different colours of another spectrum. Hence, no colour is clearly and distinctly distinguishable. That is, the spectrum becomes impure. Suppose a narrow beam of white light passes through the slit S. The beam SA of the beam is diffracted by the prism to form the spectrum in the R1V1 segment, and the SB beam is diffracted in the R2V2 segment to form the spectrum. The spectra generated for other rays between SA and SB are within R1V2. The resulting spectra overlap and create impure spectra.
Definition: A spectrum in which the light of one colour falling on the light of another colour does not clearly show each colour is called an impure spectrum.
The spectrum in which the light of one colour does not fall on the light of another colour so that each colour can be clearly seen is called a pure spectrum.