Refraction and dispersion of light

The splitting of white light into its constituent spectrum of colours when it travels through a glass prism is known as ‘dispersion of light’ .  At various frequencies and angles, white light splits into its constituent colours. Refraction is the bending of a ray when it passes at an angle from one medium into another with a different speed (as when light passes from air to water).

Dispersion of Light and Rainbow

A rainbow appears when it rains and the sun shines at the same moment. When compared to a glass prism, each raindrop has a different form and substance, but it still affects light in the same way. 

As white sunlight travels from air into a drop of water, the constituent colours of light slow down to varying rates and frequencies. As violet light penetrates the raindrop, it bends at a sharp angle. Some of the light is reflected back to the observer, while the remainder is released into the atmosphere.

Some of the reflected light escapes the drop and bends as it returns to the atmosphere. In this way, each raindrop disperses the white sunlight into its constituent hues. We observe wide bands of colour because we only see one colour from each raindrop, as if various moist spots were propagating a different single hue.

On rare occasions, a double rainbow can be seen, consisting of a bright rainbow with a fainter rainbow above it. The fainter rainbow appears in the same way as the sharper rainbow, except that the light is reflected twice inside the raindrop rather than once.As a result of the twofold reflection, the light exits the raindrop at opposite angles, and we see it above the sharper rainbow.

Refraction and dispersion  through a Glass Prism

Refraction through glass prism

A glass prism is made up of three rectangular lateral surfaces and two triangular bases that are all tilted at an angle. This angle is known as the ‘Angle of the Prism.’ Consider a triangular prism with a light beam travelling through it.  The angle of the prism is represented by D in the diagram.

According to Snell’s law, light bends towards normal when travelling from a rarer medium to a denser medium, and away from normal when travelling from a denser medium to a rarer medium. Because glass is denser than air, when a light ray strikes the prism’s surface, it bends towards the normal. As a result, when ray JP hits the prism’s surface, it bends towards the normal GP, as shown in the diagram.

The emergent ray is turned on. It bends away from the norm as it transitions from glass to air. The angle of deviation, XFN, indicates how much the emerging ray has diverged from the incident beam. When the angle of incidence equals the angle of emergence, the angle of deviation is the smallest. JPG = YFN in the diagram, and XFN is the angle of departure, which is the smallest. The refracted ray PG’ is parallel to side EF in this situation.

This is how light (white light) disperses when it passes through a prism, splitting into its seven constituent colours. When white light is refracted, it breaks into its constituent hues due to the varied degrees of deviation experienced by different colours of different wavelengths.

Dispersion through  prism

The wavelength of the light determines the degree to which it refracts. Because each light wave has a unique wavelength, it will vary in a variety of ways. White light is composed of various wavelengths (colours) of light, including violet, indigo, blue, green, yellow, and red. Red has the longest wavelength, whereas violet has the shortest.

The wavelength is inversely proportional to the variance in the path of light. Red has the least deviated light, whereas violet has the most deviated light. When white light is passed through a prism, it produces a spectrum of seven hues, suggesting that white light is composed of seven separate colours.

Prism just acts as a channel for light dispersion in the seven main hues. Refraction happens when light strikes the prism. Because of the prism’s velocity differential, these deviated colours have discrete wavelengths and frequencies, and they deviate differently at different angles. As a result, red deviates the least since it has the longest wavelength, whereas violet deviates the most because it has the shortest.

Difference between dispersion and scattering of light

Dispersion of light is a phenomenon in which the phase velocity of a light wave depends on its frequency, whereas scattering of light is a phenomenon in which moving light radiation is forced to deviate from a straight trajectory by localised non-uniformities in the medium through which the light passes.

Conclusion

Light refraction can be noticed in many places in our daily lives. It alters the perspective of objects beneath the surface of the water, making them appear closer than they are. It serves as the foundation for optical lenses, which enable the operation of equipment such as glasses, cameras, binoculars, microscopes, and the human eye. Refraction is responsible for rainbows and the transformation of white light into a rainbow spectrum as it travels through a glass prism.