Refraction and dispersion of light through a prism

Definition of dispersion of light: The phenomenon in which the white light is splitting into seven different colours is known as the dispersion of light. The medium which conducts dispersion is called the dispersive medium. 

What is a Prism?

A prism is a solid geometric figure with equal and parallel rectilinear figures on all of its end sides. The sides of a prism are always parallelograms or rectangles. Prisms come in a variety of shapes, including triangles, squares, and rectangles. When used for refraction, prisms must be manufactured of transparent material. A prism’s at least two flat surfaces that form an acute angle(less than 90 degrees).

Let’s look at the refraction of light through a glass prism! 

Refraction Through a Prism

Let’s assume the ABC prism, which is a glass triangle prism. Let BC be the starting point, and AB and AC be the refracting surfaces. The line OP represents the incident light ray. The incident ray is the light that enters the prism. The speed of the light ray changes as it approaches the new surface on the prism’s side, and it bends, forming a new beam within the prism, which is known as the refracted ray.

This refracted ray will be referred to as PQ. The refracted light ray then passes through the prism and emerges from the opposite side. Because it emerges from the surface, the light ray is referred to as an emergent ray.

QR is the designation given to the emerging ray. Because of the shift in medium, the emergent ray QR bends at an angle from the refracted ray PQ.

When a ray of white light passes through a prism, it scatters into seven different colours. When white light is refracted, it breaks into its constituents due to the varying degrees of deviation experienced by different hues of light waves.

Phenomena caused by Refraction:

Many additional phenomena are caused by refraction.

• When looking at a straw dipped in a glass of water, the part in the air and the part in the water appear to be different straws! It appears to be warped.

• Travellers in the desert may mistakenly see water or trees on the ground when there is none. The term for this phenomenon is ‘mirage.’

• Some other natural phenomena, such as the twinkling of stars and the production of rainbows, are also caused by refraction.

• When sunlight passes through water droplets, it disperses into the seven hues that make up a rainbow. In truth, when light passes through a prism, rainbow-like things can form. When light passes through a prism, it might disperse due to the geometry of the prism. To understand this, consider how light refracts when it passes through a glass prism.

What happens in a prism?

The “Dispersion of light rays” is the main phenomenon that occurs in a prism. The main source of dispersion is a change in media. When a light beam changes medium, dispersion is the amount by which it is refracted depending on its frequency and wavelength. When a light ray passes through a prism, it is refracted and distributed into a band of seven colours known as VIBGYOR(Violet, indigo, blue,  green, yellow, orange, red) or the rainbow colours when it emerges from the prism medium.

In the case of a primary rainbow, the sunlight experiences total internal reflection once and refraction of light through prism twice before emerging from the water droplet. In the case of a secondary rainbow, the sunlight experiences total internal reflection twice before emerging out of the water droplet. All the water droplets at an angle of 42∘ in any direction from the observer cause a primary rainbow. All the water droplets at an angle of 52∘ in any direction from the observer cause a secondary rainbow.

As any water droplet at a specific angle in any direction causes a rainbow, the rainbow is a circular arc. Therefore, a secondary rainbow is wider and darker as compared to primary rainbow and thus is more difficult to see.

What is the dispersion of light by Prism?

Newton observed that when an inverted prism is kept in the path of dispersed light, they reform to form white light after going through the prism. Newton is the first to obtain a spectrum of sunlight by using a glass prism experiment. He tried to split the spectrum of light more by using another prism similar to the first prism, but he could not get any more colours. He repeated the same experiment using the second prism in an inverted position with respect to the first prism. It allowed all the colours of the spectrum to pass through a second prism. He established that white light is emerging on the other side of the second prism. He concluded that the Sun is made up of 7 visible colours called VIBGYOR.

Conclusion

A prism is a solid geometric figure with equal and parallel rectilinear figures on all of its end sides. Prisms are formed from materials that are permeable to the wavelengths for which they were created. Glass, plastic, and fluorite are among the most common materials used to construct prisms.  Refraction of light through a prism occurs when light passes through a glass prism and refracts both entering and exiting the prism. The emergent and incident rays are not parallel to each other because the entry and exit (refracting surfaces) are not parallel to each other.