The Human Eye

Refraction of Light through a prism

When light rays enter from a rarer medium to denser medium it bends towards normal and vice versa. This phenomenon is called refraction of light.

Prism

An object made of transparent material like glass or plastic that has at least two flat surfaces that form an acute angle (less than 90 degree) is called a prism.

Refraction of Light through a Prism

When a light ray from air enters in a glass prism it bends towards normal. And when this light rays emerges out from the glass prism and enters in the air, it bends away from the normal.

This refraction of light even through a prism follows Snell's Law which states that

(i) The angle of incident is equal to the angle of reflection, and

(ii) The incident rays, the normal to the refracting surface at the point of incidence and the refracted or reflected ray, all lie in the same plane.

Let consider a light ray PQ enters in the glass prism ABC from a point E. After entering in the prism from air, this light ray bends towards normal NN' and travels in prism through a path EF. When this light ray EF emerges out from the glass prism, i.e. enter from glass prism to air, bends away from normal MM' and goes towards RS.

Here,

∠A is the angle of prism

PQ or PQE or PE is the incident ray

E is the point of incident at one surface of prism

NN' is the normal to the point of incidence E

∠ i is the angle of incidence

EF is the refracted ray

PS is the emergent ray

∠ e is the angle of emergence

MM' is the normal at point of emergence F

FS or RS or FRS is the emergent ray

GH which is shown with dotted lines, is the actual path of light ray PQ

GS is the path of light ray after deviation

∠D is called the angle of deviation.

Thus, a light ray deviates at a ∠D which is called angle of deviation, after entering emerging out from a prism i.e. after refraction through a glass prism. This happens because of acute angle made by surface of prism. While a light ray emerges out from a rectangular glass slab, it goes parallel to the incident ray.

Dispersion of While Light By a Glass Prism

Splitting of white light in its constituent colors (seven colors) is called the dispersion of light.

When a white light is refracted through a prism it produces a band of seven colors. These colors are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The acronym of these seven colors is VIBGYOR.

Full form of Acronym VIBGYOR

V - Stands for Violet

I - Stands for Indigo

B - Stands for Blue

G - Stands for Green

Y - Stands for Yellow

O - Stands for Orange

R - Stands for Red

The band of colors in which a white light splits after refraction through a prism is called its Spectrum.

Why a white light dispersed into seven colors?

Light travels with different speed in different medium. The speed of light is highest in vacuum and speed of light decreases with increase the density of medium, for example the speed of light is less in the water than air; because water is denser than air.

Thus, because of change of speed light gets refracted at a different angles when enter into a new medium.

The degree of bending of light path depends upon following two things

(i) Angle of incidence and

(ii) The ratio between the refractive indices of given pair of media

The refractive index varies with the wave light or color of light for different media. Thus, the variation of refractive indexes of wavelength or color light for the given pair of media is called the dispersion of light.

This cause light of different colors to be refracted differently when enters from air to a prism and to leave the prism at different angles, creating an effect similar to rainbow. And after leaving from a prism a beam of light is separated into its constituent spectrum of colors.

Isaac Newton and Spectrum of Light

Earlier it was believed that white light was colorless and it was a prism that produced the color. It was Isaac Newton who demonstrated and concluded that white light comprises of seven constituent colors and gives spectrum of those colors when refracted through a prism.

A white light is a mixture of different frequencies and after refraction through a prism light of different frequencies get bent slightly differently because of slowing down their frequencies, i.e. speed.

Red color has greatest frequency and bends at lowest angle while violet color of light has lowest frequency and bends at highest angle.

This is the cause that spectrum obtained has violet color at lower and red color is at upper most end. In a rainbow or spectrum of light obtained from a prism has red color at upper end while violet color has lower end.

Why red color is used as danger or to stop at traffic signals?

Red color has highest frequency resulting in travel to more distance. This means red color can be seen from more distance compare to other color because of its frequency. Thus, red color is used for danger sign or to stop at traffic signals as it can be seen from more distance clearly.

Rainbow

A rainbow is a natural spectrum appearing in the sky after a rain shower. Rainbow forms or appears because of dispersion of light through tiny droplets of rain left suspended in atmosphere after rain. Rainbow is always formed at the opposite side to that of the Sun.

Tiny droplets of rain act like small prisms. When sunlight passes through those droplets, beam of sun light gets reflected internally and finally refracted and dispersed and gives spectrum at the opposite side of the Sun. This spectrum is called Rainbow which we can see through our naked eyes.

How to make an Artificial Rainbow?

Take a tumbler made of plain glass.

Fill it with clean water.

Let beam of sun light coming through slit of window or door to pass through the glass filled with water. A torch with clear white light can also be used to obtain beam of white light.