Dispersion Of White Light By A Glass Prism
What happens when light falls on the surface of a compact disc (CD) or a prism? We see that when light falls on a CD or. a prism, it splits up into its many colours. The process of splitting up of white light into many colours is called dispersion.
The band of colours produced when white light is split up is called a spectrum. A spectrum is formed when white light passes through a prism. A rainbow is an example of a spectrum. Water droplets in the sky split up the sun’s rays and form a spectrum, which is called a rainbow.
When a ray of white light (sunlight) enters a glass prism (denser medium). It emerges out and broken into seven colours. This phenomenon, due to which different components of a white light are separated is called (dispersion)
Explanation: It is due to different refractive index of different components of white light. White light has seven colours, namely, violet indigo, blue, green, yellow, orange and red (remembered by the word VIBGYOR). In air (strictly in vacuum) light waves of all colours have same velocity (3 × 108 m/s). But in a denser medium, their velocities become less and different. Red light waves, being longest in length, travel fastest and have maximum velocity. Violet light waves, being shortest in length, travel slowest and have minimum velocity in the denser medium.
The refractive index (µ) of a medium for a wave is given by the relation.
\(\mu =\frac{\text{Velocity}\,\text{of}\,\text{Wave}\,\text{in}\,\text{air}\,\text{(or}\,\text{vacuum)}}{\text{Velocity}\,\text{of}\,\text{wave}\,\text{in}\,\text{the}\,\text{medium}}=\frac{c}{v}\)
As µ is maximum for violet so it bends maximum
When this dispersed white light is made to fall on a white screen, we get a seven coloured band or light. This coloured band is called spectrum.
Activity
Aim: To observe the spectrum.
Materials needed: A plane (transparent, without designs) glass/plastic container/tumbler, a plane mirror with the edges filed (long enough for it to rest on the edge of the container at an angle of about 45°), and water.
Method:
- Fill the glass/plastic container with water (about three-fourth).
- Dip the mirror, so that it rests at an angle (of about 45°) on the wall of the container.
- Place this container, with the mirror facing the sun, near a window.
- Adjust the direction of the mirror by moving the container so that it reflects the sunlight on the wall next to the window. Use a plain white sheet, if the wall or wall colour is not convenient.
Observation: You will see a lovely spectrum (like an inverted rainbow) on the wall. Shift the container until you get a sharp spectrum.
Note: This activity is best done early in the morning or late in the evening, when the sun’s rays are coming in at a large angle.