Chapter 6: Use Your Brain Power! (77)

1. From incident white light, how will you obtain white emergent light by making use of two prisms?
→ If we arrange two prisms as shown below, we can obtain emergent white light. Procedure: Take a prism. Allow white light to fall on it. Obtain a spectrum. Take a second identical prism. Place it parallel to the first prism in an upside down position with the first prism. Allow the colours of the spectrum to pass through the second prism. Obtain the beam of light emerging from the other side of the second prism. The beam of light emerging from the other side of the second prism is a beam of white light. Explanation: White light is made up of seven colours. The first prism produces dispersion of white light while the second prism combines different colours to produce white light again. The net deviation of a ray of light is ZERO. Sir Isaac Newton demonstrated this property first. This experiment proved, that it was not the prism which added colours to the white light, but it was the property of the white light itself.
→ No. The light emitted by LED (Light Emitting Diode) does not contain all the wavelengths of the visible light (400 nm – 700 nm). Hence, the spectrum of LED is not the same as that of a light spectrum from a tungsten bulb or sunlight. 2 Marks

1. From incident white light, how will you obtain white emergent light by making use of two prisms?

→ If we arrange two prisms as shown below, we can obtain emergent white light.

Procedure:

Take a prism.
Allow white light to fall on it.
Obtain a spectrum.
Take a second identical prism.
Place it parallel to the first prism in an upside down position with the first prism.
Allow the colours of the spectrum to pass through the second prism.
Obtain the beam of light emerging from the other side of the second prism.
The beam of light emerging from the other side of the second prism is a beam of white light.

Explanation:

White light is made up of seven colours.
The first prism produces dispersion of white light while the second prism combines different colours to produce white light again.
The net deviation of a ray of light is ZERO.
Sir Isaac Newton demonstrated this property first. This experiment proved, that it was not the prism which added colours to the white light, but it was the property of the white light itself.

2. You must have seen chandeliers having glass prisms. The light from a tungsten bulb gets dispersed while passing through these prisms and we see coloured spectrum. If we use an LED light instead of a tungsten bulb, will we be able to see the same effect?

→ No. The light emitted by LED (Light Emitting Diode) does not contain all the wavelengths of the visible light (400 nm – 700 nm). Hence, the spectrum of LED is not the same as that of a light spectrum from a tungsten bulb or sunlight.
2 Marks