Ncert Exercises & Solutions

What happens to the intensity of light from a bulb if the distance from the bulb is doubled? As a laser beam travels across the length of the room, its intensity essentially remains constant. What geometrical characteristic of the LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?

Hint: The intensity of light of the bulb depends on the distance from the bulb.

Step 1: Find the variation in the intensity of light of the bulb.

As the distance is doubled, the area of the spherical region

(4 {πr}^{2})

will become four times, so the intensity becomes one-fourth of the initial value

(∵ I \propto \frac{1}{r^{2}})

but in the case of laser, it does not spread. So its intensity remains the same.

Step 2: Find the characteristics of the LASER beam.

Geometrical characteristics of the LASER beam which are responsible for the constant intensity are as following:

(i) Unidirectional

(ii) Monochromatic

(iii) Coherent light

(iv) Highly collimated

These characteristic are missing in the case of light from the bulb.

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