Question

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 a room, its intensity essentially remains constant.
What geometrical characteristic of LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?

Answer 1

Given,
For a light Bulb (point source) intensity at a distance r



$I=\frac{P}{A}=\frac{P}{4\pi r^2}$

$I\alpha \frac{1}{r^2}$

As distance is doubled
$\frac{I_2}{I_1}={\left(\frac{r_1}{r_2}\right)}^2$

$\frac{I_2}{I_1}={\left(\frac{r}{2r}\right)}^2$

$I_2=\frac{I_1}{4}$

Some geometrical characteristics of LASER beam which are responsible for the constant intensity is
(i) Unidirectional
(ii) Monochromatic
(iii) Coherent light
(iv) Highly collimated

These characteristics are missing in the case of normal light from the bulb.

Final Answer: Intensity is reduced to one fourth. This is because the light beam spreads, as it propagates into a spherical region of area $4\pi r^2$, but LASER does not spread and hence its intensity remains constant.