Chapter 28, Problem 24Q

To determine

Why the street lamp intensity is much lesser than the laser beam intensity.

Answer to Problem 24Q

Solution:

The intensity of the laser beam is higher than the white light. This is because, laser beam is plane wave source and monochromatic and the white light is spherical wave source and composed of different wavelengths.

Explanation of Solution

The laser beam is a plane wave source and therefore, its intensity will not change as the wave travels away. As the wave travels a distance $r$ from the source, the intensity will be $0.0005\text{W}$ in the case of the laser beam.

The street lamp produces spherical waves as white light and therefore, its intensity will be decreasing in the order of $\frac{1}{r^2}$ as it travels away from the source. Therefore, its intensity will be less than that of the laser beam.

Another factor is wavelength. The laser is a monochromatic source but the white light constitute of different wavelengths. Therefore, the intensity of the white light is the total intensity of the constituent wavelengths.