Pointing vectors $S$ is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propagation. Mathematically, it is given by$S=\frac{1}{\mu }E\times B$. Show the nature of$S$ vs $t$ graph.

Electromagnetic waves with wavelength
(i) ${\lambda }_1$is used in satellite communication.
(ii) ${\lambda }_2$ is used to kill germs in water purifier.
(iii) ${\lambda }_3$ is used to detect leakage of oil in underground pipelines.
(iv) ${\lambda }_4$ is used to improve visibility in runways during fog and mist conditions.

(a) Identify and name the part of electromagnetic spectrum to which these radiations belong. Also, write one more application of each.

(b) Arrange these wavelengths in ascending order of their magnitude.

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?