Question

When light propagates in vacuum there is an electric field and a magnetic field. These fields
(a) are constant in time
(b) have zero average value
(c) are perpendicular to the direction of propagation of light.
(d) are mutually perpendicular.

Answer 1

(b) have zero average value

(c) are perpendicular to the direction of propagation of light.

(d) are mutually perpendicular.

EXPLANATION:   

The electric and magnetic fields vary with time and position, hence (a) is not true.          Like in sinewave where the average value of displacement within a wavelength is zero, the average value of electric and magnetic fields are zero in the light waves. (b) is true.       

The direction of electric and magnetic fields are perpendicular to the direction of propagation of light. Option (c) is true.         

The electric and the magnetic fields are also mutually perpendicular to each other. i.e. if light travels in x-direction then electric and magnetic fields will be either in y and z-direction or in z and y directions. Option (d) is true.