Q.1. Which colour of the light has the longest wavelength? (2019)
Red has the longest wavelength among the given options.
Q.2. An em wave is propagating in a medium with a velocityThe instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along :- (2018)
A: –z direction
B: +z direction
C: –y direction
D: –x direction
(Therefore, electric field vector is along + y axis)
i.e., direction of magnetic filed vector is along + z direction.
Q.3. In an electromagnetic wave in free space the root mean square value of the electric field is Erms = 6V/m. The peak value of the magnetic field is :- (2017)
A: 2.83 × 10–8 T
B: 0.70 × 10–8 T
C: 4.23 × 10–8 T
D: 1.41 × 10–8 T
Q.4. Out of the following options which one can be used to produce a propagating electromagnetic wave ? (2016)
A: An accelerating charge
B: A charge moving at constant velocity
C: A stationary charge
D: A charge less particle
An electric charge at rest has electric field in the region around it, but magnetic field is absent. A moving charge produces both electric and magnetic field. When a charge is moving with constant velocity, electric and magnetic field will not change with time. Therefore, no electromagnetic wave will be produced.
But, if the charge is moving with non-zero acceleration, electric and magnetic field will change with space and time. Electromagnetic waves will be produced.
Therefore, accelerated charge emits electromagnetic waves.