NCERT Physics Class 12 Exemplar Ch 8 Electromagnetic Waves Part 6 (For CBSE, ICSE, IAS, NET, NRA 2023)
Doorsteptutor material for CBSE/Class-12 is prepared by world's top subject experts: get questions, notes, tests, video lectures and more- for all subjects of CBSE/Class-12.
Q 32 A plane EM wave travelling along z direction is described by Show that
(i) The average energy density of the wave is given by
.
(ii) The time averaged intensity of the wave is given by
- E-field contributation is
B-field contributation is
Total energy density (1)
- The values of and vary from point to point and from moment to moment. Hence, the effective values of and are their time averages.
- The graph of and are identical in shape but shifted by , so the average values of and are also equal over any integral multiple of π.
And also
So by symmetry the average of = average of
Substuting in Equation (1) ,
(2)
(b) We know
Therefore,