# Physics Class 12 NCERT Solutions: Chapter 8 Electromagnetic Waves Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 9. The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula (for energy of a quantum of radiation: photon) and obtain the photon energy in units of eV for different parts of the electromagnetic spectrum. In what way are the different scales of photon energies that you obtain related to the sources of electromagnetic radiation?

Answer:

Energy of a photon is given as:

Where,

Wavelength of radiation

The given table lists the photon energies for different parts of an electromagnetic spectrum for different .

1 | |||||||

The photon energies for the different parts of the spectrum of a source indicate the spacing of the relevant energy levels of the source.

Q: 10. In a plane electromagnetic wave, the electric field oscillates sinusoidal at a frequency of and amplitude

(A) What is the wavelength of the wave?

(B) What is the amplitude of the oscillating magnetic field?

(C) Show that the average energy density of the E field equals the average energy density of the B field.

Answer:

Frequency of the electromagnetic wave,

Electric field amplitude,

Speed of light,

(A) Wavelength of a wave is given as:

(B) Magnetic field strength is given as:

(C) Energy density of the electric field is given as:

And, energy density of the magnetic field is given as:

Where,

Permittivity of free space

Permeability of free space

We have the relation connecting E and B as:

Where,

Putting equation (2) in equation (1) , we get

Squaring both sides, we get