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NCERT Class XII Chapter
Electromagnetic Waves
Questions With Solutions

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Question : 9
Total: 15
The terminology of different parts of the electromagnetic spectrum is given in the NCERT text. Use the formula E = hυ 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?
Solution:  
The photon of given energy are released during transition between energy levels in the atom and the emitted photon energy is equal to difference of energies of energy levels, among which the transition has taken place.
For example photon energy for
λ = 1 m can be calculated
E = hυ =
hc
λ
 =
6.63×10−34×3×108
1
 J
E =
19.88×10−26
1.6×10−19
 eV = 12.425 × 10–7 eV
So, for emission of radio waves the energy difference between energy levels should be 12.425 × 10–7 eV.
Similarly, photon energy for other wavelengths can also be calculated
Type of radiation Wavelength of radiation Photon energy
1. Radio waves 1 m 12.425 × 10–7 eV
2. Micro waves 3 × 10–3 m 4.14 × 10–4 eV
3. Infrared waves 3 × 10–5 m 4.14 × 10–2 eV
4. Visible waves5 × 10–7 m 2.485 eV
5. Ultraviolet rays 3 × 10–7 m 4.14 eV
6. X-rays 10–10 m 12.425 × 103 eV
7. γ rays 10–12 m 12.425 × 105 eV

So, for emission of γ -rays the energy difference among energy levels should be of the order of MeV, where for visible radiation it should be of the order of eV.
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