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NCERT Class XII Chapter
Electromagnetic Waves
Questions With Solutions

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Question : 13
Total: 15
Use the formula λmT = 0.29 cm K to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?
Solution:  
A black body at very high temperature produce a continuous spectrum. Using Wien’s displacement law we can calculate the wavelength corresponding to maximum intensity of radiation emitted Required absolute temperature
λmT = 0.29 cm K or T =
0.29×10−2
λm
 K
Using above formula, the temperature of black body required for various wavelength is calculated.
Type of radiation Wavelength of radiation Temperature required
1. Radio waves 1 m 29 × 10–4 K
2. Micro waves 3 × 10–3 m 1 K
3. Infrared waves 3 × 10–5 m 102 K
4. Visible waves 5 × 10–7 m 6 × 103 K
5. Ultraviolet rays 3 × 10–7 m 104 K
6. X-rays 10–10 m 29 × 106 K
7. γ rays 10–12 m 29 × 108 K

To produce electromagnetic radiations of different wavelength, we need temperature ranges. To produce visible radiation of l = 5 × 10–7 m, we need to have source at temperature of 6000 K. A source at lower temperature will produce this wavelength but not with maximum intensity
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