Class 12 Physics Electromagnetic Waves Electromagnetic Spectrum

Electromagnetic Spectrum

  • Electromagnetic spectrum is the classification of EM waves according to their frequency or wavelength.
  • Based on the wavelength EM waves are classified into different categories.This classification is known as electromagnetic spectrum.
  • Different categories of EM waves in decreasing order of their wavelength:-
    1. Radio waves > 0.1m
    2. Microwaves 0.1 m - 1mm
    3. Infra-Red 1mm – 700 nm
    4. Visible light 700nm – 400 nm
    5. Ultraviolet 400nm- 1nm
    6. X-rays 1nm – 10-3nm
    7. Gamma rays <10-3nm
  • These 7 waves together constitute the electromagnetic spectrum.


  • To remember the order of wavelength of each wave, we can just write the initial letter of all the waves and they are in the order of decreasing wavelength.
  • R (max wavelength), M, I, V, U, X and G (minimum wavelength).
  • It can be remembered like this Red Man In Violet Uniform X Gun.

The electromagnetic spectrum, with common names for variouspart of it. The various regions do not have sharply defined boundaries.


Problem:- A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?

Answer:- A radio can tune to minimum frequency, ν1 = 7.5 MHz= 7.5 × 106 Hz

Maximum frequency, ν2 = 12 MHz = 12 × 106 Hz

Speed of light, c = 3 × 108 m/s

Corresponding wavelength for ν1 can be calculated as:

λ1 = (c/v1) = (3x108)/ (7.5x106) = 40m

Corresponding wavelength for ν2 can be calculated as:

λ2 = (c/v2) =(3x108)/ (12x106) =25m

Thus, the wavelength band of the radio is 40 m to 25 m.


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