Class 12 Physics Electromagnetic Waves Electromagnetic energy of each wave

Electromagnetic energy of each wave in Electromagnetic Spectrum

Electromagnetic waves energy can be described by frequency, wavelength or energy.

  1. Frequency- Both micro and radio waves are described in terms of frequencies.
    1. Frequency is number of crests that pass a given point within one second.
    2. Consider a wave which has 3 crests which pass a point in 1 second. Therefore frequency=3Hz.
  2. Wavelength-Infrared and visible waves are generally described in terms of wavelength.
    1. Wavelength is the distance between consecutive crests or troughs.
    2. Wavelength can vary from small value to a large value.
    3. I unit: - meter.


  1. Energy- X-rays and Gamma rays are described in terms of energies.
    1. An EM wave can be described in terms of energy –in units of eV.
    2. eV is the amount of kinetic energy needed to move 1 electron through a potential of 1 volt.
  • Moving along the EM spectrum energy increases as the wavelength decreases.

Relation between Wavelength and Frequency

  • c=νλ
    • Whereλ =wavelength and ν= frequency.
  • =>λ = (c/ν)
  • E=hν
  • =(hc/λ)
  • => E ∝ν and E ∝ (1/λ)
  • Therefore from EM spectrum
    • Decreasing order of wavelength->R, M, I, V, U, X and G
    • In terms of increasing order of frequency -> G,X,U,V,I,M,R.

Problem:- The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B0 = 510 nT. What is the amplitude of the electric field part of the wave?

Answer:- Amplitude of magnetic field of an electromagnetic wave in a vacuum,

B0 = 510 nT = 510 × 10−9 T

Speed of light in a vacuum, c = 3 × 108 m/s

Amplitude of electric field of the electromagnetic wave is given by the relation,

E = cB0 = 3 × 108 × 510 × 10−9 = 153 N/C

Therefore, the electric field part of the wave is 153 N/C.

Problem:- A plane electromagnetic wave travels in vacuum along z-direction.What can you say about the directions of its electric and magneticfield vectors? If the frequency of the wave is 30 MHz, what is itswavelength?

Answer:- The electromagnetic wave travels in a vacuum along the z-direction. The electric field (E) andthe magnetic field (H) are in the x-y plane. They are mutually perpendicular. Frequency ofthe wave, ν = 30 MHz = 30 × 106s−1

Speed of light in a vacuum, c = 3 × 108 m/s Wavelengthof a wave is given as:

λ =(c/ ν) = (3x108)/ (30x106)

λ =10m

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