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__.

- Frequency- Both micro and radio waves are described in terms of frequencies.
- Frequency is number of crests that pass a given point within one second.
- Consider a wave which has 3 crests which pass a point in 1 second. Therefore frequency=3Hz.

- Wavelength-Infrared and visible waves are generally described in terms of wavelength.
- Wavelength is the distance between consecutive crests or troughs.
- Wavelength can vary from small value to a large value.
- I unit: - meter.

- Energy- X-rays and Gamma rays are described in terms of energies.
- An EM wave can be described in terms of energy –in units of eV.
- 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 B

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

B_{0} = 510 nT = 510 × 10^{−9} T

Speed of light in a vacuum, c = 3 × 10^{8} m/s

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

E = cB_{0} = 3 × 10^{8} × 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 × 10

Speed of light in a vacuum, c = 3 × 10^{8} m/s Wavelengthof a wave is given as:

λ =(c/ ν) = (3x10^{8})/ (30x10^{6})

λ =10m

.