i. Wavelength (λ) :

• The distance between two consecutive crests or two consecutive troughs in a wave is called wavelength.
• It is represented by Greek letter λ (lambda).
• The SI unit for wavelength is metre (m).

Note : The other units include Angstrom, nanometre, picometer (1 pm = 10^-12 m) and micron (1µ = 10^-6m). 

1Å = 10^-8 cm = 10^-10m 

1nm = 10^-9m = 10Å

ii. Frequency (ν) :

• The number of waves that pass a given point in one second is called frequency. 
• It is represented by Greek letter ‘ν’ (nu). 
• The SI unit of frequency is Hertz (Hz) or s^-1.

Note : 1 Hz = 1 cycle per second (1 cps) 

The units, kilo Hertz (kHz) and mega Hertz (mHz) are commonly used.

1 kHz = 10³ Hz = 10³ cps 

1 mHz = 10⁶ Hz = 10⁶ cps

iii. Wavenumber (\(\bar{\nu}\)) :

• The number of wavelengths per unit length is called the wavenumber.
• It is represented by \(\bar{\nu}\) (nu bar).
• The commonly used unit for wavenumber is cm^-1 while its SI unit is m^-1.
• Wavenumber of a wave is related to the wavelength as follows : \(\bar{\nu}\) = \(\frac{1}{\lambda}\)

iv. Amplitude (A) :

• The height of a crest or the depth of a trough from the line of propagation of the wave is called amplitude.
• It is represented by letter ‘A’.
• The square of the amplitude represents the intensity (brightness) of the radiation.

v. Velocity (c) :

• The distance travelled by a wave in one second is called the velocity of the wave.
• It is denoted by letter c.
• It is the product of the frequency and wavelength. Hence, c = νλ
• The velocity of all types of electromagnetic radiations (in space or in vacuum) is the same and it is equal to the velocity of light (3 × 10¹⁰cm s^-1 or 3 × 10⁸ m s^-1. However, they may have different wavelengths and frequencies.