ELECTROMAGNETIC  RADIATION

Light is a form of energy called electromagnetic energy, which travels in form of waves. It means the electromagnetic energy or radiation defines the wave nature of light. Other types of electromagnetic waves are radio, microwave, infra-red (heat), ultraviolet (UV), X-ray & Gamma ray radiation. Here light refers to visible light white, which consists of seven colors added together. The seven colors are Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).

WAVE : We can differentiate waves by their wavelengths. Wavelength is the distance between two crests. Crest is the highest point of a wave. The trough is the lowest point of a wave. Amplitude is the highest value of the wave and Frequency is the number of waves in a given period of time. Or we can define frequency mathematically as –         

                                                 fλ = c …………(i)

Where,  f = frequency

              λ = wavelength

              c = speed of light in vacuum

It means, from the formula we can say that frequency and wavelength of an electromagnetic wave is inversely proportional. The shorter the wave length and the greater the frequency.

Relation between Energy and Frequency and Wavelength :

We know that the energy of a photon is,  E = hf

Where,  h = Plank’s constant=  6.626176 x 10^-34 joule-seconds.

              f = frequency

  putting equation (i) in this equation –

                                                                  E = hc/λ

It means, the shorter the wavelength and the greater the frequency, the higher the energy.

THE ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum is the range of frequencies of electromagnetic radiations or waves and their respective wavelengths and photon energies. By seeing the EM spectrum we can differentiate between every EM waves according to their frequencies, respective wavelengths and photon energies.

HISTORY : In 1800 William Herschel discovered infrared radiation. The next year, Johann Ritter discovered ultraviolet radiation. Then Maxwell’s equations predicted an infinite number of frequencies of electromagnetic waves. In this way the electromagnetic spectrum is build.

VISIBLE LIGHT SPECTRUM

COLOR	WAVELENGTH (nm)	FREQUENCY (THz)	PHOTON ENERGY (eV)
Violet	380-450	668-789	2.75-3.26
Blue	450-495	606-668	2.50-2.75
Green	495-570	526-606	2.17-2.50
Yellow	570-590	508-526	2.10-2.17
Orange	590-620	484-508	2.00-2.10
Red	620-750	400-484	1.65-2.00

There is one more color in Rainbow that is Indigo. The frequency of Indigo is 425-450 nm.