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.