Light

Light is a form of energy which his propagated as electromagnetic waves in the spectrum of electromagnetic waves it lies between ultra-violet and infra-red region and has wavelength between 3900 A˚ to 7800 A˚.

Electromagnetic waves are transverse, hence light is transverse wave.

Wave nature of light explains rectilinear propagation, reflection, refraction, interference, diffraction and polarization of light. The phenomena like photoelectric effect, Compton Effect are not explained on the basis of wave nature of light. These phenomena are explained on the basis of wave nature of light. These phenomena are explained on the basis of wave nature of light. These phenomena are explained on the basis of quantum theory of light as proposed by Einstein.

In quantum theory, light is regarded as a packet or bundle of energy called photon. Photon is associated with it an energy E where E= hƱ.

Clearly light behaves as wave and particle both. Thus light has dual nature.

Speed of light was first measured y Roemer. (1678 AD).

Speed of light is maximum in vacuum and air (3 x 10⁸ m/s)

Refractive index: R.I. of a medium is defined as the ratio of speed of light in vacuume to the speed of light in the medium.

μ= c/ v     = Speed of light in vacuum   / speed of light in the medium

Speed of light is different in different media. Velocity of light is large in a medium which has small refractive index.

Speed of light in different mediums

Medium                                 Speed of light (m/s)

Vacuum                                     3 x 10⁸

Water                                        2.25 x 10⁸

Rock salt                                  1.96 x 10⁸

Glass                                           2 x 10⁸

Terpentine oil                           2.04 x 10⁸

Nylon                                       1.96 x 10⁸

Light takes 8 minute 19 second (499 second) to reach from sun to earth.

The light reflected from moon takes 1.28 second to reach earth.