Wave nature of light Light is an electromagnetic wave. EM waves are those waves in which there are sinusoidal variation of electric and magnetic fields at right angles to each other as well as to the direction of propagation of wave.

WAVE OPTICS WAVEFRONT The locus of all the points where the waves from a given source reach at given instant of time.

Spherical wavefront

Cylindrical wavefront

1. Plane wavefront

Diffraction and Interference Two phenomenon explaining wave nature of light. Diffraction and interference are similar phenomena. Interference is the effect of superposition of 2 coherent waves. Diffraction is the superposition of many coherent waves

Huygens’ principle Every point on the given wave front acts a fresh source of new disturbance called secondary wavelets which travel in all directions. The new wavefront at any instant will be the envelope of the secondary wavelets at that instant

Huygen’s Picture of a Plane wave

Huygen’s Explanation of Reflection

Huygen’s Explanation of Refraction

Young’s double slit experiment: classic wave effect

Young’s double slit experiment Interference of light is the phenomenon of redistribution of light energy in a medium on account of superposition of light waves from two coherent sources

Formation of crests n troughs

Double slit experiment with particles (e.g. electrons) Interference pattern with one slit blocked

Double slit experiment with particles (e.g. electrons) Interference pattern observed on the screen When both the slits are open

Diffraction When light waves hit an obstacle it bends around the edges of the obstacle. This property of bending of light is called diffraction. For example, if light of red wavelength from a laser source hits a small hole, it bends around the edges of the hole and forms a pattern called an Airy disc pattern.

Single slit diffraction Diffraction occurs on account of mutual interference of secondary wavelets starting from the portions of wave front which are allowed to pass from the aperture.

The pattern spreads out due to Diffraction. Wave picture Light spreads out when passed through small aperture.

Position of minima: a sin θ n = nλ Position of maxima a sin θ n= (2n+1)λ/2

If unpolarized light is incident on 1 polarizer, the intensity of the light passing through is I= ½ I 0. Polarization

Light scattered at 90 degrees is 100% polarized. Polarization & Scattering

A polarizer will transmit linear polarized waves in the same direction independent of the incoming wave.