• Light waves are transverse in nature. The direction of its vibration is in the plane which is perpendicular to the direction of propagation of the wave. Thus wave can vibrate in any plane (infinite possibilities) which is perpendicular to the direction of propagation of the wave. Such a light wave is called unpolarized light.

#### Experiment to Explain Transverse Nature of  Light Waves:

• When light waves are passed through two crystalline slits say A and B (These slits are the tourmaline plates cut parallel to the axis of crystal). Ordinary light, say from the sun is incident on the crystal A.
• When crystal A and crystal B are parallel to each other, the intensity of the light emerging from crystal A is constant at any orientation of A (hence B parallel to it) and passes through crystal B without any change.

• When crystal B is rotated w.r.t. the crystal A, the intensity of emerging light from crystal B decreases and becomes zero when crystal B is at a right angle w.r.t crystal A.

• This experiment proves the transverse nature of the light waves. Crystal A is called polariser and crystal B is called analyser.

#### The Terminology of Polarization:

• The phenomenon of restriction of the vibrations of light waves in a particular plane particular plane perpendicular to the direction of propagation of wave motion is called polarization.
• The substances which bring about the plane polarization of light are called as polarisers. e.g. Tourmaline crystal, Nicol prism.
• When the vibrations of electric vectors are confined in one plane, the light is called plane polarized light.
• The plane in which the vibration of polarized light takes place is called the plane of vibration.
• The plane perpendicular to the plane of vibration in which there are no vibrations of polarized light is called the plane of polarization and the light is said to be polarized in it.

#### Brewster’s law:

• The tangent of the polarizing angle is equal to the refractive index of the material of the surface from which reflection is taking place.
• Mathematically,     μ = tan ip.
• This angle characteristic of the medium.

ip = angle of incidence and r = angle of refraction

• Proof:
• Let us consider the unpolarized monochromatic light incident in air at the polarizing angle ip on the plane surface XY of the transparent medium of refractive index μ. Experimentally Brewster proved that ∠ SQR = 90°.

• Brewster further proved that polarizing angle is a function of a wavelength of the light.
• This law is not obeyed by polished metallic surfaces.

### Polaroid:

• Polaroid is a large sheet of synthetic material packed with tiny crystals of a dichroic substance oriented parallel to one another so that it transmits light only in one direction of the electric vector.
• The property by which some doubly refracting crystals absorb the ordinary rays (O-rays) completely and extraordinary rays whose direction is perpendicular to the optic axis when passing through the crystal, is called dichroism.
• The crystal exhibiting the property of dichromism is called dichroic crystal. e.g. tourmaline. It absorbs O – light and transmits E – light.
• Polaroids are artificially prepared dichroic substances. In 1852, W. H. Herapath discovered synthetic crystalline material iodosulphate of quinine known as herapathite which exhibits dichroism. In 1934 E.H. Land embedded tiny herapathite crystals in cellulose acetate with their optic axis parallel. The layers of the crystal were mounted between two glass sheets for protection. This act as a sheet of the polarizer and called as polaroid.
• It can be used as polarizer as well as an analyzer.

#### Uses of Polaroids:

• Polaroids are used in sunglasses to cut off the glare reflected by horizontal surfaces.
• Polaroids are used in glass windows of an aeroplane to control the intensity of light entering the aeroplane.
• Polaroid glasses are used to view three-dimensional pictures and movies.
• Polaroids are used as a filter in the photographic camera.
• They are used in production and analyzation of plane polarized light.
• They are used to improve colour contrast in old oil paintings.
• They are used in calculators, watches, monitors of laptops which have LCD screens.

### Doppler Effect in Light:

• The apparent change in the frequency of the light observed by an observer, due to relative motion between the source of the light and the observer, is called the Doppler effect.
• One major difference between Doppler effect exhibited by sound and light is as follows. In the case of sound, the frequency change depends on whether the source is moving or the observer is moving even if their relative velocities are the same. In the case of light, the Doppler effect depends only on the relative velocity of the source and the observer, irrespective of which of the two is moving. Hence the Doppler effect exhibited by light is symmetric.
• Red Shift of Light:
• When the source and observer move away from each other, the wavelength in the middle of the spectrum will be shifted towards red. This phenomenon is called red shift due to Doppler effect.
• When the source and observer move away from each other, the observer observes the lower frequency than the actual frequency of the light (towards red).
• Blue Shift of Light:
• When the source and observer move towards each other, the wavelength in the middle of the spectrum will be shifted towards blue. This phenomenon is called blue shift due to Doppler effect.
• When the source and observer move towards each other, the observer observes the higher frequency than the actual frequency of the light (towards blue).
• The measurement of Doppler shift helps in the study of motions of stars and galaxies.

#### Applications of Doppler Effect in Light:

• It is used to measure the speed of rotation of the sun.
• It is also used in the measurement of plasma temperature.
• Edwin Hubble (1889-1953) found a method to red shift and blue shift to study the relative motion of the stars and galaxies.
• It is used in tracking satellites.
• They are used in speed guns to measure the speed of cricket and tennis ball.