WEEK #2 Physics-II (Ph-1002) Dispersion Polarization polarizing filters Polarization by reflection Scattering of light and its types Huygens principle Book: “University Physics” by Young & Freedman 13th Edition

Dispersion The speed of light in vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. “The dependence of wave speed and index of refraction on wavelength is called dispersion.”

Dispersion The index of refraction for a material usually decreases with increasing wavelength. Violet light refracts more than red light when passing from air into a material.

Angle of deviation The amount the ray is bent away from its original direction is called the angle of deviation (δ).

Chromatic aberration A very common optical problem in which there is a failure of a lens to focus all colors to the same convergence point.

Spherical Aberration An optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens

Spherical Aberration It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image.

Achromats An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration.

Examples of Dispersion Prism Spectrometer A prism spectrometer uses a prism to cause the wavelengths to separate. The instrument is commonly used to study wavelengths emitted by a light source.

ii. Rainbow A ray of light strikes a drop of water in the atmosphere ii. Rainbow A ray of light strikes a drop of water in the atmosphere. It undergoes both reflection and refraction.

Rainbow Formation (steps) Light entering the raindrop is first split into all the colors of the rainbow by dispersion (I've only shown red and violet to keep the diagram simple). Inside the raindrop, the red was refracted least, and violet the most. When the colors of light hit the back of the raindrop, they reflect (total internal reflection). When they exit on the other side, only certain colors will be at the correct angle to enter the persons eye; drops near the top will appear red, through the other colors, to violet at the bottom.

How Rainbow forms?

Polarization In an unpolarized transverse wave, oscillations may take place in any direction at right angles to the direction in which the wave travels. Direction of propagation of wave

Linear Polarization If the oscillation does take place in only one direction then the wave is said to be linearly polarized (or plane polarized) in that direction.

Polarization

Polarization Polarization can be obtained from an unpolarized beam by: Selective absorption Reflection Scattering

i. Polarization by Selective Absorption A number of crystalline materials absorb more light in one incident plane than another. so that light progressing through the material become more and more polarized as they proceed.

ii. Polarization by Reflection Unpolarized light can be polarized, either partially or completely, by reflection. The amount of polarization in the reflected beam depends on the angle of incidence.

Brewster’s law It is found that experimentally when the reflected ray is perpendicular to the refracted ray, the reflected light will be completely plane-polarized.

Polarizing angle (Brewster’s angle) The angle of incidence at which the reflected light is completely plane-polarized is called the polarizing angle (or Brewster’s angle). By Snell’s law, Since and Then we get

iii. Polarization by Scattering When a light wave passes through a gas, it will be absorbed and then re-radiated in a variety of directions. This process is called scattering. Unpolarized sunlight Gas molecule Light scattered at right angles is plane-polarized O y z x

Polarization by Scattering Consider a gas molecule at point O. The electric field in the beam of sunlight sets the electric charges in the molecule into vibration. Since light is a transverse wave, the direction of the electric field in any component of the sunlight lies in the yz-plane, and the motion of charges take place in this plane. There is no electric field, and hence no motion of charge in the x-direction. The molecule re-emits the light because the charges are oscillating. But an oscillating charge does not radiate in the direction of its oscillation so it does not send any light to the observer directly below it. Therefore, an observer viewing at right angles to the direction of the sunlight will see plane-polarized light.

Why is Scattering Important? The world with scattering The world without scattering

Types of scattering Rayleigh scattering Elastic scattering of light by molecules and particulate matter much smaller than the wavelength of the incident light. Partical size << , particles ;gases, atmospheric molecules. Scattering intensity (I) Mostly affects blue light

Why Sky looks blue? As the original beam of sun light passes though the atmosphere, its intensity decreases as its energy goes into the scattered light. Detailed analysis of the scattering process shows that the intensity of the light scattered from air molecules increases in proportion to the fourth power of the frequency (inversely to the fourth power of the wavelength). Thus the intensity ratio for the two ends of the visible spectrum is (700 nm/400 nm)4 = 9.4. Roughly speaking, scattered light contains nine times as much blue light as red, and that's why the sky is blue. This scattering, called Rayleigh scattering, is more effective at short wavelengths

Why Clouds looks white? Clouds contain a high concentration of water droplets or ice crystals, which also scatter light. Because of this high concentration, light passing through the cloud has many more opportunities for scattering than does light passing through a clear sky. Thus light of all wavelengths is eventually scattered out of the cloud, so the cloud looks white.

2) Mie Scattering Broad class of scattering of light by spherical particles of any diameter. Partical size  , Mean diameter 0.1 to 10m, Type of particles: dust, smoke, soot, volcanic ash, water vapor, polen Scattering Intensity (I) to

Polaroid Sunglasses The glare from reflecting surfaces can be diminished with the use of Polaroid sunglasses. The polarization axes of the lens are vertical, as most glare reflects from horizontal surfaces.

Polaroid Sunglasses

Liquid Crystal Liquid crystal is a substance that behaves something like a liquid and something like a solid. The shape of its molecules are long and thin. Properties of Liquid Crystal Their orientations can be aligned with one another in a regular pattern. A particular sort of liquid crystal, called twisted nematics (TN), is naturally twisted. Applying an electric current to these liquid crystals will untwist them to varying degrees, depending on the current's voltage.

Twisted Nematics (Greek word means thred like) They can rotate the plane of oscillation of polarized light passing through them. Light passes through the cell with its plane of polarization turned through 90° Light cannot pass through since the line does not rotate the plane of polarization

Liquid Crystal Display (LCD)

(Dutch scientist Christian Huygens in 1678) Huygens’s principle (Dutch scientist Christian Huygens in 1678) This is a geometrical method for finding, from the known shape of a wave front at some instant, the shape of the wave front at some later time. Huygens assumed that every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave. The new wave front at a later time is then found by constructing a surface tangent to the secondary wavelets or, as it is called, the envelope of the wavelets.

MIRAGE A mirage is an optical phenomenon that creates the illusion of water and results from the refraction of light through a non-uniform medium. Mirages are most commonly observed on sunny days when driving down a roadway.

MIRAGE

That’s all for today ! 