PowerPoint Presentation

Book: University Physics by Young & Freedman 13th Edition

DispersionPolarization polarizing filters Polarization by reflectionScattering of light and its typesHuygens principlePhysics-II (Ph-1002)WEEK #2

1DispersionThe speed of light in vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths.

2The dependence of wave speed and index of refraction on wavelength is called dispersion.

DispersionThe 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.

3

Angle of deviationThe amount the ray is bent away from its original direction is called the angle of deviation ().4

5

Chromatic aberrationA very common optical problem in which there is a failure of alenstofocusallcolors to the same convergence point.6

Spherical Aberration7

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

8

AchromatsAn achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration.9

Examples of DispersionPrism SpectrometerA prism spectrometer uses a prism to cause the wavelengths to separate. The instrument is commonly used to study wavelengths emitted by a light source.10

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

11

12Rainbow 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?

1314

15

PolarizationIn an unpolarized transverse wave, oscillations may take place in any direction at right angles to the direction in which the wave travels.

16 Direction of propagation of waveLinear PolarizationIf the oscillation does take place in only one direction then the wave is said to be linearly polarized (or plane polarized) in that direction.

17

Polarization

18PolarizationPolarization can be obtained from an unpolarized beam by:

Selective absorptionReflectionScattering

19i. Polarization by Selective AbsorptionA 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. 20

ii. Polarization by Reflection

21Unpolarized light can be polarized, either partially or completely, by reflection.

The amount of polarization in the reflected beam depends on the angle of incidence.Brewsters law 22It is found that experimentally when the reflected ray is perpendicular to the refracted ray, the reflected light will be completely plane-polarized.

Polarizing angle (Brewsters angle)The angle of incidence at which the reflected light is completely plane-polarized is called the polarizing angle (or Brewsters angle).By Snells law,

Sinceand

Then we get

23iii. 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.UnpolarizedsunlightGas moleculeLight scattered at right anglesis plane-polarizedOyzx24

25Polarization 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.

The world with scatteringThe world without scatteringWhy is Scattering Important?26Types of scatteringRayleigh scatteringElasticscattering of light by molecules andparticulate mattermuch smaller than the wavelength of the incident light. Partical size