Showing posts with label Class Notes: Physics I. Show all posts
Showing posts with label Class Notes: Physics I. Show all posts

### What are dextrorotatory and laevorotatory substances?

Dextrorotatory and laevorotatory substances are optically active substances.

## Dextrorotatory  substances

Substances which rotate the plane of polarization of the light towards the right are known as dextrorotatory substances.

## Laevorotatory substances

Substances which rotate the plane of polarization of the light towards the left are known as laevorotatory substances.

## Optical activity

Optical activity is the ability of a substance to rotate the plane of polarization of plane polarized light. Substances which have this ability are called optically active substance. Tartaric acid is an example of optically active liquid.

## Nicol prism

Nicol prism was invented by William Nicol in 1828. It is an optical device used for producing and analyzing plane polarized light. The nicol prism is made in such a way that it eliminates one of the two rays by total internal reflection. It is generally found that it eliminates the ordinary ray and only the extraordinary ray is transmitted through the prism.

## Double refraction

Erasmus Bartholinus discovered in 1669, that, when a ray of light is refracted by a crystal of calcite it gives two refracted rays. This phenomenon is called double refraction. Calcite is crystallized calcium carbonate (CaCO3). It crystallizes in many forms and can be reduced by cleavage or breakage into a rhombohedra.

## Phase velocity

Phase velocity is the velocity with which a plane progressive wave front travels forward. Phase velocity is symbolized by . It has a constant phase =

## Group velocity

The group velocity () is the speed with which the point of reinforcement of the resultant wave travels. The group velocity is different from the individual wave velocity.

## Phase angle

Phase angle is defines as the fraction of the time interval that has lapsed since the particle has crossed the mean position of rest in the positive direction. It is also equal to the angle swept by the radius vector since the vibrating particle last crossed its mean position of rest.

### Wavelet:

Wavelets are tiny parts of waves.

### Secondary wavefront:

The envelop of all the secondary wavelets in the forward direction is called the secondary wavefront. The backward envelop is not a secondary wavefront.

### Wavefront

A wavefront can be defined as the locus of all the points of the medium which are vibrating in phase and are also displaced at the same time.

If the distance of the source is small the wavefront is spherical. But when the distance is large then any small portion of the wavefront can be considered plane.

### Diffraction grating

Diffraction grating is an extremely useful device. In one of its form it consists of a very large number of narrow slits side by side. The slits are separated by opaque spaces.

### What do you mean by Fresnel and Fraunhofer class of diffraction?

Diffraction phenomena can conveniently divided into two groups.
1. Fresnel Diffraction Phenomena: In the Fresnel class of diffraction, the source or the screen or the both are at finite distances from the aperture or obstacle causing diffraction. In this case, the effect at a specific point on the screen due to the exposed incident wavefront is considered and no modification is made by lenses and mirrors.
2. Fraunhofer Diffraction Phenomena: In the Frauhofer class of diffraction phenomena, the source and the screen on which the pattern is observed are at infinite distances from the aperture or the obstacle causing diffraction. The incoming light is rendered parallel with a lens and the diffracted beam is focused on the screen with another lens.

### Diffraction

The rhythmic variations in intensity and the bending of light around the corners of an obstacle or the encroachment of light into the region of geometrical shadow constitute a class of phenomena known as the diffraction of light.

### Newton’s rings and its formation :

When a Plano-convex lens of long focal length is placed on a plane glass plate, a thin film of air is enclosed between the lower surface of the lens and the upper surface of the plate. The thickness of the air film is very small at the point of contact and gradually increases from the center outwards.

### Fringe width

The distance between any two consecutive bright or dark fringe is known as fringe width. Symbolically the fringe width is β and

### Why two independent source of light can not produce interference?

Two independent sources of light can not produce interference because a large number of difficulties are involved. The two sources may emit light waves of largely different amplitude and wave length and the phase difference between the two may change with time. That’s why interference of light is produced by two coherent sources which are virtual.

### Coherent sources

Two sources are said to be coherent if they emit light waves of same frequency, nearly equal amplitude and are always in phase with each other.

### Interference of light:

Wave emitted from two coherent sources having the same wavelength, equal or nearly equal amplitude when passes through a point in a medium, then superposition takes place.

During superposition, if the two waves reaching the point in the same phase then the intensity will be maximum that is bright. But if the two waves reaching the point in opposite phase then the point will have minimum intensity that is dark. This variation of intensity due to superposition is called interference of light.

The interference is said to be constructive or destructive when the intensity is respectively more or less than that given by one beam alone.

### Huygens principle

To explain the propagation of light through ether, Huygens proposed the following principles for the construction of a wavefront at a subsequent time from the known position of the wavefront at any given instance.

### Stationary waves:

When two simple harmonic waves of the same amplitude, frequency and time period travel in opposite directions in a straight line, the resultant wave obtained is called a stationary or a standing wave. The formation of stationary waves is due to the superposition of the two waves on the particles of the medium.

### Transverse waves:

In transverse wave motion, the particles of the medium vibrate at right angles to the direction of propagation of wave. Example: Light waves are transverse waves.

### Longitudinal wave motion:

In longitudinal wave motion, particles of the medium vibrate along the direction of propagation of the wave. Example: Sound waves are longitudinal waves.

### Wave motion:

Wave motion is a form of disturbance produced in the medium by the repeated periodic motion of the particles of the medium. Here, only the waves travel forward whereas the particles of the medium vibrate about their mean positions.