How does rainbow forms

1। Light from sun strikes raindrop.

White light from the Sun has to hit the raindrops at a certain angle before a rainbow is possible. It is best if the sun is fairly low in the sky such as dawn and late afternoon. The angle is important as it effect the direct the light travels after it hits the raindrops and that determines whether or not we will see a rainbow.

2. Some of the light is reflected.

It is possible to see through a glass window but, at the same time, see your own reflection. This is because the window both transmits and reflects light. Water can do this too - that is why you can see a reflection in a pool of clean water and also see the bottom.

When light from the sun hits a water droplet, some of the light is reflected. This light will obey the Law of Reflection.

3. The rest of the light is refracted.

The light that is not refracted crosses the air-water interface (boundary layer). When this happens it slows down because the water is more dense than the air. The reduction of speed cause the path of the light to bend - this is called refraction. In this case the path of the light rays bends toward the normal line.

4. White light splits into component colours.

White light is made up of a spectrum of colours, each with its own wavelength. Different wavelengths travel at different speeds and when they encounter a change to medium that is more dense or less dense, the speeds are efected by different amounts. Hence, the colours separate. This phenomenon is know as Dispersion.

5. Light is reflected at rear of raindrop (TIR).

At the rear of the raindrop, the light hits the water-to-air interface. If the angle of incidence is greater than the critical angle, Total Internal Reflection will occur. A rainbow will only be seen if this happens, otherwise the light will continue out the other side of the raindrop and continue to move away from the would-be viewer.

6. Light is refracted again as it leaves raindrop.

Just as the light changed speed as it entered the raindrop, its speed changes again as it leaves. Here, the light is moving from a more dense medium (water) to a less dense medium (air). As it does so, it speeds up and its path bends. In this case the path of the light rays bends away from the normal line. This is another example of refraction.

7. Colours are further dispersed.

As the rays are refracted once again, the various wavelengths are effected to different extents. The overall result of this is increased separation of the component colours of white light. This is Dispersion.

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Plane polarized light:

when light travels along a certain direction the vibration takes place in a direction at right angles to the direction of propagation. If the vibration of the ether particles are linear and take place parallel to a plane through the axis of the beam or the direction ofpropagation,light is said to be "plane polarized."


Huygen's theory of Double refraction:


Huygen's theory states that every point on a wave front is a source of secondary wavelets which radiates in all direction from their centres with the speed of the propagation of wave. The wave front is the envelop of the secondary wavelets. If we imagine a point source within the uniaxial Crystal(calcite), the wave front at any time correspnding to the O-rays is a sphere about the point as centre and the wave front corresponding to the E-rays is spheroid because the E-rays have minimum velocity along optic axis and minimum velocity in all direction
perpendicular to the optic axis.


Fraunhofer and Fresnel diffraction:

In the Fraunhofer diffraction, the source of the light and the screen are effectively at infinite distance from the diffracting obstacle or aperture. This is achieved by placing the source and the screen in the focal planes of two lenses. In this case the incident wave frony is plane.

In the Fresnel's diffraction, the source of light and screen on which diffraction pattern is observed are finite distances from obstacle or aperture. In this case no lenses are used. In this case the incident wave front is either spherical or cylindrical.


Spontaneous Emission:


When an atom in an excited state (higher state) E2 jumps to the lower energy state E1 by emitting a photon of frequency v, the process is known as spontaneous emission. If there is an assembly of atoms, the radiation emitted spontaneously by each atom has a random direction and a random phase and is therefore incoherent from one atom to
another.



Stimulated Emission:


When an atom in an excited state E2 interacts with an incident photon of right frequency v and is there by induced to move to ground state E1 by emitting the difference of energy of a photon of same frequency v, the process is known as stimulated emission.




Dichonism:

Some doubly-refracting crystals have the properly of absorbing strongly one of the two refracting rays, while allowing the other to emerge with little loss. This selective absorbtion by the crystal is known as "dichonism". The best example of such crystal is tourmaline.


Interference of light:


When two wave of same frequency travel in approximately the same direction and have a phase difference that remain constant with time, the resultant intensity of light is not distibuted uniformly in space. The non uniform distribution of light intensity due to the superposition of two wave is called interference. At same points the intensity is a maximum,and the interference at these points is called "Constructive interference". At other some point the intensity is minimum, and the interference at these points is called destructive interference.