SCATTERING OF LIGHT

About light scattering Light scattering is a form of scattering in which light in the form of propagating energy is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media. Deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering. When these irregularities are considered to be random and dense enough that their individual effects average out, this kind of scattered reflection is commonly referred to as diffuse reflection.Most objects that one sees are visible due to light scattering from their surfaces. Indeed, this is our primary mechanism of physical observation. Scattering of light depends on the wavelength or frequency of the light being scattered.

DIFFERENT TYPES OF SCATTERING:1) Rayleigh scattering

2) Mie scattering

3) Tyndall scattering

4) Brillouin scattering

5) Raman scattering

Light scattering is responsible for various spectacular phenomena in nature. The blue colour of sky, colour of water in deep sea, the reddening of the sun at sunrise and sunset can be explained on the basis of scattering of light caused by the Earth’s atmosphere.

Tyndall effect The scattering of light by particles in its path is called Tyndall effect.When a beam of sunlight enters a dusty room through a window, then its path becomes visible to us. This is because the tiny dust particles present in the room scatter the beam of light all around the room. When this scattered light enters our eyes, we can see the beam of light. Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest. Here, tiny water droplets in the mist scatter sunlight.

In 1859, Tyndall discovered that when white light consisting of seven colours is passed through a clear liquid is passed through a clear liquid having small suspended particles in it, then the blue colour of white light having shorter wavelength is scattered much more than red colour having longer wavelength.

Blue light has longer wavelength, so it is scattered more easily easily. On other hand, red light has longer wavelength, so it is not scattered much.

The colour of scattered light depends on the size of scattering particles

The dust particles and water droplets suspended in the atmosphere are much larger than the wavelength range of visible light. When white light coming from the sun hits these larger particles, it gets reflected or scattered in different directions. Due to this, the scattered light appears white. Thus, when white sunlight falls on larger particles present in the atmosphere, it is scattered as such, so the scattered light also appears white.

The air molecules present in the atmosphere are smaller than the wavelength range of visible light. So, when light coming from the sun hits these very small air molecules, it behaves differently. Since the different colours of white light have different wavelengths, so they are affected differently. The lower wavelength lights (blues) are scattered much more by the air molecules but the higher wavelength lights (reds) are scattered much less. Thus, when white sunlight falls on the extremely small particles like air molecules, it is not scattered as white light. The molecules of air scatter mainly the lower wavelengths of light which have blue shades.

Why the sky is blue The scattering of blue component of the white sunlight by air molecules (nitrogen and oxygenmolecules) present in the atmosphere causes the blue colour of the sky.

When sunlight passes through the atmosphere, most of the longer wavelength lights present in it do not get scattered much by the air molecules and hence pass straight though. The shorter wavelength blue light is, however, scattered all around the sky by air molecules in the atmosphere and enters ours eyes, making the sky appear blue.

Also, only a little of the blue light present in white light is scattered by the atmosphere which makes the sky appear blue. Most of the blue light remains behind unscattered due to which the composition of sunlight remains unaltered. Because of this the direct sunlight coming through the sky still appears to be white.

Why the sun appears red at sunrise and sunset

The sun and the surrounding sky appears red at sunrise and at sunset because at that time, sunlight has to travel the greatest distance through the atmosphere to reach to us. During this long journey, most of the blue colour present in sunlight has been scattered out and away from our line of sight. So, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red colour due to which the sun and surrounding sky appears red.

But when the sun is overhead in sky (at noon), the light coming from it has to travel a relatively shorter distance through the atmosphere to reach us. During this short journey, only a little of the blue colour of the white light is scattered. Since the light coming from the overhead sun has almost all its component colours in the right proportion, it appears white to us.

Experiment to study the scattering of light

An arrangement for observing the scattering of light in colloidal solution to show how the sky appears blue, and the Sun appears red at sunrise and sunset.

When a light from a source S enters convex lens L1, the light rays become parallel and enters transparent glass tank. The tank contains 200 gram of sodium thiosulphide dissolved in water, along with 1-2 ml conc. Sulphuric acid. On entering the colloidal solution, the short wavelength blue colour of light gets scattered and can be observed by looking at sides of glass tank. This is how the sky looks blue. The long wavelength red colour remains unscattered. It enters through a split in card board into another convex lens L2. The red colour can be observed as a red patch on screen placed at other side of lens. This is how the Sun look red during sunrise and sunset.

Following is the setup for demonstrating the experiment,

THANK YOUMADE BY-1) Rivan Patel2) Dhruvil Shah3) Jeet Patel4) Jivansu Vyas 5) Nimay Shah6) Mahir Desai7) Rushil Dalal8) Nachiket Raval