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Scattering of Light

Q: At noon the sun appears white because:

At noon because the sun is overhead, the light is scattered the least and hence appears white. When it is overhead, it has lesser air to travel through and the scattering from dust and other particles is reduced if the distance to be travelled in air is reduced.

Q: Blue colour of sky is due to :

Hint:Rayleigh Scattering Theory suggests that scattering of light is inversely proportional to fourth power of wavelength of light.Correct Option is B.Explanation for the correct answer:\\(\bullet\\) Dispersion of Light is a phenomenon in which white light is dispersed into seven colors, when passed through a glass prism. This phenomenon is the reason for formation of rainbow, not for blue color of sky. Option A is incorrect.\\(\bullet\\) Scattering of light is the phenomenon in which path of light gets deviated by striking and obstacle like dust particles. The particles present in our atmosphere are of very small size. Thus, they can deviate only that part of light whose wavelength is closer to their size.\\(\bullet\\) White light coming from Sun is made up of seven colors. Wavelength of Red color is the largest and that of violet and blue is the smallest. Thus, the violet and blue color are scattered most by these particles, thus the sky appears to be blue. Option B is correct.\\(\bullet\\) Reflection of light is the phenomenon in which light bounces back after striking a surface. And retraction of light is bend in path of light when it the medium of its propagation changes. They are not responsible for blue color of light Option C and D are incorrect.Thus, only Option B is correct.

Q: The clear sky appears blue because:

The scattering of light depends on the size of the dust particles and the wavelength of the light. The amount of scattering is inversely proportional to the fourth power of wavelength.Since, violet and blue lights get scattered more than lights of all other colours by the atmosphere the sky appears blue.

Q: Which colour of light is scattered the most (maximum) in the atmosphere?

The molecules of air and other fine particles in the atmosphere have their size smaller than the wavelength of visible light. Thus, they are more effective in scattering light of shorter wavelengths, e.g. blue light. The red light has a larger wavelength than blue light. Thus, when sunlight is passing through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red.

Q: How can we explain the reddish appearance of sun at sunrise or sunset?

During sunrise or sunset, the light has to pass through greater distance in the atmosphere. The blue light is removed as it gets scattered the most while the red colour is less scattered and reaches the observer. Thus, we find reddish colour of the sun during sunrise or sunset.

Q: The bluish colour of water in deep sea is due to:

As we look deep into the sea in a direction away from the sun as the blue light is more scattered more than red. The light reaching our eyes has more of violet and blue and hence the deep sea appears blue.

Q: Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?

We see red colour of the sun at sunrise or sunset as the sun at horizon and light rays need to travel a greater distance. In the process of scattering, violet, blue and green rays in the original sunlight are removed and the transmitted beam has yellow and red dominant.

Q: One cannot see through fog because .......... .

Answer is D.We cannot see through fog because of scattering.Atoms and molecules in the air, including anything carried in the air like dust or smoke, will scatter light. Water droplets, as they are present in fog, also scatter light. The light falling on an object and reflected to a viewer can be scattered to heck and back before it gets to the place where it can be 'seen' by an observer. So the observer just sees a 'whiteout' instead of being able to make out anything beyond a few meters or so.

Q: Why are red coloured lights are used in traffic signals to stop the vehicles?

We know among the seven colours of white light, red colour has maximum wavelength. That is why red colour is least scattered by atmospheric dust and other particles. Hence red colour is used to indicated any danger sign.

Q: Why does sun appear reddish at sunrise and sunset?

Near the horizon sunlight passes through thicker layers of air having larger particles that scatter light of longer wavelength such as the red end of the spectrum. Also it travels larger distance at horizon before reaching the earth. However, light from the Sun overhead travels relatively shorter distance. At noon, the Sun appears white as only a little of the blue color is scattered whereas, near the horizon, most of the light of shorter wavelength at the blue end of the spectrum are scattered away by the small particles of the atmosphere. Therefore, the light that reaches our eyes is of longer wavelength which gives rise to the reddish appearance of the Sun.