Atmospheric Refraction & Scattering of Light | Science Class 10 PDF Download

Atmospheric Refraction

The refraction by different layers of the atmosphere is called atmospheric refraction.

• Apparent flickering of objects placed behind a hot object or fire.
• Stars near the horizon appear slightly higher than their actual position.
• Advanced sunrise and delayed sunset.
• Apparent flattering of sun’s disc.
• The twinkling of stars.

1. An object placed behind the fire or a hot surface appears to flicker when seen through the air.

• The air above the hot surface becomes hot and rises. The space is occupied by cool air. The refractive index of hot air is less than that of cool air. 
• So, the physical condition of the medium is not constant. 
• Due to the changing Refractive Index (RI) of the medium, the light appears to come from different directions.
• It results in fluctuation in the apparent position of the object.

2. Stars, when seen near the horizon, appear slightly higher than their actual position due to atmospheric refraction.

• The refractive index of the earth’s atmosphere, in general, increases from top to bottom.
• So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards the normal.
• As a result, the star appears higher.

3. Advanced sunrise

• The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset.
• When the sun is below the horizon, the rays have to pass from rarer to denser medium.
• So, rays bend towards the normal. As a result, the sun appears higher than its actual position.

4. Twinkling of stars

• Stars are very far from us, so they behave as a point source of light.
• Since the physical conditions of the Earth’s atmosphere are not constant, the light from
• Stars appear to come from different directions.
• This results in fluctuation of the apparent position of the star.
• The amount of light coming from stars also varies due to the changing Refractive Index of the atmosphere.
• The star appears bright when more light from the star reaches our eyes, and the same star appears dull when less amount of light reaches our eyes.
• Both these effects are responsible for the twinkling of stars.

Scattering effect

Spreading of light in various directions by colloid particles.

Scattering ∝ 1/wavelength

Tyndall effect: When light passes through a colloid its path becomes visible. This is called Tyndall effect.
Example:
(i) Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.
(ii) Path of light becomes visible when light passes through dense canopy of trees in a forest.

Dependence of colour of scattered light

(i) If particles are very fine, they scatter mainly the blue colour of light (shorter wavelength).
(ii) Medium sized particles scatter mainly the red colour (longer wavelength).
(iii) Even larger particles scatter all the colours of light that is why it appears white.

Wavelength of red light is about 1.8 times to that of blue light.

 Anger signs are made in red colour.

• Red is the least scattered colour. It is least scattered by fog and smoke and can be seen in the same colour over a long distance. So, danger signs are made in red colour.

 Colour of sky appears blue on a clear day.

• The upper layer of atmosphere contains very fine particles of water vapours and gases. These particles are more effective in scattering light of shorter wavelengths, mainly blue, than larger wavelengths. So, the sky appears blue.

 Appearance of sky to an astronaut in the space 

• The sky would appear dark to an astronaut in the space as scattering is not very prominent at such high altitude due to absence of particles.

 Clouds appear white

• Clouds are formed by water vapours. Water vapours condense to form water droplets due to the larger size of droplets; all colours of light are scattered, and clouds appear white.

 Colour of sun appear red during sunrise and sunset (Old Syllabus)

• At sunset and sunrise, the colour of the sun and its surroundings appear red. During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in atmosphere.
• Due to this most of the blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why the sun appears red in colour.