MATHPAL CLASSES & SACHDEVA STUDY ZONE&
CATALYST INSTITUTE
CLASS 10
HUMAN EYE AND COLORFUL WORLD
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1. Why does the star appear slightly higher than its actual position?
Light from stars undergoes continuous refraction as it enters the earth’s atmosphere. Since the retracted ray bends towards the normal, the stars appear at different position and appear slightly higher than their actual positions. The apparent position of the star also changes gradually due to change in the earth’s atmosphere, Therefore, the apparent position of the star fluctuates. As a result, the path of light from the star varies.
2. Advanced sunrise and delayed sunset:
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. The density of the air in the atmosphere is not the same everywhere. At some places it is higher and in some places it is less. The higher the density, the greater is the refractive index of air. At sunrise or at sunset, the sun is either at the horizon or below the horizon. Hence the light rays from the sun has to travel a larger distance through the atmosphere, As the days travels from rarer to denser medium, they start bending more and more towards the normal(observer).
Hence, there is change in the altitude of the sun and they appear at a higher position than they actually are. The sun is visible to us about 2 minutes before actual sunrise and 2 minutes after sunset because of atmospheric refraction. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
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(3) Twinkling of stars:
The twinkling of star is due to the atmospheric refraction. Light from the stars undergoes continuous atmospheric refraction as it passes through the layers of the atmosphere. Since the atmosphere bends the refracted rays towards the normal, the apparent position of the star is slightly different from its actual position. So, the star appears slightly above than
its actual position when viewed near horizon. The apparent position of the star changes
gradually due to the change in the earth ’s atmosphere, Therefore, the apparent position of
the star changes and the path of light from the star also vary. Due to this change in the brightness of light front the star, the star appears to be twinkling.
(4) Planets don't twinkle:
The Planets are very close to earth as compared to the stars. So, the intensity of light we receive from the planets is very large. Therefore, small variations in their positions and brightness are not noticeable. Thus, the brightness of the planet seems to remain the same. The continuously changing earth’s atmosphere is unable to cause variations in the light rays coming from the planet. Therefore, planets do not twinkle
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(5) Flickering of flame:
The air above the fire is hotter than surrounding air.
The hotter air is lighter(less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air above it.
Since the physical conditions of the refracting medium (air) are not stationary, the
apparent position of the objects, as seen through the hot air, fluctuates.
Scattering of the light:
Scattering of the light is a very important phenomenon of our daily life.
Throwing of light in different directions on passing through regions of small particles is called scattering of light.
This scattering is the result of the presence of various particles in the atmosphere.
Our planet earth consists of various mixtures of particles like smoke, molecules of air, dust particles and water droplets.
These diffused particles reflect the light before it reaches the earth.
The scattering of the light by the colloidal particles is known as the Tyndall effect.
The size of the particles determines the colour of the scattered light.
Some are large particles like dust, water droplets while others are small particles like oxygen molecules, nitrogen molecules etc. Large particles scatter all colours while small particles scatter blue colour the maximum and red light gets scattered less.
Red light is used for showing danger signs.
The primary reason why the color red is used for danger signals is that red light is scattered the least by air molecules, The wavelength of red light is more than any other colour. So red light is able to travel the longest distance through fog, rain, and the alike.
