# GA Study Notes On Light and its phenomena (Part 1) : SSC CGL Tier-1 2017

LIGHT – Reflection and Refraction

REFLECTION OF LIGHT

(i) The angle of incidence is equal to the angle of reflection
(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
Uses of concave mirrors
• Concave mirrors are commonly used in torches, search lights and vehicles headlights to get powerful parallel beams of light.
•  They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients.
• Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Uses of convex mirrors
• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving.Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
REFRACTION OF LIGHT

The following are the laws of refraction of light.
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
(ii) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.
The one with the larger refractive index is optically denser medium and the other medium of lower refractive index is optically rarer. The speed of light is higher in a rarer medium. Thus, a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

You might have observed these due to Refraction of light.
• when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.
• The bottom of a tank or a pond containing water appears to be raised.
•  A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water.
• A lemon kept in water in a glass tumbler appears to be bigger than its actual size when viewed from the sides.
• A paper viewed through the lens happen to burn the paper.This is because the light from the Sun constitutes parallel rays of light. These rays are converged by the lens at the sharp bright spot formed on the paper. In fact, the bright spot you got on the paper is a real image of the Sun.The concentration of the sunlight at a point generated heat. This caused the paper to burn.
Refraction of Light through a Prism
Dispersion

The prism split the incident white light into a band of colours VIBGYOR. The splitting of light into its component colours is called dispersion.
Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most.
Rainbow
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop  Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
Phenomenon due to Atmospheric Refraction.
• Wavering of the object when seen through fire.

The air just above the fire becomes hotter than the air further up. 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.Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object, as seen through the hot air, fluctuate. This wavering is thus an effect of atmospheric refraction.

• Twinkling of stars

The twinkling of a star is due to atmospheric refraction of starlight.The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. As the path of rays of light coming from the star goes on varying slightly, the starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.

• Advance 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 time difference between actual sunset and the apparent
Scattering of Light
• The blue colour of the sky
• The colour of the water in the deep sea.
• The reddening of the sun at sunrise and the sunset.
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than the light of longer wavelengths at the red end. Thus, when sunlight passes through the atmosphere, the fine particles in the air, scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes.
If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.

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