CLASS X | SCIENCE | Colourful World
notes prepared by subhankar Karmakar
GLASS PRISM:
A prism is a wedge shaped portion of a transparent refracting medium bounded by two plane faces inclined to each other at a certain angle.
The two plane faces (ABED and ACFD) inclined to each other are called refracting faces of the prism.
The line (AD) along which the two refracting faces meet is called the refracting edge of the prism.
The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.
The angle A included between the two refracting faces is called angle of the prism.
Any section of the prism cut by a plane perpendicular to the refracting edge is called principal section of the prism.
The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.
ANGLE OF DEVIATION FOR A PRISM:
The angle of deviation D is the angle between the incident ray and the emergent ray.
Difference between when light travels through a glass slab and light travels through a prism:
In refraction through a glass slab , the emergent ray is parallel to the incident ray, but in refraction throigh a glass prism, the emergent ray is not parallel to the incident ray.
DISPERSION OF LIGHT:
The phenomenon of splitting of white light into its component colours on passing through a refracting medium is called "dispersion of light." The pattern of the coloured bands obtained on the screen is called "spectrum."
CAUSES OF DISPERSION:
The dispersion of white light occurs because different colours of white light travel at different speeds through the glass prism.
"More is the speed, less is the deviation"
(i) Red colour travels with maximum speed in glass prism, hence, red colour deviates least or minimum.
(ii) Violet colour travels with minimum speed in glass prism, therefore, violet colour deviates maximum.
RECOMBINATION OF LIGHT:
Opposite of dispersion is called recombination of light.
Where as splitting of white light into seven colours is called dispersion, but when seven colours combined together to give white light, it is called "recombination" of light.
RAINBOW FORMATION:
The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun's light by raindrops in the atmosphere. Therefore, we can say, a rainbow is produced by the dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum of seven colours.
(i) Why planets don't twinkle
In comparison to the stars planets are very near to us. Therefore size of the planets seem to bigger to us. The continuously changing atmosphere is unable to cause variations in the light coming from a big sized planet because of which the planet does not twinkle at all.
(ii) Atmospheric Refraction:
The refraction of light caused by the earth's atmosphere give two different layers of varrying optical densities is called atmospheric refraction
The different phenomena occur due to optical refraction of light
1) twinkling of stars
2) the stars seem higher then they actually are
3) advance sunrise and delayed sunset
(iii) Why does twinkling of a star occur?
Twinkling of a star occurs due to atmospheric refraction of star's light. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. Therefore the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and hence the star appears to twinkle at night.
In the fig the actual position of a star is given, but due to atmospheric refraction the light from the star gradual curved as shown in the figure. When it ultimately reach our eyes, we see the stars at a height which is more than its actual height. The light bends due to the difference of optical densities of different layers of atmosphere. Hence ,"The Stars Seem Higher Than They Actually Are."
(iv) Advance Sunrise and Delayed Sunset :
We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.
So we observe an advance sunrise and a delayed sunset.
When the sun is below the horizon actually the rays of sunlight get bend due to atmospheric refraction and reaches our eyes and we observe the sun rise although the sun is still below the horizon.
Similarly when the Sun already goes below the horizon after actual sunset, but still rays from the sun get bend due to atmospheric refraction and we observe that sunset still not happen. Therefore, we see a delayed sunset.
SCATTERING OF LIGHT
Q1. What is scattering of light?
Ans. Scattering of light means to through light in various random directions. When light rays fall on various types of suspended particles in its path, it is scattered in random directions. Depending on the size of particles, different colour of light are scattered. The most prominent examples of scattering of light are (i) blue colour of the sky, (ii) red colour of the sun at sunrise as well as at sunset.
Q2. What is Tyndall effect?
Ans. The scattering of light by particles in its path is called tyndall effect. When a beam of sunlight enters a dusty room through his window the tiny dust particles present in the air of room scatter the beam of light all around the room, and thus its path becomes visible to us.
Q3) On what factor the colour of scattered light depends upon. Explain.
Ans. The colour of scattered light depends upon the size of the scattering particles in the atmosphere. Like, the larger particles of dust and water droplets present in the atmosphere scatter the light of all the colours due to which the scattered light appears to be white. As larger particles scatter all the colours present in the white light. But air molecules in the upper atmosphere where the amount of dust particles are low, they are very small compared to air particles and hence they scatter only light with low wavelength like blue. Due to this reason, the atmosphere looks blue to us.
Q4. Why the colour of the sky is blue?
The scattering of blue component of the white sunlight by air molecules of small size are responsible for blue colour of the sky. As we know scattering of light depends upon the size of the scattering particles. Where as dust particles are larger and scattered all the colours of visible light but air molecules like nitrogen and oxygen are much smaller in size hence they only scatter colours with low wavelengths like blue. It is the real reason behind the fact that our sky looks blue in colour.
Q5. Why the sun appears red at sunrise and sunset.
Ans. The sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.
Q6. Perform an experiment to study the scattering of light.
Ans.
To show the scattering of light we can perform an experiment. For this we shall take a source of light S placed at the focus of a convex lens L1 so as to produce a parallel beam of light rays. A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc d having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to focus the light rays to form an image on the screen R. If we dissolve about 200 grams of sodium thiosulphate in water of the glass tank. Then add 1 or 2 ml of concentrated sulphuric acid to the water, we will see the fine microscopic particles of sulphur begin to form in water and colloidal solution is obtained. As the sulphur particles begin to form in water we will see the blue light coming from the sides of the glass tank. This is due to the scattering of short wavelength blue light by the minute colloidal sulphur particles. This is how the sky looks blue.
©Subhankar Karmakar
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