Physics for students of class X
Score ‘n’ Learn
CONTENTS Chapter 1
Light
7
Chapter 2
The Human Eye and the Colourful World
45
Chapter 3
Electricity
67
Chapter 4
Magnetic Effects of Electric Current
97
Chapter 5
Sources of Energy
119
The following 2 pages will give you a glimpse of how effectively we have used illustrations along with simple language to explain a concept. The following pages are from the part of the physics book for X grade, these pages detail “Atmospheric Scattering of light�
5. 6. 7. 8. 9. 10. 11.
Which colours undergo the maximum and the minimum deviation during dispersion? What are the necessary conditions for observing a rainbow? Why do stars twinkle? How many minutes before the actual sunrise time can the sun be seen? Why do stars seem higher than they actually are? What are the various colours of the spectrum? What are the advantages of having two eyes?
Atmospheric Scattering of light
Some natural phenomena arising because of the scattering of light can be easily understood by understanding the Tyndall effect. Tyndall Effect The Tyndall effect is named after the British physicist John Tyndall (1820-1893). He discovered that the path of a beam of light becomes visible when it passes through a heterogeneous mixture. This happens due to the scattering of light by the particles suspended in the medium. The phenomenon of the scattering of light by the particles in a medium is called the Tyndall effect. This effect becomes more pronounced as the number of suspended particles in the medium increases. In our daily lives, we observe this effect in the following situations: A beam of sunlight becomes visible when it enters a closed, dusty room through a small crack. The tiny dust particles in the room scatter the beam of light. Hence, the beam of light is visible. We can see the light from the headlights of a car on foggy days. Little sunlight passes through the thick canopy in a dense forest. However, it is scattered by the tiny droplets of water in the atmosphere. Why is the colour of the clear sky blue? You have learnt that red-coloured light has the longest wavelength and violet or blue-coloured light has the shortest wavelength. Since the colour of scattered light also depends on the size of the suspended particles, blue light will be more scattered by a minute particle, while red light will be more scattered by a relatively large-sized particle. The sizes of the molecules of air and other particles present in the atmosphere are smaller than the wavelengths of visible light. Hence, these particles scatter lights of shorter wavelength (i.e., blue) more effectively than lights of longer wavelengths (i.e., red). When sunlight passes through the atmosphere, the fine particles present in the atmosphere scatter blue light more effectively than red light. The scattered blue light makes the sky appear blue to our eyes. If the Earth lacked atmosphere, then it would have looked dark as there would have been no scattering of light. This was noticed by the astronauts who travelled into the space. To them, the sky appeared to be dark because light cannot be scattered in the outer space as it lacks atmosphere. The scattering of red-coloured light finds its application in various fields. It is used in danger signals because it is least scattered by fog, smoke, or the dust particles present in the air. PHYSICS
Tyndall effect
62 Score ‘n’ Learn
Why does the Sun Appear to be Red at Sunrise and Sunset? Sunlight travels a great distance through the atmosphere to reach us at sunrise and sunset. Blue colour, which has a shorter wavelength, gets scattered away in the atmosphere. Hence, the light that reaches us consists mainly of red colour, which has a longer wavelength. This is why the sun appears to be red in colour at sunrise and sunset. At noon, sunlight travels a relatively shorter distance through the atmosphere to reach us as the sun is directly overhead. In this case, only a small portion of blue light gets scattered in the atmosphere. This is the reason why the sun appears to be white at noon.
Recall Scattering is a very useful phenomenon. Rutherford used this in his famous gold foil experiment to determine the structure of an atom.
Figure 32: Colour of sun To understand the scattering of light further, let us undertake an activity.
The Idea box
Take a strong source of light and focus its light with the help of a converging lens. Allow the light beam from the lens to pass through a glass tank containing clear water. After this, allow the beam to fall on a cardboard having a circular hole. Obtain an image of the circular hole on a screen with the help of a second converging lens. After this, add 2 mL of sulphuric acid and about 200 g of sodium thiosulphate in the water and look at the image. What do you observe on the screen?
Sectional Assessment 4
1. 2. 3. 4. 5. 6. 7. 8.
Time: 25 minutes
What is the function of the optic nerve? Name the two types of cells that are present in the retina of the eye. What are the three defects of vision? What do you understand by the scattering of light? Briefly explain why the colour of the sky is blue. Describe Tyndall effect. Why is red colour commonly used in danger signals? Why does the sky appear dark to an astronaut?
63 THE HUMAN EYE AND THE COLOURFUL WORLD
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