Solutions of The Human Eyes And The Colorful Wor (Page No- 293 & 297) - Physics By Lakhmir Singh | Extra Documents, Videos & Tests for Class 10 PDF Download

Lakhmir Singh Physics Class 10 Solutions Page No:293

Question 20: We know that light refracts (or bends) when it goes from one medium to another. Now, the atmosphere contains only air. Then how does light get refracted on passing through only air in the atmosphere ?

Solution : The atmosphere only contains air but all the air in the atmosphere is not at the same temperature. Some of the air layers are cold whereas others are comparatively warm. The cooler air layers of the atmosphere behave as optically denser medium for the light rays whereas the warmer air layers behave as optically rarer medium. So, light gets refracted on passing through these layers of air in the atmosphere.

Question 21: By how much time the day would have been shorter if the earth had no atmosphere ?

Solution : By about 4 minutes

Question 22: A student claims that because of atmospheric refraction, the sun can be seen after it has set, and the day is, therefore, longer than if the earth had no atmosphere.
 (a) What does the student mean by saying that the sun can be seen after it has set ?
 (b) Do you think that the students’ conclusion is correct ?

Solution : (a) This means that due to atmospheruc refraction we continue to see the sun about two minutes after the actual sunset. No such atmospheric refraction could have been possible if the earth had no atmosphere. Hence, the day is longer due to the atmosphere of earth.
(b) Yes

Lakhmir Singh Physics Class 10 Solutions Page No:297

Question 1: What is the colour of the sunlight :
 (a) scattered by the dust particles in the atmosphere ?
 (b) scattered by the air molecules in the atmosphere ?

Solution : (a) White
(b) Blue

Question 2: Which of the two is scattered more easily : light of shorter wavelengths or light of longer wavelengths ?

Solution : Light of shorter wavelengths

Question 3: State whether the following statements are true or false :
 (a) The scattering away of red light makes the sky appear blue during the day time.
 (b) The scattering away of blue light makes the sun appear red at sunset.

Solution : (a) False
(b) True

Question 4: What colour does the sky appear to an astronaut ?

Solution : Dark or black

Question 5: Which effect is illustrated by the observation that when a beam of sunlight enters a dusty room, then its path becomes visible to us.

Solution : Tyndall effect

Question 6: State two effects produced by the scattering of light by the atmosphere.

Solution : Two effects produced by the scattering of light by the atmosphere are:
-Sky appears blue.
-Sun appears red at sunrise and sunset.

Question 7: What is tyndall effect ? Explain with an example

Solution : The scattering of light by particles in its path is called Tyndall effect.
Ex. When a beam of sunlight enters a dusty room through a window, then its path becomes visible to us due to the scattering of the light by the dust particles present in the air.

Question 8: What happens when a beam of sunlight enters a dusty room through a window ? 

Solution :. When a beam of sunlight enters a dusty room through a window, then its path become visible to us. The tiny dust particles present in the air of room scatter the beam of light all around the room.

Question 9: Why does the sky appear blue on a clear day ?

Solution : The sky appears blue on a clear day because of the scattering of blue component of white sunlight by air molecules presents in the atmosphere. When sunlight passes through the atmosphere, most of the longer wavelength lights do not get scattered much and hence pass straight through the atmosphere. The shorter wavelength blue light is, however, scattered all around the sky and whichever direction we look, some of this scattered blue light enters our eyes.

Question 10: Why does the sky appear dark (or black) to an astronaut instead of blue ?

Solution : To an astronaut, the sky looks dark and black instead of blue because there is no atmosphere containing air in the outer space to scatter sunlight. So, there is no scattered light to reach our eyes in outer space, therefore the sky looks dark and black there.

Question 11: Why does the sun appear red at sunrise ?

Solution : The sun and the surrounding sky appear red at sunrise because at that time most of the blue color present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red color in the direct sunlight beam that reaches our eyes.

Question 12: Why does the sun appear red at sunset ?

Solution : The sun and the surrounding sky appear red at sunset because at that time most of the blue color present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red color in the direct sunlight beam that reaches our eyes.

Question 13: Why are the ‘danger signal’ lights red in colour ?

Solution : ‘Danger’ signls are red in colour because the red coloured light having longer wavelength is the least scattered by fog or smoke particles. Due to this the red light can be seen in the same colour even from a distance.

Question 14: (a) Draw a neat and labelled diagram of the experimental set up for observing the scattering of light in a colloidal solution of sulphur to show how the sky appears blue, and the sun appears red at sunrise and sunset.
 (b) Out of blue light and red light, which one is scattered more easily ?
 (c) Which component of sunlight is scattered away when the sun appears red at sunrise or sunset ?
 (d) What causes the scattering of blue component of sunlight in the atmosphere ?

Solution :

(b) Blue light scatters more easily due to its smaller wavelength

(c) Shortest wavelength component i.e. blue light scatters away when the sun appears red at sunrise or sunset,

(d) Gas moleccules present in the air.

Question 20: In an experiment to study the scattering of light by passing a beam of white light through a colloidal solution of sulphur in a transparent glass tank :
 (a) Which colour is observed from the front of the glass tank ? Does this colour   correspond to the   colour of
 sky on a clear day or the colour of sky around the sun at sunset ?
 (b) Which colour is observed from the sides of the glass tank ? Does this colour   correspond to the   colour of
 sky on a clear day or the colour of sky around the sun at sunset ?

Solution : (a) Red colour is observed from the front of the glass tank. This colour corresponds to the colour of sky around the sun at sunset.
(b) Blue colour is observed from the sides of the glass tank. This colour corresponds to the colour of sky on a clear day.

Question 21: Explain why, when the sun is overhead at noon, it appears white, but when the same sun is near the horizon at sunset, it appears red.

Solution : When the sun is overhead, then the light coming from the sun has to travel a relativity shorter distance through the atmosphere to reach us. During the shorter journey of sunlight, only a little of the blue color of the white light is scattered. Since light coming from the overhead sun has almost all its component colors in the right proportion, therefore the sun in the sky overhead appears white to us.

But when the same sun is near the horizon at sunset, the sunlight has to travel the greatest distance through the atmophere to reach us. During this long journey of sunlight, most of the shorter wavelength blue colour present in it is scattered out and away from our line of sight. So, the light reaching us directly from the setting sun consists mainly of longer wavelength red colour due to which the sun appears red.

Question 22: Complete the following statements :

When the sun is setting, the light from it has to travel a ……………… thickness of the earth’s atmosphere and only…….. wavelength……… light is able to reach us. Sunset is therefore……….

Solution : greater; longer; red; red