Chapter 17. STARS AND THE SOLAR SYSTEM | Science | CBSE Class VIII (8th) | Solved Exercises

Things to remember....

1. The phases of the moon occur because we can see only that part of the moon which reflects the light of the Sun towards us.
2. Stars are celestial bodies that emit light of their own. Our sun is also a star.
3. It is convenient to express distances of stars in light years.
4. Stars appear to move from east to west.
5. The pole star appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth.
6. Constellations are groups of stars that appear to form recognizable shapes.
7. The solar system consists of eight planets and host of asteroids, comets and meteors.
   
8. A body revolving around another body is called a satellite.
9. Moon is the natural satellite of the Earth.
10. Some planets also have natural satellites.
11. Venus is the brightest planet in the night sky.
12. Jupiter is the largest planet of the solar system.
13. The artificial satellites revolve around the Earth. They are much closer than the moon.
14. Artificial satellites are used for weather forecasting, long distance communication and remote sensing.

Choose the correct answer in Questions 1-3 :
Question 1. Which of the following is NOT a member of the solar system?

1. (a) An asteroid
2. (b) A satellite
3. (c) A constellation
4. (d) A comet

Answer : (c) A constellation

Question 2. Which of the following is NOT a planet of the sun?

1. (a) Sirius
2. (b) Mercury
3. (c) Saturn
4. (d) Earth

Answer : (a) Sirius

Question 3. Phases of the moon occur because

1. (a) we can see only that part of the moon which reflects light towards us.
2. (b) our distance from the moon keeps changing.
3. (c) the shadow of the Earth covers only a part of moon’s surface.
4. (d) the thickness of the moon’s atmosphere is not constant.

Answer : (a) we can see only that part of the moon which reflects light towards us.

Question 4. Fill in the blanks:

1. (a) The planet which is farthest from the Sun is ____________ .
2. (b) The planet which appears reddish in colour is ____________ .
3. (c) A group of stars that appear to form a pattern in the sky is known as a ____________ .
4. (d) A celestial body that revolves around a planet is known as __________.
5. (e) Shooting stars are actually not ____________.
6. (f) Asteroids are found between the orbits of _________ and _________ .

Answer :

1. (a) The planet which is farthest from the Sun is Neptune .
2. (b) The planet which appears reddish in colour is Mars .
3. (c) A group of stars that appear to form a pattern in the sky is known as a Constellations .
4. (d) A celestial body that revolves around a planet is known as satellite.
5. (e) Shooting stars are actually not stars.
6. (f) Asteroids are found between the orbits of Mars and Jupiter .

Question 5. Mark the following statements as true (T) or false (F):

1. (a) Pole Star is a member of the solar system. [ ]
2. (b) Mercury is the smallest planet of the solar system. [ ]
3. (c) Uranus is the farthest planet in the solar system. [ ]
4. (d) INSAT is an artificial satellite. [ ]
5. (e) There are nine planets in the solar system. [ ]
6. (f) Constellation Orion can be seen only with a telescope. [ ]

Answer :

1. (a) Pole Star is a member of the solar system. [ false (F)]
2. (b) Mercury is the smallest planet of the solar system. [true (T) ]
3. (c) Uranus is the farthest planet in the solar system. [false (F) ]
4. (d) INSAT is an artificial satellite. [ true (T)]
5. (e) There are nine planets in the solar system. [false (F) ]
6. (f) Constellation Orion can be seen only with a telescope. [false (F) ]

Question 6. Match items in column A with one or more items in column B:

A	B
(i) Inner planets (ii) Outer planets (iii) Constellation (iv) Satellite of the Earth	(a) Saturn (b) Pole Star (c) Great Bear (d) Moon (e) Earth (f) Orion (g) Mars

Answer :

A	B
(i) Inner planets	(e) Earth (g) Mars
(ii) Outer planets	(a) Saturn
(iii) Constellation	(c) Great Bear (f) Orion
(iv) Satellite of the Earth	(d) Moon

Question 7. In which part of the sky can you find Venus if it is visible as an evening star?
Answer : Venus, if it is visible as an evening star, can be found in the western part of the sky only, just 1-3 hours after sunset

Question 8. Name the largest planet of the solar system.
Answer : Jupiter is the largest planet of the solar system.

Question 9. What is a constellation? Name any two constellations.
Answer : Constellations are groups of stars that appear to form recognisable shapes.Constellations were devised by ancient people to easily recognise stars in the sky. The constellations were named as per theirs shapes resembling familiar objects of that time. We can easily identify some constellations in the night sky if know how a particular constellation looks like and where to look for it. Two constellations are Ursa Major and Orion

Question 10. Draw sketches to show the relative positions of prominent stars in
(a) Ursa Major and (b) Orion

Answer : 

(a) Ursa Major

 

Orion

Question 11. Name two objects other than planets which are members of the solar system.

Answer : (1.) Asteroids   (2.) Comets

Question 12. Explain how you can locate the Pole Star with the help of Ursa Major.
Answer : Ursa Major is one of the prominent constellations, which we can see during summer time in the early part of the night. To locate the pole star, with the help of Ursa Major, We will first, look at the two stars at the end of Ursa Major, then will the Imagine a straight line passing through these stars as shown in the fig. Now looking in the direction of this imaginary line towards the north direction, we can see the Pole Star,which is not too bright and located at a distance about five times the distance between the two inline stars of Ursa Major. The Pole Star is not visible from the southern hemisphere

Question 13. Do all the stars in the sky move? Explain.
Answer : Yes, in real, all the stars in the sky move. As we know, all the celestial bodies comprising our solar system, Stars, Constellations and galaxies, are part of big Universe, which is ever expanding. Every celestial object with in Universe, in order to over come the gravitational pull of other celestial object around, has to move in some relative motion. The pole star, which is also moving, but appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth. From earth, all others stars appear to move from east to west as earth makes rotation from west to east around its axis.


Question 14. Why is the distance between stars expressed in light years? 
What do you understand by the statement that a star is eight light years away from the Earth?

Answer : The stars are located at far greater distance from each other.It is convenient to express distances of stars in light years. For example, The Sun is nearly 150,000,000 kilometres away from the Earth. Alpha Centauri, the next nearest star , is at a distance of about 40,000,000,000,000 km from the Earth. This distance is not convenient to express in normal units of length. To measure such large distances, we make use of another unit known as light year, which is the distance traveled by light, in one year with a speed of 300,000 km per second . So using this unit, the distance of Alpha Centauri is about 4.3 light years, which is very convenient to express.
A star is eight light years away from the Earth implies that, for light, which travels with speed of 300,000 km per second, it takes 8 years to reach Earth. In other way, the distance between star and earth is around 9460800000000 km (365 X 24 X 60 X 60 X 3,00, 000 Km).

Question 15. The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Answer :

Let the radius of the Earth	= r
∴ The radius of Jupiter	=11r
Volumes of earth	= 4/3√π(r)3
volumes of Jupiter	=4/3√π(11r)3
Ratio of the volumes of Jupiter and the Earth	4/3√π(11r)3 = 4/3√π(r)3
	=1331 : 1
∴  Jupiter can accommodate 1331 numbers of Earths

Question 16. Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it. 

Answer : The Sketch is incorrect. The Correct one is as shown below : -

Chapter 16. LIGHT | Science | CBSE Class VIII (8th) | Solved Exercises

Question 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.

Answer : We can not see objects in the dark room without the presence of light or a lighted object it self . We may see an object only when, the light from an object enters our eyes. The light may have been emitted by the object directly (luminous objects.), or may have been reflected by it ( illuminated objects). Similarly, we can not see outside objects, sitting inside the darkroom, if there is no opening or passage for any incidental light from out side. How ever, out side the darkroom, in open, we can easily see the objects, due to either reflected light from the surface of illuminated objects or direct light from luminous objects such as Sun, candle, torch etc. Nearly everything we see around is seen due to reflected light.

Question 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer :

	Regular reflection	Diffused reflection
(1.)	When all the parallel rays reflected from a plane surface are parallel, the reflection is known as regular reflection	When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection
(2.)	Reflection from a smooth surface like that of a mirror is an example of regular reflection Images are formed by regular reflection.	Reflection from a rough surface, like that of a cardboard, is an example of irregular reflection. It is caused by the irregularities in the reflecting surface .
(3.)	Images are formed by regular reflection.	Images are not formed by irregular reflection.

Question 3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

1. (a) Polished wooden table
2. (b) Chalk powder
3. (c) Cardboard surface
4. (d) Marble floor with water spread over it
5. (e) Mirror
6. (f) Piece of paper

Answer :

1. (a) Polished wooden table -----> regular reflection
2. (b) Chalk powder -----> diffused reflection
3. (c) Cardboard surface -----> diffused reflection
4. (d) Marble floor with water spread over it -----> regular reflection
5. (e) Mirror -----> regular reflection
6. (f) Piece of paper -----> diffused reflection

In case of objects with smooth, polished and shiny surfaces, like Polished wooden table, Marble floor with water spread over it and Mirror, when a incident beam of light strikes on their surfaces, regular reflection takes place. Whereas, in case of objects with rough surfaces, such as Chalk powder, Cardboard surface, Piece of paper etc, when a incident beam of light strikes on their surfaces, irregular reflection takes place. It is caused by the irregularities in the reflecting surface

Question 4. State the laws of reflection.

Answer : There are two laws of reflection as given below :

1. (i) The angle of incidence is equal to the angle of reflection.
2. (ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane

Question 5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Answer : When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane.

When we bend the paper we create a plane different from the plane in which the incident ray and the normal lie.

 Then we do not see the reflected ray. What does it indicate? It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is second law of reflection.

Similarly, in activity below, as the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane, we can clearly see the glare of led torch

Where as, in activity below, as the incident ray, the normal at the point of incidence and the reflected ray all do not lie in the same plane, therefor,  the glare of led torch is not as powerful.

Question 6. Fill in the blanks in the following :

Answer :

1. (a) A person 1 m in front of a plane mirror seems to be 2 m from his image.
2. (b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with left hand
3. (c) The size of the pupil becomes large when you see in dim light.
4. (d) Night birds have  small number of cones than rods in their eyes.

Choose the correct option in Questions 7 – 8
Question 7. Angle of incidence is equal to the angle of reflection

1. (a) Always
2. (b) Sometimes
3. (c) Under special conditions
4. (d) Never

Answer : (a) Always

Question 8. Image formed by a plane mirror is

1. (a) virtual, behind the mirror and enlarged
2. (b) virtual, behind the mirror and of the same size as the object
3. (c) real at the surface of the mirror and enlarged
4. (d) real, behind the mirror and of the same size as the object.

Answer : (b) virtual, behind the mirror and of the same size as the object

Question 9. Describe the construction of a kaleidoscope.

Answer : A kaleidoscope, is made up of three rectangular mirror strips about 15 cm long and 4 cm wide each. This mirrors, as shown in the figure, are Joined together to form a prism. This assembly, may be housed inside a a circular cardboard tube or tube of a thick chart paper.

The length of tube is slightly longer than the mirror strips. One end of the tube is covered by a cardboard disc having a hole in the center, through which we can see . Other end of the cardboard tube is covered by a circular plane glass plate touching the edge of mirror strips. Now several small pieces of coloured glass (broken pieces of coloured bangles) are put inside . Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.
Your kaleidoscope is ready. When we peep through the hole, we will be able to see a variety of patterns in the tube. Interesting feature of a kaleidoscope is that we will never see the same pattern again.
Question 10. Draw a labeled sketch of the human eye.

Answer :

Question 11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teachers advise?

Answer : The intensity of laser light is extremely high. If we throw beam from laser torch directly on the eye, the eyes may be injured or blinded temporarily or partially due to sudden contraction of pupil and damage of  retina

Question 12. Explain how you can take care of your eyes.

Answer : We can take proper care of our eyes in following ways :

1. (1) In case of any problem we should go to an eye specialist. Beside, we should have a regular eye checkup
2. (2) In case, after an eye checkup, if advised bye an eye specialist, we should use suitable spectacles.
3. (3) We should avoid bright or dim light. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
4. (4) We should not look at the sun or a powerful light directly.
5. (5) We should not rub our eyes. If particles of dust go into our eyes, we should wash our eyes with clean water. If there is no improvement then we should go to a doctor.
6. (6) We should Wash our eyes frequently with clean water.
7. (7)We should always read at the normal distance for vision. Do not read by bringing our book too close to our eyes or keeping it too far.
8. (6) Our Diet should include food items rich in vitamin A

Question 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer : As per the laws of reflection
The angle of incidence ∠i = angle of reflection ∠r
Here as given ∠i + ∠r = 90°
  ∴ ∠i + ∠i = 90 °
or ∠i = 90/2 = 45 °
Hence angle of incidence of a ray = 45 degree

Question 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer : if a candle is placed between two parallel plane mirrors, we will see an infinite numbers of images of candles

Question 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

Fig. 16.19

Answer :

Question 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer :

 As Boojho is not standing in front of plane mirror, Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, does not lie in the same plane, therefor he can not see him self. He can see the image of objects situated at P, Q as Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane. Similarly, incidental ray from R, will not be reflected to reach point A, hence will not be seen by Boojho.

Question 17.

1. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
2. (b) Can Paheli at B see this image?
3. (c) Can Boojho at C see this image?
4. (d) When Paheli moves from B to C, where does the image of A move?
5.  
   

Answer :

1. (a) A virtual and lateral image of an object situated at A, will be formed , behind the mirror surface. The image will be exactly at the same distance , as the distance of real object at position A and reflecting surface of the mirror
2. (b)Yes, as incidental ray from object A, is reaching at B as reflected ray, therefor Paheli at B, can see this image
3. (c) Boojho can also see this image, as reflected rays from object at A is reaching at C
4. (d) The point of convergence, of reflected rays reaching at point B and C, is same and at same distance behind the mirror. Therefor, When Paheli moves from B to C, the image of A will not change its position

---------------------------------------------------------------------------------------------------
CBSE Class VIII (8th) | Science | Chapter 16. LIGHT |  |  Solved Exercises

Things to remember....

1. Light is reflected from all surfaces.
2. Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
3. Diffused/irregular reflection takes place from rough surfaces.
4. Two laws of reflection are :
   1. The angle of incidence is equal to the angle of reflection.
   2. Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
5. Image formed in a plane mirror undergoes lateral inversion.
6. Two mirrors inclined to each other give multiple images.
7. Beautiful patterns are formed in a kaleidoscope because of multiple reflections.
8. Sunlight, called white light, consists of seven colours.
9. Splitting of light into its constituent colours is known as dispersion.
10. Important parts of the eye are cornea, iris, pupil, lens, retina and optic nerve.
11. A normal eye can see nearby and distant objects clearly.
12. Visually challenged persons can read and write using Braille system.
13. Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.

CBSE Class VIII (8th) | Science | Chapter 16. LIGHT |  |  Solved Exercises

Chapter 15. SOME NATURAL PHENOMENA |Science | CBSE Class VIII (8th) | Solved Exercises

Select the correct option in Questions 1 and 2.
Question 1. Which of the following cannot be charged easily by friction?

1. (a) A plastic scale
2. (b) A copper rod
3. (c) An inflated balloon
4. (d) A woolen cloth.

Answer : (b) A copper rod

Question 2. When a glass rod is rubbed with a piece of silk cloth the rod

1. (a) and the cloth both acquire positive charge.
2. (b) becomes positively charged while the cloth has a negative charge.
3. (c) and the cloth both acquire negative charge.
4. (d) becomes negatively charged while the cloth has a positive charge.

Answer : (b) becomes positively charged while the cloth has a negative charge.

Question 3. Write T against true and F against false in the following statements:

1. (a) Like charges attract each other (T/F)
2. (b) A charged glass rod attract a charged plastic straw (T/F)
3. (c) Lightning conductor cannot protect a building from lightning (T/F)
4. (d) Earthquakes can be predicted in advance (T/F)

Answer :

1. (a) Like charges attract each other (F)
2. (b) A charged glass rod attract a charged plastic straw (T)
3. (c) Lightning conductor cannot protect a building from lightning (F)
4. (d) Earthquakes can be predicted in advance (F)

Question 4. Sometime, a crackling sound is heard while taking off sweater during winters. Explain.

Answer : We know th at some objects can be charged by rubbing with other objects.The electrical charges produced by rubbing are called static charges. A woolen sweater while taking off comes in contact with the surface of inner clothes like shirts etc which are either made of cotton or synthetic material. Due to rubbing , or friction between the surfaces of cloth materials, where inner cloth lose some charge in the form of lose electrons, and the sweeter acquires excess of static charge. This excess of static charge, accumulated in sweater, when it comes near or in contact with body, gets discharged to either back or to ground through body, producing electric sparks with crackling sound.

Question 5. Explain why a charged body loses its charge if we touch it with our hand.

Answer : When we touch a charged body, with our hand, the excess of accumulated charge or static charges on it, gets transfer to ground through our body. Thus the charged body loses its charge, and becomes neutral


Question 6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Answer : Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 3 on Richter scale, may be recorded by a seismograph and it is not likely to cause much damage as it will result in  mild tremors of very low intensity on the earth surface. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.


Question 7. Suggest three measures to protect ourselves from lightning.

Answer : The three measures to protect ourselves from lightning, are

1. We should avoid driving or sitting in Open vehicles, like motorbikes, tractors, construction machinery, open car etc as they being made up of metal, may induce lightening strike, hence are not safe. Similarly Open fields, tall trees, shelters in parks, elevated places do not protect us from lightning strokes.
2. We should secure, high rise building with lightening conductor. At the same time, We should avoid carrying umbrella during thunderstorms as lightening may strike its pointed end of metal pipe
3. In a forest, we should take shelter under shorter trees to an extent as possible. In an open field, we should stay away from trees, metal poles or structures and squat low on the ground, Placing hands on knees with head between the hands
4. In an open field, we should stay away from trees, metal poles or structures and squat low on the ground, placing hands on knees with head between the hands

Question 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Answer : We know that like charges repel and unlike charges attract each other. Therefor a charged balloon is repelled by another charged balloon, whereas an uncharged balloon, in the presence of a charged balloon gets induced with opposite charge, is attracted by another charged balloon


Question 9. Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer : The instrument which is used to detect a charged body, is called electroscope. Such instrument can be made very easily. As shown in the figure, the mouth of empty jam bottle is covered with a piece of cardboard slightly bigger in size than the mouth of the bottle.

A stretched paper clip, is inserted in the middle of cardboard as shown. Two small strips of aluminium foil about 4 cm × 1 cm each are Hanged by the lower end of paper clip inside the bottle. When a charged refill is brought in contact with the upper end of paper clip and as metals are good conductors of electricity, the aluminium foil strips receive the same charge from the charged refill through the metal body of paper clip . The aluminium foil strips carrying similar charges repel each other and they become wide open, thus showing the presence of a charge in refill body.


 Question 10. List three states in India where earthquakes are more likely to strike.

Answer :

1. Jammu & Kashmir
2. Gujrat
3. Rajsthan

Question 11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answer : The precautions, which we would take to protect our self are :

1. We would find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
2. If we are in a car or a bus, we would not come out. If moving in vehicle, would ask the driver to drive slowly to a clear spot and wait there inside the vehicle till the tremors stop.

Question 12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Answer : At the event of thunderstorm prediction, while going out, We should avoid carrying an umbrella, as lightening has great affinity to strike at any available metal object in the open. An umbrella with pointed metal tube and wire frame structure inside, offers and ideal situation for a lightening strike in a thunderstorm.

Things to remember....

1. Some objects can be charged by rubbing with other objects.
2. There are two kinds of charges — positive charge and negative charge
3. Like charges repel and unlike charges attract each other.
4. The electrical charges produced by rubbing are called static charges.
5. When charges move, they constitute an electric current.
6. An electroscope may be used to detect whether a body is charged or not.
7. The process of transfer of charge from a charged object to the earth is called earthing.
8. The process of electric discharge between clouds and the earth or between different clouds causes lightning.
9. Lightning strike could destroy life and property.
10. Lightning conductors can protect buildings from the effects of lightning.
11. An earthquake is a sudden shaking or trembling of the earth.
12. Earthquake is caused by a disturbance deep inside the earth’s crust.
13. It is not possible to predict the occurrence of an earthquake.
14. Earthquakes tend to occur at the boundaries of earth’s plates. These boundaries are known as fault zones.
15. Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.
16. We should take necessary precautions to protect ourselves from earthquakes.

CBSE Class VIII (8th) |Science |  Chapter 15. SOME NATURAL PHENOMENA  |  Solved Exercises