Sunday, February 19, 2012

CBSE-class 8- Science - Ch16 - Light - ncert book Solution

Q1. 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 see any object, when light reflected by that object reaches our eyes. But in the dark room, no light is reflected by the object. Hence, we are unable to see the objects in dark room. But if there is light outside the room, we can see the objects lying there.

Q2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:
S.No. Regular reflection Diffused reflection
1. It takes place from a smooth or regular surface. It takes place from a rough surface.
2. All reflected rays are parallel to each other. The reflected rays are not parallel to each other.
3. Reflected rays go in one direction. Reflected rays are scattered in different directions.
4. e.g. it is caused by smooth surfaces such as mirror. e.g. It is caused by the irregularities in the reflecting surface, like that of a cardboard.
(Draw fig 16.8 and 16.7 from the book)

In diffused reflection, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated. 

Q3: 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.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper




Answer:
(a) A polished wooden table is an example of smooth surface. So, regular reflection will take place.
(b) Chalk powder has irregular surface. So, when a beam of light reflects from it, diffused reflection will take place.
(c) A cardboard has irregular surface. So, when a beam of light reflects from it, diffused reflection will take place.
(d) A marble floor with water spread over it provides a smooth surface. So, regular reflection will take place.
(e) A mirror has a smooth surface. So, regular reflection will take place.
(f) A piece of paper may appear smooth but it has many minor irregularities. So, when a beam of light reflects from it, diffused reflection will take place.

Q4: State the laws of reflection.
Answer:
  • The angle of incidence is equal to the angle of reflection. i.e. ∠i = ∠r
  • The incident ray, the reflected ray and the normal to the surface at the point of incidence lie in the same plane.
Q5: 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: Refer to Activity 16.2 of NCERT book.

Q6: Fill in the blanks in the following:
Answer: (a) A person 1 m in front of a plane mirror seems to be _2_ m from his image.
Explanation: (Distance of the object and its image from the plane mirror is same. When the person is standing 1m away from the mirror, its image is also 1 m away. So, the distance between the person and its image appears to be 2 m.)

(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_.
(Explanation: The image formed by plane mirror is laterally inverted)

(c) The size of the pupil becomes _large_ when you see in dim light.
(Explanation: The pupil expands to increase the amount of light which is entering into our eyes in dim light.)

(d) Night birds have _less__ cones than rods in their eyes.
(Explanation:Night bird has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more cones and fewer rods.)

Choose the correct option in Questions 7 - 8
Q7. Angle of incidence is equal to the angle of reflection

(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Answer: (a) Always.
According to first law of reflection, the angle of incidence is always equal to the angle of reflection.

Q8. Image formed by a plane mirror is(a) virtual, behind the mirror and enlarged
(b) virtual, behind the mirror and of the same size as the object
(c) real at the surface of the mirror and enlarged
(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
Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object


Q9. Describe the construction of a kaleidoscope.

Answer: The kaleidoscope is a simple toy that uses the idea of multiple images formed by the combination of plane mirrors. It forms many beautiful and fascinating coloured patterns. It is made by taking three plane mirror strips and joining them to form a triangle. A piece of cardboard is wrapped around the mirrors. On one side of the tube so formed, we place pieces of glass of different colours between two transparent sheets. The other side is closed with a plane transparent sheet. When we see through this end, and rotate the kaleidoscope, we see beautiful coloured patterns. (Refer to figure 16.12 in NCERT book)
(refer Video Link showing construction)
Q10. Draw a labeled sketch of the human eye. 
Answer:
(Ref Fig 16.14 in the book)
  

Q11: 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: Intensity of laser beam is very high, as it carries large amount of energy. It is harmful for eyes and can cause permanent damage. One should not look laser beam directly or for a longer period.

Q12. Explain how you can take care of your eyes.

Answer: We should take the following precautionary measures for our eyes:

  1. We should wash our eyes two or three times a day.
  2. We should never touch our eyes with dirty hands.
  3. We should never rub our eyes.
  4. We should read books and watch TV at proper distance.
  5. We should consult eye specialist in case of any redness or irritation in the eyes.
Q 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: According to the laws of reflection, angle of incidence = angle of reflection.
As given, ∠i + ∠r = 90
It implies ∠i = ∠r = 90/2 = 45 degrees.

Angle of incidence = 45 degrees.

Q14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer: Infinite number of images will be formed if a candle is placed between two parallel plane mirrors.

Q15. 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.

Answer: Laws of reflection will apply here. Applying triangle law properties, we can easily find out, the emergent ray from mirror B, will reflect at 60 degrees.

Q16: 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: Boojho himself can't see his image because he is not not standing in front of the mirror.However, he is able to see objects at P and Q because the reflected rays from P and Q reach his eyes. If the ray from object R does not reflect and reaches Boojho, he cannot see object R as well.


Q17. (a) Find out the position of the image of an object situated at A in the plane mirror.
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?


Answer: (a) The image of the object at position A will be formed behind the mirror. It will be at the same distance away from mirror as the object is.

(b) Yes Paheli at B can see the object because reflected ray from A will reach B.

(c) Boojho can also see the image because his eyes receives the reflected ray from A.

(d) If we trace the reflected rays from B and C backwards, they converge at point behind the mirror. Position of image A remains fixed even if Paheli moves from B to C.
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