## Thursday, 7 July 2011

### Chapter 15 . Light | Science | CBSE Class VII (7th) | Lesson Exercises

Exercises : Solution
1. Fill in the blanks:
(a) An image that cannot be obtained on a screen is called a virtual image.
(b) Image formed by a convex  mirror  is always virtual and smaller in size.
(c) An image formed by a plane mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a real image.
(e) An image formed by a concave lens cannot be obtained on a screen.

2. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) We can obtain an enlarged and erect image by a convex mirror. (T)
(b) A concave lens always form a virtual image. (T)
(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T)
(d) A real image cannot be obtained on a screen. (T)
(e) A concave mirror always form a real image. (F)

3. Match the items given in Column I with one or more items of Column II.

 Column I Column II (a) A plane mirror (b) A convex mirror (c) A convex lens  (d) A concave mirror  (e) A concave lens (i) Used as a magnifying glass.  (ii) Can form image of objects (iii) Used by dentists to see enlarged image of teeth. (iv) The image is always inverted and magnified.                       (v) The image is erect and of the same size as the object. (vi) The image is erect and smaller

Solution :
 Column I Column II (a) A plane mirror (v) The image is erect and of the same size as the object. (b) A convex mirror (vi) The image is erect and smaller (c) A convex lens (i) Used as a magnifying glass. (d) A concave mirror (iii) Used by dentists to see enlarged image of teeth. (e) A concave lens (vi) The image is erect and smaller

4. State the characteristics of the image formed by a plane mirror.
1. An image formed by a plane mirror is erect and of the same size as the object.
2. The image formed by the plane mirror is at the same distance behind the mirror as the object is in front
of it.
3. The sides of an image formed by a plane mirror gets interchanged. In the mirror the ‘right’ appears ‘left’ and the ‘left’ appears ‘right’.
4. Only sides are interchanged; the image does not appear upside down.
5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

Answer : The image of following  English Alphabet, formed in a plane mirror  appears exactly like the letter itself :
1. A
2. H
3. I
4. M
5. O
6. T
7. U
8. V
9. W
10. X
6. What is a virtual image? Give one situation where a virtual image is formed.
Answer : Virtual Image : An image which cannot be obtained on a screen is called a virtual image.
A virtual image can be formed  by a plane mirror, convex mirror, concave mirror, convex lens and concave lens.  Polished Metallic surface or liquid surface such as lakes or water bodies can also form a virtual image

7. State two differences between a convex and a concave lens.
Answer : Two difference between a convex and a concave lens are as given below :
 Convex Lens Concave Lens Convex lenses are thicker in the middle than at the edges. A convex lens can forms real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. A convex lens can be used as a magnifying glass A convex lens converges (bends inward) the light generally falling on it Concave lenses are thinner in the middle than at the edges. A concave lens always forms erect, virtual and smaller image than the object. A concave  lens can not be used as a magnifying glass A concave lens diverges (bends outward) the light.

8. Give one use each of a concave and a convex mirror.

1. Concave Mirror : It is used by dentists to see enlarged image of teeth.
2. Convex Mirror :It is used as view mirror in bus, cars or side mirrors in scooters.  Convex mirrors can
form images of objects spread over a large area. So, these help the drivers to see the traffic behind them .
9. Which type of mirror can form a real image?
Answer : A concave mirror can form a real and inverted image. When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.

10. Which type of lens forms always a virtual image?
Answer : A concave lens always forms a
virtual, erect,  and smaller image than the object.
Choose the correct option in questions 11–13
11. A virtual image larger than the object can be produced by a
(i) concave lens (ii) concave mirror
(iii) convex mirror (iv) plane mirror

Correct Option : (ii) concave mirror
12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
(i) 3 m (ii) 5 m
(iii) 6 m (iv) 8 m

13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be
(i) 1 m/s (ii) 2 m/s
(iii) 4 m/s (iv) 8 m/s

Please Note : The car driver will see the image of parked truck in his rear-view mirror, approaching at a speed of 2 meters per second (with respect to him). Where as, for an bystander on the ground (or the driver with respect to ground)... the  image in the mirror will be moving with an apparent speed of 4 m meters per second  to catch up the car, which is already reversing with a speed of 2 meters per second.

From the point of view of the driver, the image in his rear-view mirror will approach at a speed of 2 meters per second. I agree with you on this.

From the point of view of a physicist standing by with a pencil, a piece of paper and an optics textbook the virtual image in the mirror will move at a speed of 4 meters per second relative to the ground.

From the point of view of the driver, the image in his rear view mirror will be approaching at an apparent speed of 2 meters per second as above while the ground viewed through his windshield is receding at a speed of 2 meters per second. This adds up to a closing rate of 4 meters per second. [The distinction between a speed and a closing rate is a not important if we're sticking to classical physics]

From the point of view of the driver, the image in his rear-view mirror will approach at a speed of 2 meters per second. I agree with you on this.

From the point of view of a physicist standing by with a pencil, a piece of paper and an optics textbook the virtual image in the mirror will move at a speed of 4 meters per second relative to the ground.

From the point of view of the driver, the image in his rear view mirror will be approaching at an apparent speed of 2 meters per second as above while the ground viewed through his windshield is receding at a speed of 2 meters per second. This adds up to a closing rate of 4 meters per second. [The distinction between a speed and a closing rate is a not important if we're sticking to classical physics]

From the point of view of the driver, the image in his rear-view mirror will approach at a speed of 2 meters per second. I agree with you on this.

From the point of view of a physicist standing by with a pencil, a piece of paper and an optics textbook the virtual image in the mirror will move at a speed of 4 meters per second relative to the ground.

From the point of view of the driver, the image in his rear view mirror will be approaching at an apparent speed of 2 meters per second as above while the ground viewed through his windshield is receding at a speed of 2 meters per second. This adds up to a closing rate of 4 meters per second. [The distinction between a speed and a closing rate is a not important if we're sticking to classical physics]

---------------------------------------------------------------------------
Chapter 15 . Light  | Science | CBSE NCERT Class VII (7th) | Lesson  Exercises
=======================================
Did you know?
The mirrors can be used as weapons. Archimedes, a Greek scientist, is said to have done just that more than two thousand years ago. When the Romans attacked Syracuse, a coastal city-state in Greece, Archimedes used mirrors arranged as shown in Fig. 15.34. The mirrors could be moved in any direction. They were positioned such that they reflected the sunlight on the Roman soldiers. The soldiers were dazzled by the sunlight. They did not know what was happening. They got confused and ran away. This was an example of triumph of ideas over military might.

1. The science was so hard for me with this site it is too easy for me

2. This sight is so great

3. ya really now it is so easy....:)

4. really nice site , its helpful

5. no need for finding answers in the book...!!)

6. this questions are very helpful for me

7. i prepared for my sa 2 very well thank u so much
www.samratnachiyappan.webs.com

8. can somebody give methe solution for 13th answer.I think it is 4m/s

9. It is simple and very good.

10. its very helpful & I completed my science power-point presentation

11. super and good

12. Didn't like the site all that much ...... The questions should have been a lille more challenging but otherwise the site is alright

13. it is a superb site should be recommended for everyone