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NCERT Class 7 Science Chapter 15 Light
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Light
Chapter: 15
NCERT TEXT BOOK EXERCISES
Q. 1. Fill in the blanks:
(a) An image that cannot be obtained on a
screen is called ________.
Ans: Virtual image.
(b) Image formed by a convex ________ is always virtual and smaller in size.
Ans: Mirror.
(c) An image formed by a _______ mirror is always of the same size as that of the object.
Ans: Plane.
(d) An image which can be obtained on a screen is called a __________ image.
Ans: Real.
(e) An image formed by a concave ______ cannot be obtained on a screen.
Ans: Lens.
Q. 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.
Ans: False.
(b) A concave lens always form a virtual image.
Ans: True.
(c) We can obtain a real, enlarged and inverted image by a concave mirror.
Ans: True.
(d) A real image cannot be obtained on a screen.
Ans: False.
(e) A concave mirror always form a real image.
Ans: False.
Q.3. Match the Items given in Column with one more items of Column II
| Column – I | Column – II |
| (i) Used as a magnifying glass. | (i) Used as a magnifying glass. |
| (ii) Can form image of objects spread over a large area. | (ii) Can form image of objects spread over a large area. |
| (iii) Used by dentists to see enlarged image. | (iii) Used by dentists to see enlarged image. |
| (iv) The image is always inverted and magnified. | (iv) The image is always inverted and magnified. |
| (v) The image is erect and of the same size as the object. | (v) The image is erect and of the same size as the object. |
| (vi) The image is erect and smaller in size than the object. |
Ans:
| Column – I | Column – II |
| (i) Used as a magnifying glass. | (v) The image is erect and of the same size as the object. |
| (ii) Can form image of objects spread over a large area. | (ii) Can form image of objects spread over a large area. |
| (iii) Used by dentists to see enlarged image. | (i) Used as a magnifying glass. |
| (iv) The image is always inverted and magnified. | (iii) Used by dentists to see enlarged image. |
| (v) The image is erect and of the same size as the object. | (vi) The image is erect and smaller in size than the object. |
Q. 4. State the characteristics of the image formed by a plane mirror.
Ans: (i) Plane mirror forms an erect image.
(ii) It forms a virtual image.
(iii) Size of the image is same as that of the object.
(iv) Image gets formed at the same distance behind the mirror as the object stands in front of it.
(v) Image formed is a laterally inverted image. i.e. right hand side of the object seems to be the left hand side and vice versa.
Q. 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.
Ans: Letters like A, H, I, M, O, T, U, V, W etc., appear same when seen through a plane mirror.
Q. 6. What is a virtual image? Give one situation where a virtual image is formed.
Ans: The image which cannot be taken on a screen is called a virtual image. When some object is placed very close to the concave mirror we do not get any image of that object on the white screen placed behind the mirror. Such image is called a virtual image.
Q. 7. State two differences between a convex and a concave lens.
Ans:
| Convex Lens | Concave Lens |
| (i) Convex lens can form both real and virtual images. | (i) Concave lens always form a virtual image. |
| (ii) It can form mag- nified image. | (ii) Image is always diminished in size. |
Q.8. Give one use each of a concave and a convex mirror.
Ans: Use of Concave Mirror: Concave mirror is used by dentists to examine the teeth.
Use of Convex Mirror: Convex mirror is used as side view mirror in vehicles.
Q. 9. Which type of mirror can form a real image?
Ans: Concave mirror can form a real image.
Q. 10. Which type of lens forms always a virtual image?
Ans: Convex lens always form a virtual image.
Choose the correct option in questions 11-13:
Q. 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.
Ans: (ii) Concave mirror.
Q. 12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves Im 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
Ans: (i) 3 m.
Q. 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
Ans: (iii) 4 m/s.
ADDITIONAL IMPORTANT QUESTIONS
Very Short Type Questions Answer
Q. 1. If an object is placed at a distance of 5 cm from a plane mirror, how far would it be from the image?
Ans: 10 cm.
Q. 2. What reflects light when we see the images of trees and other objects in a lake?
Ans: Source of water in the lake.
Q. 3. Define the term reflection.
Ans: The bouncing back of light when it strikes a smooth or polished surface is called reflection of light.
Q. 4. What type of mirror is used to obtain a real image?
Ans: A concave mirror.
Q. 5. Which mirror has a large field of viewing?
Ans: A convex mirror.
Q. 6. Which mirror is used by dentists?
Ans: A concave mirror.
Q. 7. Name the phenomenon responsible for the following effect:
When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand.
Ans: Lateral inversion.
Q. 8. Name the lens which can converge sun’s rays to a point and burn a hole in a piece of paper.
Ans: Convex lens.
Q. 9. If the image formed by a lens is always virtual, erect and smaller than the object, what is the nature of the lens?
Ans: Concave lens.
Q. 10. Which type of lens can form real and inverted images, as well as virtual and erect image of an object?
Ans: Convex lens.
Q. 11. Name any two letters of English alphabet in which the image formed in a plane mirror appears exactly like the letters.
Ans: A and H.
Q. 12. Which kind of lens can form:
(a) an inverted image smaller than the object?
Ans: Convex lens.
(b) an erect image smaller than the object?
Ans: Concave lens.
Q. 13. Why do drivers prefer to use a convex mirror or a back view mirror in automobile?
Ans: It produces an erect and a diminished image.
Q. 14. How will you identify a convex lens by touching it?
Ans: A convex lens is thicker at the center and thinner at the edges.
Q. 15. How will you identify a concave lens by touching it?
Ans: A concave lens is thinner at the center and thicker at the edges.
Short Type Questions Answer
Q.1. State four characteristics of the image formed in a plane mirror.
Ans: The image formed in a plane mirror is:
(i) always virtual and erect.
(ii) of the same size as the object.
(iii) at the same distance behind the mirror as the object is in front of it.
(iv) laterally inverted.
Q. 2. What is the difference between virtual images produced by concave, plane and convex mirrors?
Ans: Virtual image produced by concave mirror is magnified, that produced by plane mirror is of the same size and the virtual image produced by convex mirror is diminished.
Q. 3. What kind of a lens can form:
(a) an inverted image larger than the object?
Ans: Convex lens.
(b) an erect image larger than the object?
Ans: Convex lens.
(c) an inverted image smaller than the object?
Ans: Convex lens.
(d) an erect image smaller than the object?
Ans: Concave lens.
Q. 4. State one way in which the image formed in a convex mirror is similar to that in plane mirror and one way in which it is different.
Ans: Similarity: Both convex mirror and plane mirror always form a virtual and erect image of an object.
Difference: Convex mirror always forms an image which is smaller than the object but a plane mirror always forms an image which is of the same size as the object.
Q. 5. What happens to the image formed by a convex lens if its lower part is blackened?
Ans: Every part of a lens forms a complete image. If the lower part of the lens is blackened the complete image will be formed but its intensity will decreases.
Q. 6. How will you distinguish between a convex and a concave lens without touching them?
Ans: We will keep the lens close to some page of a book and see the print of the book through it. If the letters of the book appear enlarged, it is a convex lens and if they appear diminished, it is concave lens.
Q. 7. Define ‘dispersion’ of white light. Which colour bends:
(i) the least.
(ii) the most, while passing through the glass prism?
Ans: Dispersion of light is the splitting of light into its component colors on passing through a dispersion medium, e.g., a prism.
(i) Red light bends the least.
(ii) violet light bends the most while passing through a prism.
Q. 8. Write two uses of convex mirror.
Ans: Convex mirror is used:
(i) as a rear view mirror in vehicles to help the driver check the view of the traffic behind them.
(ii) as a reflector in street lamps to spread light over a wider area.
Q. 9. Name the type of mirror which is used as a reflector in the head-lights of car. Why is it used for this purpose?
Ans: Concave mirror is used as a reflector in the head-lights of a car. This is because when the source is placed at the focus of the concave mirror, a parallel beam of light is produced.
Q. 10. Which type of mirror is used as a shaving mirror? Support your answer with reason.
Ans: Concave mirrors are used as shaving mirrors to see a large image of the face. This is because when the face is held within the focus of a concave mirror, then an enlarged image of the face is seen in the concave mirror. This helps in making a smooth shave.
Short Type Questions Answer
Q. 1. The mirror outside the driver seat of a vehicle is usually a convex mirror. Printed on such a mirror is usually the warning, “Vehicles in this mirror are closer than they appear.” What is the reason for this warning?
Ans: The reason for this warning is that the virtual image produced in a convex mirror is reduced in size and therefore looks smaller, just as a distant object will appear in a plane mirror. A driver, thinking that the side view mirror is a plane mirror, might incorrectly deduce from the small size of the image that the vehicle behind is far enough away to ignore.
Q. 2. Name the type of mirror used in the following situations:
(a) Head lights of a car.
Ans: Concave mirrors are used as head lights of a car because they reflect the light.
(b) Side/rear-view mirror of a vehicle.
Ans: Concave mirror is used in side/rear-view mirror of a vehicle because it produces an erect image of the objects and gives a field of view of the traffic behind.
(c) Solar furnace.
Ans: Concave mirror is used in solar furnace because concave mirror focuses the sun rays on the objects to be heated.
Q.3. How is rainbow formed?
Ans: Rainbow is formed in a direction opposite to that of the sun. Just after rain, there are a large number of small water droplets in the atmosphere which act like small prism. They refract and disperse the incident sunlight, reflect it internally, and finally refract it again when it comes out of the rain drop. Due to the dispersion of light and internal reflection, different colors form rainbow.
Fig 15. 32. Rainbow formation.
Q.4. What are the characteristics of images formed by a concave mirror?
Ans: The characteristics of images formed by a concave mirror are:
(i) The image is not always the same size as the object. It can be larger or smaller depending on the distance of the object to the mirror.
(ii) The image is formed on the screen. Such type of image that can be formed on a screen is called real image.
(iii) The image is inverted when the object is kept far from the mirror. When it is brought very close to the concave mirror, an erect and enlarged image is formed.
Q. 5. What are the characteristics of images formed by a convex mirror?
Ans: The characteristics of images formed by a convex mirror are:
(i) the image is erect.
(ii) the image is smaller than the object.
(iii) the image formed is virtual.
Long Answer Type Questions
Q. 1. How can you distinguish between plane mirror, convex mirror and concave mirror by merely looking at the image formed in each case?
Ans: To distinguish between a plane mirror, a convex mirror and a concave mirror, the given mirror is held near the face and the image is seen.
(i) If the image is upright, if same size as the object and it does not change in size when the mirror is moved, then the mirror is a plane mirror.
(ii) If the image is upright magnified and it becomes inverted when the mirror is moved away from the face, then the mirror is a concave mirror.
(iii) If the image is upright and diminished and it remains upright when the mirror is moved away from the face, then the mirror is a convex mirror.
Q.2. Can a plane mirror form:
(a) a real image. and
(b) an inverted image?
Ans: (a) Yes, when the object is virtual. A plane mirror forms a virtual image of a real object. So in accordance with the principle of reversibility of light if the object is virtual the image will be real. The real image formed is erect and of the same size as that of the object.
(b) Yes, when an object is placed upright on a plane mirror. This is based on the fact that the image formed by a plane mirror is as far behind the mirror as the object is in front of it. The lower part of the object, being nearest to the mirror, forms an image closer to it while the upper part being farthest from it forms an image farther away, thereby resulting in an inverted image of the object. Such a situation arises when we look for our image in water.
Q. 3. Explain the image formed by a plane mirror.
Ans: In a plane mirror, an image is formed due to regular reflection.
Let the point object O be placed in front of plane mirror M,M. Consider rays OP and OQ as incident rays from a point object O. OP and OQ obey laws of reflection and bounce back as reflected rays PR and QS, but they do not intersect each other in front of the mirror. However, when produced back the reflected rays appear to come from the point ‘T’ behind the mirror. Thus the reflection of O is seen in the mirror at I which is behind the mirror.
Fig. 15.33. Formation of image in a plane mirror.
HOTS QUESTIONS
Q. 1. We wish to obtain an erect image of an object by using a concave mirror of focal length 10 cm.
(a) What should be the distance of the object from the concave mirror?
Ans: Less than 10 cm from concave mirror.
(b) Is the image real or virtual?
Ans: Virtual.
(c) Is the image larger or smaller than the object?
Ans: Larger than the object.
Q. 2. An object is placed at the following distances from a concave mirror of focal length 10 cm.
(a) 8 cm.
(b) 15 cm.
(c) 25 cm.
Which position of the object will produce:
(i) a real image larger than the object?
(ii) a virtual image larger than the object?
(iii) a real image smaller than the object?
Ans: (i) 15 cm.
(ii) 8 cm.
(iii) 25 cm.
Q. 3. A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain, why it is so?
Ans: When white light enters the glass slab, dispersion of light takes place. The angle of refraction for violet colour is more than for red colour on entering the glass slab. But all colours of light return to the original direction of propagation while refraction from other side of the glass slab and thus white light emerges out of the glass slab. Hence, glass slab does not produce any spectrum.
Q. 4. The rear view mirror of a car is a plane mirror. The driver can see the image of a bus parked behind his car in the rear view mirror. If the driver reverses his car at a speed of 0.75 m/s at which speed the image of bus will appear to approach the car driver?
Ans: 1.50 m/s.
Q. 5. An object is placed at the following distances from a convex lens of focal length 10 cm:
(a) 8 cm.
(b) 15 cm.
(c) 25 cm.
Which position of the object will produce:
(i) a diminished image?
(ii) a magnified real image?
(iii) a magnified virtual image?
Ans: (i) 25 cm.
(ii) 15 cm.
(iii) 8 cm.
SKILL BASED QUESTIONS
Q.1. Draw a diagram to show a:
(i) concave mirror.
(ii) convex mirror.
Define these terms and write one important difference between them.
Ans:
Fig. 15. 34. A concave and a convex mirror.
(i) Concave mirror: If reflecting surface of a spherical mirror is concave, it is called a concave mirror.
(ii) Convex mirror: If reflecting surface of a spherical mirror is convex, it is called convex mirror.
Difference: The concave mirror is converging and convex mirror is diverging in nature?
Q.2. Draw a diagram to show converging nature of convex lens and diverging nature of a concave lens.
What do you mean by converging and diverging nature?
Ans:
Fig 15.35.
• Convex lens is called converging lens. It means that it bends light rays inward.
• Concave lens is called diverging lens. It means that it bends light rays outward.
Q. 3. Draw ray diagrams showing the image formation by a convex mirror when an object is placed.
(a) infinity.
(b) at finite distance from the mirror.
Ans: (a)
Fig. 15. 36.
FORMATIVE ASSESSMENT
1. Fill in the following blanks with suitable words:
(i) Light travels in _________ lines.
Ans: Straight.
(ii) The change in direction of light by a mirror is called ________ of light.
Ans: Reflection.
(iii) A ________ mirror can form a real and inverted image.
Ans: Concave.
(iv) Convex mirrors are _________ in the middle than at the edges whereas concave lenses are _________ in the middle than at the edges.
Ans: Thicker, thinner.
(v) The splitting up of white light into seven colors on passing through a glass prism is called _________.
Ans: Dispersion.
(vi) The image formed by a plane mirror as far ________ the mirror as the ________is in front of it.
Ans: Behind, object.
(vii) The image formed in a plane mirror is ________ inverted.
Ans: Laterally.
(viii) The rear view mirror/side mirror in automobiles is a ________ mirror.
Ans: Convex.
(ix) The color of the light that deviates through the largest angle while passing through a prism is ___________.
Ans: Violet.
(x) If the image formed is always virtual, the mirror can be _________.
Ans: Convex or plane.
2. State whether the following statements are true or false:
(i) The image of an object formed by a plane mirror is virtual.
Ans: True.
(ii) A concave mirror can not be used as a magnifying mirror.
Ans: False.
(iii) The image formed in a plane mirror is erect, enlarged and virtual.
Ans: False.
(iv) The objects having polished, shining surfaces reflect more light than objects having unpolished dull surfaces.
Ans: True.
(v) The image formed by a plane mirror is inverted laterally.
Ans: True.
(vi) Convex mirror can be used to see large image of small object.
Ans: False.
(vii) A convex mirror always forms a virtual and erect image.
Ans: True.
(viii) A concave mirror can be used to obtain a parallel beam of light from a small lamp.
Ans: True.
(ix) If a surface does not reflect any light at all, it looks black.
Ans: True.
(x) A lens which is thinner, in the middle and thicker at the edges, is a convex lens.
Ans: False.
3. Match the following:
| Column – A | Column – B |
| 1. Real image | (a) cannot be formed on the screen |
| 2. Magnified image | (b) upside down image |
| 3. Erect image | (c) smaller than the object |
| 4. Diminished image | (d) upright image |
| 5. Virtual image | (e) bigger than the object |
| 6. Inverted image | (f) formed on the screen |
Ans:
| Column – A | Column – B |
| 1. Real image | (f) formed on the screen |
| 2. Magnified image | (e) bigger than the object |
| 3. Erect image | (d) upright image |
| 4. Diminished image | (c) smaller than the object |
| 5. Virtual image | (a) cannot be formed on the screen |
| 6. Inverted image | (b) upside down image |
4. Write one word for the following:
(i) A beam of light consisting of rays coming closer to each other.
Ans: Convergent beam.
(ii) A mirror that forms virtual image to the same size.
Ans: Plane mirror.
(iii) The image that cannot be seen on a screen.
Ans: Virtual image.
(iv) A lens that can be used as a simple microscope.
Ans: Convex lens.
(v) Splitting of white light into its constituent colors by a prism.
Ans: Dispersion of white light.
(vi) Any smooth polished surface which can return the rays of light into the same medium.
Ans: Mirror.
5. Multiple Choice Questions:
(i) A virtual image:
(a) can be formed on the screen
(b) cannot be formed on the screen
(c) is formed only by the plane mirror
(d) is formed only by the convex mirror
Ans: (b) Cannot be formed on the screen.
(ii) Which of the following would you prefer to use while reading small letters found in a dictionary?
(a) a concave mirror
(b) a concave lens
(c) a convex mirror
(d) a convex lens.
Ans: (d) A convex lens.
(iii) The image of an object formed by a plane mirror is:
(a) virtual
(b) real
(c) diminished
(d) upside down
Ans: (a) Virtual.
(iv) A diverging mirror is:
(a) a plane mirror
(b) a convex mirror
(c) a concave mirror
(d) none of the above
Ans: (b) A convex mirror.
(v) The image formed by spherical mirror is virtual. The mirror will be:
(a) concave
(b) convex
(c) either concave or convex
(d) none of the above
Ans: (c) either concave or convex.
