DAV Class 7 Science Chapter 12 Question Answer – Light
Introduction
Light helps us see the objects around us. It travels in straight lines and gets reflected when it falls on a reflecting surface. In this chapter, we learn about reflection of light, plane mirrors, spherical mirrors, real and virtual images, and the uses of concave and convex mirrors.
Something To Know
A. Fill in the Blanks
1. An image, which can be obtained on a screen, is called __________ image.
Answer: Real
Explanation:
A real image is formed when reflected rays actually meet at a point. It can be obtained on a screen. For example, a concave mirror can form a real image when the object is placed beyond its focus.
2. The angle of reflection __________ the angle of incidence.
Answer: Equals
Explanation:
According to the first law of reflection, the angle of incidence is equal to the angle of reflection.
3. The incident ray, the __________ at the point of incidence and the reflected ray all lie in the same __________.
Answer: Normal, plane
Explanation:
According to the second law of reflection, the incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
4. The point, where a concave mirror forms the image of a very distant object, is known as the __________ of the mirror.
Answer: Focus
Explanation:
When parallel rays coming from a very distant object fall on a concave mirror, they converge at a particular point in front of the mirror. This point is called the focus.
5. A spherical mirror that always forms a virtual, erect and diminished image of an object is a __________ mirror.
Answer: Convex
Explanation:
A convex mirror always produces an image that is virtual, erect, and smaller than the object. This is why convex mirrors are useful as rear-view and security mirrors.
6. Eye specialists use an ophthalmoscope which is fitted with __________ mirror.
Answer: Concave
Explanation:
A concave mirror can converge light and produce a magnified view when the object is placed close to it. This property makes it useful in instruments such as an ophthalmoscope.
B. Write True or False
1. The image formed by a plane mirror shows the right side of the object as its right side and left side of the object as its left side.
Answer: False
Explanation:
A plane mirror produces lateral inversion. The right side of an object appears as the left side in the image and vice versa.
2. When we stand between a pair of plane mirrors, we see an infinite number of images.
Answer: True
Explanation:
Light is reflected repeatedly between two parallel plane mirrors. These repeated reflections can produce a very large number of images; for ideal parallel mirrors, the number is infinite.
3. A concave mirror never forms a virtual image of an object.
Answer: False
Explanation:
A concave mirror can form a virtual, erect, and magnified image when the object is placed between its pole and focus.
4. We can use either a concave or a convex mirror as a reflector in torches.
Answer: False
Explanation:
A concave mirror is commonly used as a reflector in a torch because it can reflect light from a source near its focus into a nearly parallel beam.
5. Motor vehicles generally use either a plane mirror or a convex mirror as the rear-view mirror.
Answer: False
Explanation:
The statement is false as written because the usual rear-view mirror is a convex mirror, which provides a wider field of view and forms an erect, diminished image.
C. Tick (✓) the Correct Option
1. The angle of incidence equals the angle between the incident ray and the
(a) Mirror surface
(b) Normal at the point of incidence
(c) Reflected ray
(d) Line inclined at 60° to the mirror surface
Answer: (b) Normal at the point of incidence
Explanation:
The angle of incidence is measured between the incident ray and the normal drawn at the point where the ray strikes the mirror.
2. A ray of light, falling normally on a plane mirror, gets reflected in such a way that it
(a) Becomes parallel to the mirror surface after reflection.
(b) Now makes an angle of 30° with the plane mirror.
(c) Now makes an angle of 60° with the plane mirror.
(d) Just retraces its path.
Answer: (d) Just retraces its path.
Explanation:
A ray falling normally on a mirror travels along the normal. Its angle of incidence is 0°. Therefore, the reflected ray travels back along the same path.
3. The central point of the section of the sphere used for making a spherical mirror is known as the
(a) Pole of the mirror
(b) Centre of curvature of the mirror
(c) Focus of the mirror
(d) Midpoint of the mirror
Answer: (a) Pole of the mirror
Explanation:
The pole is the central point of the reflecting surface of a spherical mirror.
4. The distance between the object and its image in a plane mirror is 6 m. If the object moves 1 m towards the mirror, then the distance between object and its image will be
(a) 5 m
(b) 6 m
(c) 8 m
(d) 4 m
Answer: (d) 4 m
Explanation:
Initially, the object and its image are 6 m apart, so the object is 3 m in front of the mirror and its image is 3 m behind it.
When the object moves 1 m towards the mirror, it is 2 m in front of the mirror. Its image will be 2 m behind the mirror.
Therefore:
Distance = 2 m + 2 m = 4 m
5. The rear-view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 4 m/s. The driver sees in his rear-view mirror the image of a scooter parked behind his car. The speed at which its image appears to approach the driver will be
(a) 2 m/s
(b) 4 m/s
(c) 8 m/s
(d) 16 m/s
Answer: (b) 4 m/s
Explanation:
For a plane mirror, the image of a stationary object remains at the same distance behind the mirror as the object is in front of it. As the driver moves toward the stationary scooter at 4 m/s, the image also appears to approach the driver at 4 m/s.
D. Answer the Following Questions in Brief
1. Explain clearly the difference between a real image and a virtual image. Give one example of a virtual image.
Answer:
| Real Image | Virtual Image |
|---|---|
| It is formed by the actual meeting of reflected or refracted rays. | It is formed when rays only appear to meet. |
| It can be obtained on a screen. | It cannot be obtained on a screen. |
| It may be inverted. | It is generally erect in the examples studied here. |
Example of a virtual image:
The image formed by a plane mirror.
Explanation:
When you look into a plane mirror, the reflected rays do not actually meet behind the mirror. They only appear to come from behind it. Therefore, the image is virtual.
2. How does changing the distance of the object, from a convex mirror, affect the size and nature of the image formed by it?
Answer:
A convex mirror always forms a virtual, erect, and diminished image. When the distance of the object changes, the exact size and position of the image change, but its basic nature remains the same.
Explanation:
No matter where the object is placed in front of a convex mirror, the image remains behind the mirror, virtual, erect, and smaller than the object.
3. Draw a ray diagram to show the formation of an image when the object is placed between C and F of a concave mirror.
Answer:
When an object is placed between C and F of a concave mirror, the image is formed beyond C.
The image is:
- Real
- Inverted
- Enlarged
Explanation:
One ray parallel to the principal axis reflects through the focus. Another ray passing through C retraces its path. The reflected rays meet beyond C, forming the enlarged image.
4. Where should we keep an object, in front of a concave mirror, so as to get a virtual and magnified image of it?
Answer:
The object should be placed between the pole (P) and focus (F) of the concave mirror.
Explanation:
When an object is placed between P and F, the reflected rays diverge and appear to come from behind the mirror. The resulting image is virtual, erect, and magnified.
5. The magnified image of an object appears to be formed by an apparent intersection of the reflected rays. It is on the same side as the object. What can you say about:
(a) Nature of the image
Answer: Virtual
(b) The type of mirror used
Answer: Concave mirror
Explanation:
A virtual image formed by a concave mirror is located behind the mirror, opposite the object. Therefore, if the image is on the same side as the object, the wording of the question is inconsistent with a virtual image. For a concave mirror, a real image forms on the same side as the object, while a virtual image forms behind the mirror.
6. Rohit suggested to his father to fix a ‘security mirror’ which would provide them with a wider view of the surroundings of their shop. Name the type of mirror that Rohit must have suggested to his father. Also mention the basic characteristics of the image formed by this type of mirror.
Answer:
Rohit should suggest a convex mirror.
A convex mirror forms an image that is:
- Virtual
- Erect
- Diminished
Explanation:
A convex mirror provides a wide field of view, allowing a person to see a larger area. This makes it useful as a security mirror.
7. State the type of mirror that the following items/objects would correspond to:
(a) Inside of a stainless steel bowl
Answer: Concave mirror
(b) Reflector of a torch
Answer: Concave mirror
(c) A polished shining wooden surface
Answer: Plane mirror
Explanation:
The inner surface of a bowl is curved inward and behaves like a concave reflecting surface. A torch uses a concave reflector to produce a directed beam of light. A smooth, flat polished surface behaves approximately like a plane mirror.
E. Answer the Following Questions
1. State the laws of reflection. Describe an activity to show that incident ray, reflected ray and the normal, at the point of incidence, all lie in the same plane.
Answer:
Laws of Reflection
First law:
The angle of incidence is equal to the angle of reflection.
Second law:
The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
Activity
Take:
- A drawing board
- A plane mirror
- A white sheet of paper
- A torch
- A piece of thick cardboard
Make a small hole in the cardboard so that the torch produces a thin beam of light. Place the plane mirror vertically on the white sheet.
Direct the thin beam of light towards the mirror. Mark the paths of the incident ray and reflected ray and draw the normal at the point of incidence.
Now bend a portion of the cardboard so that the bent portion lies in a different plane. Switch on the torch again.
The reflected light does not follow the bent portion of the cardboard.
Conclusion:
The incident ray, reflected ray, and normal all lie in the same plane.
Explanation:
The activity demonstrates the second law of reflection because changing the plane of the cardboard prevents the reflected ray from travelling along the differently placed portion.
2. Trace the path of reflected ray in the following ray diagrams.
Answer:
The reflected ray should be drawn according to the laws of reflection:
- The angle of incidence equals the angle of reflection.
- The incident ray, reflected ray, and normal lie in the same plane.
- A ray falling normally on a plane mirror retraces its path.
Explanation:
To complete each ray diagram, first draw the normal at the point where the ray strikes the mirror. Then draw the reflected ray so that its angle with the normal equals the angle made by the incident ray with the normal.
3. Draw a ray diagram to show the formation of the image of a point object by:
(i) A plane mirror
Answer:
A plane mirror forms a virtual and erect image behind the mirror. The image is the same distance behind the mirror as the object is in front of it.
(ii) A convex mirror
Answer:
A convex mirror forms a virtual, erect, and diminished image behind the mirror.
Explanation:
For both mirrors, reflected rays do not actually meet behind the mirror. They only appear to come from a point behind it, so the image is virtual.
4. Three persons are standing at points P, Q and R in front of a plane mirror. Can ‘P’ see himself in the mirror? Also can he see the image of Q and R in the mirror?
Answer:
The answer depends on the positions of P, Q, R and the mirror shown in the diagram.
For the arrangement given in the question, P cannot see himself in the mirror, but he can see the images of Q and R.
Explanation:
To see an image in a plane mirror, reflected light from the object must reach the observer's eyes. Whether P can see himself or another person's image depends on the path of the reflected rays and the positions of the people.
5. With the help of a suitable diagram, explain the meaning of the following terms for a concave mirror:
(a) Centre of Curvature
Answer:
The centre of curvature (C) is the centre of the sphere of which the spherical mirror forms a part.
Explanation:
Imagine extending the curved mirror to form a complete sphere. The centre of that sphere is called the centre of curvature.
(b) Pole
Answer:
The pole (P) is the central point of the reflecting surface of a spherical mirror.
Explanation:
It is located at the middle of the mirror's reflecting surface.
(c) Principal Axis
Answer:
The principal axis is the straight line passing through the pole and the centre of curvature of a spherical mirror.
Explanation:
The principal axis is an important reference line used to describe the positions of the object, focus, centre of curvature, and image.
(d) Focus
Answer:
The focus (F) of a concave mirror is the point on the principal axis where rays parallel to the principal axis converge after reflection.
Explanation:
Parallel rays falling on a concave mirror are reflected and meet at the focus. For a convex mirror, reflected parallel rays appear to diverge from the focus behind the mirror.
6. What type of mirror is used in the following cases? Is the image formed virtual and erect in each case?
(a) ‘Make-up’ mirror
Answer: Concave mirror — Yes, the image can be virtual and erect.
Explanation:
When the face is placed close to a concave mirror, between its pole and focus, the mirror forms a magnified, virtual, and erect image.
(b) ‘Rear-view’ mirror of vehicles
Answer: Convex mirror — Yes, the image is virtual and erect.
Explanation:
A convex mirror gives a wider field of view and forms a smaller, virtual, erect image.
(c) Reflecting-type telescope
Answer: Concave mirror — The primary image is real and inverted.
Explanation:
A concave mirror is used to collect and focus light from distant objects. The primary image formed by the objective mirror is real.
(d) Shaving mirror
Answer: Concave mirror — Yes, the image can be virtual and erect.
Explanation:
When the face is placed close to a concave mirror, it produces a magnified, virtual, and erect image, making it useful for shaving.
Value-Based Questions
It was celebration time for the Menons as their son had become a full-fledged and registered dentist. The father advised his son to always maintain the same level of empathy, sincerity and dedication towards all his patients irrespective of their financial or social status. His younger sister agreed with him and compared his behaviour with the spherical mirror that he would often use because it reflects light for anyone being examined.
1. List at least two other values you may like to add to the suggestion given by the senior Mr. Menon.
Answer:
Two additional values are:
- Honesty
- Humility
Other suitable values include kindness, patience, responsibility, respect, and fairness.
Explanation:
A good healthcare professional should treat all patients respectfully and fairly. Honesty and humility help build trust between a professional and the people they serve.
2. Name the type of spherical mirror used by dentists. Draw an appropriate ray diagram in support of your answer.
Answer:
Dentists use a concave mirror.
When the mirror is held close to the tooth, with the tooth between the pole and focus, it produces a virtual, erect, and magnified image.
Explanation:
The magnification produced by a concave mirror allows the dentist to see a larger image of the tooth and examine it more easily.
Something To Do
1. List those letters in 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:
Some capital English letters that have mirror symmetry in a vertical plane are:
H, I, M, O, T, U, V, W, X, Y
Explanation:
When these letters are placed in front of a vertical plane mirror, their reflected shapes appear unchanged because they have vertical line symmetry.
2. You must have seen laughing galleries in science centres, parks or fairs where you can see your distorted and funny images in different mirrors. Using your family photographs from an old album and different strips of mirrors, try to create your own laughing gallery.
Answer:
Activity: Do it yourself.
Suggested method:
Use photographs and safely arranged mirror strips with different shapes or curvatures. Observe how the images change when the mirrors are viewed from different positions.
Explanation:
Different curved mirrors can produce images of different sizes and shapes. This demonstrates how the shape of a reflecting surface affects the image.
3. Take three long rectangular plane mirror strips and use them to make a kaleidoscope. Put some pieces of coloured bangles in it and try to draw the images which you see in it on a sheet of paper.
Answer:
Activity: Do it yourself.
Explanation:
A kaleidoscope uses repeated reflection between mirrors to produce multiple images of the coloured objects. The repeated reflections create beautiful symmetrical patterns.
4. You must have heard the story of the lion and the rabbit from Panchatantra, in which the rabbit fooled the lion by showing it its reflection in water. Try to collect such stories which underlie the principles of reflection.
Answer:
Activity: Do it yourself.
Suggested approach:
Collect stories or examples in which a reflection in water, glass, or a mirror plays an important role. Write the name of the story and briefly explain how reflection of light is involved.
Explanation:
A smooth water surface can act approximately like a plane mirror and produce a reflection when the surface is sufficiently calm.
Important Points to Remember
- Light travels in straight lines in a uniform medium.
- Reflection is the bouncing back of light from a reflecting surface.
- The angle of incidence equals the angle of reflection.
- The incident ray, reflected ray, and normal lie in the same plane.
- A plane mirror forms a virtual, erect image of the same size as the object.
- A plane mirror produces lateral inversion.
- A concave mirror curves inward.
- A convex mirror curves outward.
- A convex mirror always forms a virtual, erect, and diminished image.
- A concave mirror can form either a real or virtual image depending on the position of the object.
- The central point of a spherical mirror is called its pole (P).
- The centre of the sphere of which the mirror is a part is called the centre of curvature (C).
- The point where parallel rays converge after reflection from a concave mirror is called its focus (F).
- Convex mirrors are commonly used as rear-view and security mirrors because they provide a wide field of view.
- Concave mirrors are useful in torches, shaving mirrors, make-up mirrors, and some optical instruments.
- Dentists can use concave mirrors to obtain a magnified view of teeth.
Conclusion
Reflection of light is an important phenomenon that explains how mirrors produce images. Plane, concave, and convex mirrors have different shapes and therefore produce different types of images. Understanding their properties helps us understand their many uses in everyday life, such as rear-view mirrors, torches, shaving mirrors, security mirrors, and dental examination.
