The DAV Class 7 Science Solutions and DAV Class 7 Science Chapter 12 Question Answer – Light are essential study tools for DAV public school students in Class 7.
DAV Class 7 Science Ch 12 Question Answer – Light
DAV Class 7 Science Ch 12 Solutions – Light
Something To Know
A. Fill in the blanks.
1. An image, which can be obtained on a screen, is called ________ image.
Answer:
Real
2. The angle of reflection ________ the angle of incidence.
Answer:
Equals
3. The incident ray, the ________ at the point of incidence and the reflected ray all lie in the same
Answer:
normal
4. The point, where a concave mirror forms the image of a very distant object, is known as the ________ of the mirror.
Answer:
focus
5. A spherical mirror that always forms a virtual, erect and diminished image of an object is a ________ mirror.
Answer:
Convex
6. Eye specialists use opthalmoscope which is fitted with ________ mirror.
Answer:
Concave
B. Write True or False for the following statements.
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.
2. When we stand between a pair of plane mirrors, we see an infinite number of images.
3. A concave mirror never forms a virtual image of an object.
4. We can use either a concave or a convex mirror as a reflector in torches.
5. Motor vehicles generally use either a plane mirror or a convex mirror as the rear view mirror.
Answer:
- 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. (False)
- When we stand between a pair of plane mirrors, we see an infinite number of images. (True)
- A concave mirror never forms a virtual image of an object. (False)
- We can use either a concave or a convex mirror as a reflector in torches. (False)
- Motor vehicles generally use either a plane mirror or a convex mirror as the rear view mirror. (False)
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° the mirror surface
Answer:
(b) Normal at the point of incidence
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.
(h) Now makes an angle of 30° with the plane mirror.
(c) Now makes and angle of 60° with the plane mirror.
(d) Just retraces its path.
Answer:
(d) Just retraces its 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
4. The distance between the object and its image in a plane mirror is 6 m. if the object moves lm towards the mirror, then the distance between object and its image will be
(a) 5m
(b) 6m
(c) 8m
(d) 4m
Answer:
(d) 4m
5. The rear view mirror of a car is 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
D. Answer the following questions in brief.
Question 1.
Explain clearly the difference between a real image and a virtual image. Give one example of a virtual image.
Answer:
A real image can be obtained on a screen, while a virtual image cannot be obtained on a screen. A real image is formed in front of the mirror, while a virtual image is formed behind the mirror. The image formed by a plane mirror is a virtual image.
Question 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:
The size and nature of image formed by a convex mirror does not change with changing distance of the object. In all cases, much smaller, virtual and erect image is formed behind a convex mirror.
Question 3.
Draw a ray diagram to show the formation of an image when the object is placed between the C and
Answer:
Question 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:
When object is kept between F and P, a larger and virtual image is formed by a concave mirror. So, for obtaining a magnified and virtual image we should keep the object between F and P.
Question 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 (b) the type of mirror used?
Answer:
Since image is formed on the same side as the object, so it is a real image. Real images are formed by concave mirrors only, so it is a concave mirror.
Question 6.
Rohit suggested to his father to fix a ‘security mirror’ which would provide them with a wider view of the surrounding 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 must have suggested convex mirror. A convex mirror always makes a smaller, erect and virtual image.
Question 7.
State the type of mirror that the following items/objects would correspond to:
(a) Inside of a stainless steel bowl
(b) Reflector of a torch
(c) A polished shining wooden surface
Answer:
(a) concave mirror,
(b) concave mirror,
(c) plane mirror,
(d) convex mirror
E. Answer the following questions.
Question 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:
For this, take a drawing board, a torch, a white sheet of paper and a thick cardboard. Make a circular piece from the thick cardboard the size of which should be same as the glass in front of the torch. Make a small hole in the centre of the circular piece of cardboard. Fix this at the front of the torch. This will help in getting a thin beam of light. Keep the plane mirror vertically on the white sheet which should be fixed to the drawing board. By throwing the beam of light on plane mirror, find the incident ray, normal and reflected ray. Fold the cardboard from the edge of the drawing board; in a way the folded portion is in a different plane than the rest of the cardboard.
Once again switch on the torch to see if reflected light falls on the folded portion of cardboard or not. The light does not go along the folded portion. This shows that incident ray, normal and the reflected ray; all lie in the same plane.
Question 2.
Trace the path of reflected ray in the following ray diagrams:
Answer:
Question 3.
Draw a ray diagram to show the formation of the image of a point object by
(i) a plane mirror and
(ii) a convex mirror.
Answer:
Question 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:
P cannot see himself in the mirror. He can see the images of Q and R in the mirror.
Question 5.
With the help of a suitable diagram, explain the meaning of the following terms for a concave mirror:
(a) Centre of curvature
(b) Pole
(c) Principal axis
(d) Focus
Answer:
(a) Centre of curvature: The centre of sphere (of which the mirror is a part) is called centre of curvature of the spherical mirror.
(b) Pole: The central point of the spherical mirror is called its pole.
(c) Principal axis: The line joining the pole to centre of curvature is called the pole of the spherical mirror.
(d) Focus: In case of a concave mirror, light rays coming from infinity converge at a point after reflection. In case of a convex mirror, light rays coming from infinity appear to be diverging from a point after reflection. This point is called the focus of the mirror.
Question 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
(b) ‘rear view’ mirror of vehicles
(c) Reflecting type telescope
(d) Shaving mirror
Answer:
(a) Concave mirror, virtual image
(b) Convex mirror, virtual image
(c) Concave mirrors, real image
(d) Concave mirror, virtual image
Value Based Questions
It was celebration time for 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. ‘Yes brother’ said his younger sister. ‘You must behave like the spherical mirror that you would often use. It reflects light to any one whosoever is being examined.’
Question 1.
List at least two other Values’ you may like to add to the suggestion given by the senior Mr. Menon.
Answer:
Honesty and humility
Question 2.
Name the type of spherical mirror used by dentists. Draw an appropriate ray diagram in support of your answer.
Answer:
Dentists use concave mirror which makes an enlarged image of a tooth when it is placed close to a tooth
Something To Do
Question 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:
Letters of English alphabet which makes similar image in a plane mirror are as follows:
H, I, M, O, S, T, U, V, W, X and Y
Question 2.
You must have seen laughing galley jin science centres/parks/fairsj 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:
Do it yourself.
Question 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:
Do it yourself.
Question 4.
Your 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 underlies the principles of reflection.
Answer:
Do it yourself.
DAV Class 7 Science Chapter 12 Solutions – Light
I. Fill in the blanks.
Question 1.
Light is a form of ________
Answer:
Energy
Question 2.
A ________. image cannot be obtained on screen.
Answer:
Virtual
Question 3.
A ________ image can be obtained on screen.
Answer:
Real
Question 4.
The focus of a concave mirror is ________ of the mirror.
Answer:
In front
Question 5.
The focus of a convex mirror is ________ of the mirror.
Answer:
At the back
II. Tick ( ✓) the correct option.
1. In which of the following cases, a magnified image if formed by a concave mirror?
(a) Object at infinity
(b) Object at C
(c) Object between C and F
(d) Object at F
Answer:
(c)Object between C and F
2. In which of the following cases, a magnified image is formed by a convex mirror?
(a) Object at C
(b) Object at F
(c) Object between F and P
(d) Never
Answer:
(d) Never
3. Which type of mirror is used by dentists to see and enlarged image of teeth?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Any type
Answer:
(b) Concave mirror
4. Which type of mirror is used in rear view mirrors of vehicles?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Any type
Answer:
(c) Convex mirror
5. Which type of image is always formed by a plane mirror?
(a) Real image
(b) Virtual image
(c) Both ‘a’ and ‘b’
(d) None of these
Answer:
(c) Both ‘a’ and ‘b’
III. Answer the following questions in brief.
1. What do you understand by reflection of light?
Answer:
When light falls on a shining surface, there is a change in direction of propagation of light. This phenomenon is called reflection of light.
2. Write the laws of reflection.
Answer:
Laws of Reflection:
First Law: The incident ray, the normal at the point of .incidence and the reflected ray; all lie in the same plane.
Second Law: Angle of incidence and angle of reflection are equal.
3. Define concave mirror.
Answer:
When the reflecting surface is on the inside of spherical surface, it is called a concave mirror.
4. Define convex mirror.
Answer:
When the reflecting surface is on the outside of spherical surface, it is called a convex mirror.
5. What is centre of curvature for a spherical mirror?
Answer:
The centre of sphere (of which the mirror is a part) is called centre of curvature of the spherical mirror.
IV. Answer the following questions.
1. What are the various uses of concave mirror?
Answer:
Uses of Concave Mirror:
- As reflector in headlights and flashlights.
- By ENT specialists, to throw a beam of light inside ear.
- By dentists, to see enlarged image of the tooth.
- By hairdresser, to see enlarged image of the face.
2. What are the various uses of convex mirror?
Answer:
Uses of Convex Mirror:
- At hairpin bends, so that a driver can see the vehicles coming from other side of the bend.
- As rear view mirror in vehicles.
3. Convex mirrors are often used as security mirrors in stores. Why?
Answer:
Convex mirror has a wider field of view. Due to this, it can show image of a larger area. This is the reason, convex mirrors are often used as security mirrors in stores.
4. How does changing the shape of the mirror affect the shape and position of an image?
Answer:
Shape of mirror plays a great role in shape and position of an image. In case of a plane mirror, image is formed behind the mirror and is same sized as object. In case of concave mirror, most of the images are in front of the mirror and are smaller than the object. In case of a convex mirror, image is always formed behind the mirror and is smaller than the object.
5. State the type of mirror the following items/objects can act:
(a) Inside of a stainless steel bowl Answer: Concave mirror
(b) Reflector of a torch Answer: Concave mirror
(c) Polished wooden surface Answer: Plane mirror
(d) Shiny surface of the bicycle bell Answer: Convex mirror
V. Projects
1. Use a basket and aluminium foils to make a solar cooker. Observe the location where the utensil should be kept to get the maximum heat in the case of your solar cooker. Discuss with your classmates.
Answer:
Do it yourself.
2. Take two plane mirrors of the same size. Keep them at different angles to each other and note down the number of images of an object formed by these two mirrors.
Answer:
Do it yourself.
3. Set up a light box with a simple slit to produce a single narrow ray of light. Shine this ray of light onto different materials in your room (table, glass, dressing table, walls, almirahs). Notice the type of reflection from these reflected surfaces. Record your observations and discuss the results with your teacher.
Answer:
Do it yourself.
4. Light and heat are both forms of energy. Imagine and try writing a story/play about the life without light and heat.
Answer:
Do it yourself.