NCERT Exemplar Problems Class 7 Science – Light

Question 1:
Boojho and Paheli were given one mirror each by their teacher. Boojho found his image to be erect and of the same size, whereas Paheli found her image erect and smaller in size. This means that the mirrors of Boojho and Paheli are, respectively
(a) plane mirror and concave mirror
(b) concave mirror and convex mirror
(c) plane mirror and convex mirror
(d) convex mirror and plane mirror
Answer:
(c) As in case of plane mirror, image formed is virtual, erect and of same size and in case of convex mirror, image is virtual, erect and smaller in size.

Question 2:
Which of the following can be used to form a real image?
(a) Only concave mirror
(b) Only plane mirror
(c) Only convex mirror
(d) Both concave and convex mirrors
Answer:
(a) As only a concave mirror can form a real image. Plane mirror and convex mirror always form a virtual image of an object.

Question 3:
If an object is placed at a distance of 0.5 m in front of a plane mirror, the distance between the object and the image formed by the mirror will be
(a) 2 m
(b) 1 m
(c) 0.5 m
(d) 0.25 m
Answer:
(b) The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of it. Therefore, the distance between object and image is given by
distance between object and mirror + distance between mirror and image
= 0.5 m+ 0.5 m = 1 m

Question 4:
You are provided with a concave mirror, a convex mirror, a concave lens and a convex lens. To obtain an enlarged image of an object, you can use either
(a) concave mirror or convex mirror
(b) concave mirror or convex lens
(c) concave mirror or concave lens
(d) concave lens or convex lens
Answer:
(b) Concave mirror and convex lens can only form enlarged image of an object. As convex mirror and concave lens always form a diminished image of an object.

Question 5:
A rainbow can be seen in the sky
(a) when the sun is in front of you
(b) when the sun is behind you
(c) when the sun is overhead
(d) only at the time of sunrise
Answer:
(b) A rainbow can only be seen in the sky when the sun is behind you in rainy season.

Question 6:
An erect and enlarged image can be formed by
(a) only a convex mirror
(b) only a concave mirror
(c) only a plane mirror
(d) both convex and concave mirrors
Answer:
(b) An erect and enlarged image can only be formed by a concave mirror and it is virtual in nature.

Question 7:
You are provided with a convex mirror, a concave mirror, a convex Lens and a concave lens. You can get an inverted image from
(a) both concave lens and convex lens
(b) both concave mirror and convex mirror
(c) both concave mirror and convex lens
(d) both convex mirror and concave lens
Answer:
(c) Concave mirror and convex lens can only form real and inverted image of an object.
As convex mirror and concave lens always form a virtual and erect image of an object.

Question 8:
An image formed by a lens is erect. Such an image could be formed by a
(a) convex lens provided the image is smaller than object
(b) concave lens provided the image is smaller than object
(c) concave lens provided the image is larger than object
(d) concave lens provided the image is of the same size
Answer:
(b) A concave lens can only form virtual, erect and diminished image of an object. As virtual and erect image formed by a convex lens is always magnified.

Very Short Answer Type Questions

Question 9:
The image formed by a lens is always virtual, erect and smaller in size for an object kept at different positions in front of it. Identify nature of lens.
Answer:
Such types of lens which always forms virtual, erect and smaller image in spite of the different positions of an object is called concave lens.

Question 10:
Fill in the blanks.
(a) The inner surface of a steel spoon acts as a…………….
(b)The outer surface of a flat steel plate acts as a……………….
(c)The outer shining surface of a round bottom steel bowl acts as a
(d)The inner surface of the reflector of a torch acts as a…………..
Answer:
(a) Concave It acts as a concave mirror because it forms real and virtual inverted and erect, magnified and diminished images depending upon the position of an object.
(b) Plane It acts as a plane mirror because it forms virtual, erect and same sized image of object.
(c) Convex It acts as a convex mirror because it forms virtual, erect and diminished image in spite of the position of an object.
(d) Concave It acts as a concave mirror to obtain a parallel beam of light.

Question 11:
State whether the following statement are True or False.
(a) A concave lens can be used to produce an enlarged and erect image.
(b) A convex lens always produces a real image.
(c)The sides of an object and its image formed by a concave mirror are always interchanged.
(d)An object can be seen only if it emits light.
Answer:
(a) False, a concave lens can only form virtual, erect and diminished image in spite of the position of object.
(b) False, a convex lens can form real or virtual, inverted or erect magnified and diminished images depending upon the position of object.
(c) True, the image formed by a concave mirror shows lateral inversion, i.e. left seems to be right and right seems to be left.
(d) False, an object can be seen if it reflects the light falling on it.

Short Answer Type Questions

Question 12:
What type of mirror is used as a side mirror in a scooter? Why is this type of mirror chosen?
Answer:
Convex mirror is used as a side mirror in a scooter because it can form diminished images of objects spread over a larger area.
So, this helps the driver to have a wide field of view of traffic

Question 13:
Observe the given figures carefully.

The given figures show the path of light through lenses of two different types represented by rectangular boxes A and B. What is the nature of lenses A and 6?
Answer:
Since, in first case, light rays are converging towards a point, so the lens A will be a convex lens and in case of lens S, light rays diverge or spread out. So, the lens B will be a concave lens.

Question 14:
Boojho made light from a laser torch to fall on a prism. Will he be able to observe a band of seven colours? Explain with a reason.
Answer:
No, he will not observe band of seven colours because laser light consists of monochromatic light of a single colour.
However, seven colours of bands are observed, when a white light is passed through a prism and dispersion takes place.

Question 15:
State the correct sequence (1-7) of colours in the spectrum formed by the prisms A and B shown in the figure

Answer:
When a white light is passed through a prism, it disperses into its seven constituent colours.

Question 16:
The side mirror of a scooter got broken. The mechanic replaced it with a plane mirror. Mention any inconvenience that the driver of the scooter will face while using it.
Answer:
As we know that the side mirror of a scooter must be a convex mirror so that we can view a wide range of traffic spread over a large area.
But if plane mirror, is used, we will not be able to see large area of traffic behind us which may create a difficulty in driving vehicle and can cause accident.

Question 17:
The concave reflecting surface of a torch got rusted. What effect would this have on the beam of light from the torch?
Answer:
If the concave reflecting surface of a torch get rusted, it will produce diffused beam of light with lower intensity, the objects will not be clearly visible in this diffused and lower intensity of light.

Question 18:
An erect and enlarged image of an object is formed on a screen. Explain how this could be possible.
Answer:
An erect and enlarged image of an object can be formed by either concave mirror or convex lens if an object is placed upside down between the focus and centre of curvature of concave mirror or convex lens.

Question 19:
Two different types of lenses are placed on a sheet of newspaper. How will you identify them without touching?
Answer:
On identifying the size of the letters of newspaper, we can differentiate between the two types of lenses.
If image of letters is large or magnified, then the lens is a convex lens and if the image of letters is smaller or diminished in size, then the lens is a concave lens.

Question 20:
A shopkeeper wanted to fix a mirror which will give a maximum view of his shop. What type of mirror should he use? Give reason.
Answer:
If a shopkeeper wanted to fix a mirror which will give him maximum view of his shop, he should use a convex mirror.
As a convex mirror can form images of objects spread over a large area, i.e. it can give a wider field of view.

Question 21:
The distance between an object and a convex lens is changing. It is noticed that the size of the image formed on a screen is decreasing. Is the object moving in a direction towards the lens or away from it?
Answer:
In case of convex lens, when we move the object away from the lens, the size of image decreases and ultimately, when object is at infinite distance a point image is formed at the focus of lens.

Long Answer Type Questions

Question 22:
Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it.
Answer:
(i) In first case, we will use a concave mirror and a screen. As we know that a concave mirror forms a real image of a distant object at its focus, therefore, a real image of a distance tree will be formed on the screen placed at the focus of concave mirror.
(ii) In second case, we will use a convex lens and a screen. As convex lens forms a real image of a distant object at its focus, therefore, a real image of a distant tree will be formed on the screen placed at the focus of convex lens.

Question 23:
It was observed that when the distance between an object and a lens decreases, the size of the image increases. What is the nature of this lens? If you keep on decreasing the distance between the object and the lens, will you still able to obtain the image on the screen? Explain.
Answer:
If on decreasing the distance between the object and lens, the size of the image increases, then the nature of the lens is convex type.
No, if the distance between object and lens is less than the focal length of the lens, then it will form a virtual image which cannot be obtained on a screen.

Question 24:
You are given three mirrors of different types. How will you identify each one of them?
Answer:
We can identify the mirrors by forming images of an object which are given as below:
(i) Plane mirror In case of plane mirror, image will be virtual, erect and of same size as that of object.
(ii) Concave mirror In case of concave mirror, image may be real or virtual, inverted or erect and magnified or diminished depending upon the position of object.
(iii) Convex mirror In case of convex mirror, image formed will always be virtual, erect and diminished in spite of the position of object.