NCERT Exemplar Solutions: Light

Multiple Choice Questions

Q1. 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.

Plane Mirror, Concave Mirror and Convex Mirror

Q2. Which of the following can be used to form a real image?
(a) Concave mirror only.
(b) Plane mirror only.
(c) Convex mirror only.
(d) Both concave and convex mirrors
Ans: (a)
Explanation:
A concave mirror can form a real image when the object is placed beyond its focus; the reflected rays meet in front of the mirror and the real image can be formed on a screen. A plane mirror and a convex mirror always form virtual images (rays appear to diverge from behind the mirror) and therefore cannot produce a real image on a screen.

Q3. 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
Ans: (b)
Explanation:
In a plane mirror the image is formed as far behind the mirror as the object is in front of it. If the object is 0.5 m in front of the mirror, the image is 0.5 m behind it. Thus the total distance between object and image = 0.5 m + 0.5 m = 1 m.

Q4. 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.
Ans: (b)
Explanation:
A concave mirror can produce an enlarged image when the object is placed between the focus (F) and the centre of curvature (2F) or between F and the mirror (for virtual enlarged image). A convex lens (converging lens) produces an enlarged image when the object is placed between its focus and 2F (or closer than 2F but beyond F). A convex mirror and a concave lens normally produce diminished images.

Q5. 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.
Ans: (b)
Explanation:
A rainbow is formed by dispersion of sunlight by water droplets in the atmosphere. For a rainbow to be visible, the sun must be behind the observer so that sunlight falls on the raindrops in front of them and the dispersed colours reach the eye. Rainbows are commonly seen when the Sun is low in the sky, such as soon after rain.

Q6. 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
Ans: (b)
Explanation:
A concave mirror can form a virtual, erect and enlarged image when the object is placed between the mirror and its focal point (F). A convex mirror always gives erect but diminished images. A plane mirror gives erect images of the same size.

Q7. 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 the 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.
Ans: (c)
Explanation:
An inverted (real) image can be produced by a concave mirror (when the object is beyond the focus) and by a convex lens (when the object is placed beyond the focal length). A convex mirror and a concave lens produce only virtual and erect images.

Q8. 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 the object.
(b) concave lens provided the image is smaller than the object.
(c) concave lens provided the image is larger than the object.
(d) concave lens provided the image is of the same size.
Ans: (b)
Explanation:
A concave lens always produces a virtual, erect and diminished image of an object placed anywhere in front of it. A convex lens produces an erect virtual image only when the object is placed between the lens and its focal point; in that case the virtual image is usually larger (magnified), not smaller.

Very Short Answer Type Questions

Q1. 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 the nature of the lens.
Ans: The lens is a concave lens.

Q2. Fill in the blanks:
(a) The inner surface of a steel spoon acts as a ___________ mirror. (b) The outer surface of a flat steel plate acts as a ___________ mirror. (c) The outer shining surface of a round bottom steel bowl acts as a __________ mirror. (d) The inner surface of the reflector of a torch acts as a __________ mirror.
Ans:
(a) The inner surface of a steel spoon acts as a concave mirror.
(b) The outer surface of a flat steel plate acts as a plane mirror.
(c) The outer shining surface of a round bottom steel bowl acts as a convex mirror.
(d) The inner surface of the reflector of a torch acts as a concave mirror.

Q3. State whether the following statements 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.
Ans:

• (a) False - A concave lens produces a diminished (smaller) and erect image, not an enlarged one.
• (b) False - A convex lens can produce either a real image (when the object is outside the focal length) or a virtual erect image (when the object is inside the focal length). So it does not always produce a real image.
• (c) True - Reflection from a mirror causes lateral inversion, so the left and right sides of the object and its image are interchanged in the image formed by a mirror such as a concave mirror.
• (d) False - An object can be seen if it reflects light falling on it; it need not emit light of its own.

Short Answer Type Questions

Q1. What type of mirror is used as a side mirror in a scooter? Why is this type of mirror chosen?
Ans: A convex mirror is used as a side mirror in a scooter. This mirror is chosen because it gives a wider field of view by forming smaller (diminished), erect images of objects behind the scooter, allowing the rider to see more area and traffic behind.

Q2. Observe the figures given in Figure 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 B?
Ans:

• A: Convex lens - the rays converge after passing through the lens.
• B: Concave lens - the rays diverge after passing through the lens.

Q3. 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.
Ans: No. A laser torch emits light of a single colour (single wavelength), so when this monochromatic light passes through a prism it does not split into different colours. A prism disperses white light (which contains many colours) into a band of colours, but it will not produce a rainbow from single-colour laser light.

Q4. State the correct sequence (1-7) of colours in the spectrum formed by the prisms A and B, shown in Figure.

Ans:

Q5. 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?
Ans: If a plane mirror is used instead of a convex mirror, the driver will get a narrower field of view. A plane mirror gives an image of the same size and does not show as wide an area behind the scooter as a convex mirror does. This reduces the driver's ability to see vehicles approaching from the sides.

Q6. The concave reflecting surface of a torch got rusted. What effect would this have on the beam of light from the torch?
Ans: If the reflector inside a torch becomes rusted or rough, it will not reflect light in a well-directed manner. Instead of forming a strong, focused beam, the light will be scattered or diffused, and the beam will become weaker and less focused.

Q7. An erect and enlarged image of an object is formed on a screen. Explain how this could be possible.

Ans: This is not possible. Any image formed on a screen must be a real image, and real images produced by mirrors or lenses are always inverted. An erect and enlarged image is usually a virtual image (for example, when an object is between the focus and a concave mirror or a convex lens), and virtual images cannot be obtained on a screen.

Q8. Two different types of lenses are placed on a sheet of newspaper. How will you identify them without touching?

Ans:  Hold each lens over printed letters on the newspaper:
- If the letters appear magnified (larger), the lens is a convex lens.
- If the letters appear diminished (smaller), the lens is a concave lens.

Q9. 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.
Ans: The shopkeeper should fix a convex mirror. A convex mirror produces diminished but erect images and therefore shows a wider area. This allows the shopkeeper to see more of the shop in a single mirror.

Q10. 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?
Ans: The object is moving away from the lens. For a convex lens producing a real image on a screen, when the object moves farther from the lens (towards infinity), the image moves closer to the lens and its size decreases.

Long Answer Type Questions

Q1. Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it.
Ans: Two ways to obtain a real image of a distant tree are:
1. Using a concave mirror and a screen: A concave mirror reflects parallel rays from a distant tree to its focus and beyond, forming a real, inverted image in front of the mirror. Place a screen at the point where rays meet to catch the image.
2. Using a convex (converging) lens and a screen: A convex lens forms a real image of a distant object close to its focal plane. By placing a screen at the position where the lens forms the image, the real and inverted image of the distant tree can be obtained on the screen.

Q2. 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.
Ans: The lens is a convex lens. When the object moves closer to a convex lens (but remains outside the focal length), the image formed on the other side grows larger. If the object is moved closer than the focal length (i.e., inside the focus), the lens produces a virtual, erect and enlarged image which cannot be projected on a screen. So, you will not be able to obtain that virtual image on a screen.

Q3. You are given three mirrors of different types. How will you identify each one of them?
Ans: Identify the mirrors by observing the nature of the image formed of a test object (for example, a candle or printed letters):

• Plane mirror: The image is virtual, erect and of the same size as the object and appears as far behind the mirror as the object is in front.
• Concave mirror: The image may be real or virtual, inverted or erect, and magnified or diminished depending on the object's position relative to the focus (F) and centre of curvature (2F). For example, if the object is far away, the image is real and inverted; if it is between the mirror and the focus, the image is virtual, erect and magnified.
• Convex mirror: The image is always virtual, erect and diminished irrespective of the object position. It provides a wide field of view.