NCERT Solutions For Class 7 Science Chapter 15 Light

Correct option is (iii).

Explanation:

Speed of truck in the mirror = 2v

where v is original speed.

Speed of truck in the mirror = 2 × 2 m/s = 4 m/s

Correct option is (iii).

Explanation:

Distance between the mirror and David’s image = 4 m

If David moves 1 m towards the mirror, then his distance from the mirror = 4 m − 1 m = 3 m

Therefore, the distance between David and his image  = 3 m + 3 m = 6 m

Correct option is (ii).

Explanation:

When the object is placed very close to the concave mirror, the image formed is virtual, erect and magnified.

Concave lens always forms a virtual image.

Concave mirror can form a real image.

Use of concave mirror:

As concave mirror produces large magnified image, it is used by the doctors to examine eye, ears, nose and throat etc.

Use of convex mirror:

Convex mirror forms small and virtual image due to which it is used as the side rear view mirror in vehicles to see the large area for a better view of traffic on road.

The image which cannot be obtained on a screen is called a virtual image. When we see ourselves in a plane mirror, the image formed cannot be obtained on a screen. It is one of the situations where a virtual image is formed.

The characteristics of the image formed by a plane mirror are as follows:

1. Image is erect and virtual.
2. Image is formed at the same distance behind the mirror as the object is placed in front of the mirror.
3. Image is of the same size as that of the object.
4. Image is laterally inverted.

The two differences between a convex and a concave lens are as follows:

1. A convex lens forms a real and inverted image while a concave lens forms a virtual and erect image.
2. A convex lens can also form a virtual and erect image when object is placed very close to it whereas, a concave lens cannot form a real and inverted image at all.

The letters of English alphabets, whose image formed by the plane mirror is exactly like the letter itself, are as follows:

A, H, I, M, O, T, U, V, W, X and Y

All of these letters can be divided into two equal halves through a vertical plane. Therefore, even their images after undergoing lateral inversion would appear same as that of letter itself. Thus, we can say that all vertically symmetrical objects will produce image same as themselves when placed in front of a plane mirror.

The correctly matched contents of Column I and Column II are as follows:

(a) False

(b) True

(c) True

(d) False

(e) False

(a) An image that cannot be obtained on a screen is called virtual image.

(b) Image formed by a convex mirror is always virtual and smaller in size.

(c) An image formed by a plane mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a real image.

(e) An image formed by a concave lens cannot be obtained on a screen.