Class 7 Science Chapter 15: Light - NCERT Solutions

(a) Virtual image
Virtual image cannot be obtain on screen. A virtual image can be seen only by looking into a mirror. The image of our body formed by a plane mirror cannot be received on a screen, it can be seen only looking into the mirror.

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

(c) Plane
Plane mirror always form an image which is exact to the size of the object.

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

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

(a) False
This statement is false because a convex mirror always forms an virtual and an erect image.

(b) True
The concave lens always forms a virtual, erect and diminished image as we can observe in the ray diagram which is provided below concave lens spreads out light rays that are than refracted through it.

(c) True
We can obtain a real, enlarged and inverted image by a concave mirror.

(d) False
A real image is always obtain on a screen.

(e) False
Concave mirror can form both real and virtual images.

Column I                                          Column II

(a) A plane mirror                              (v) The image is erect and of the same size as the object.
(b) A convex mirror                            (ii) Can form image of objects spreads over a large area.
(c) A convex lens                               (i) Used as a magnifying glass.
(d) A concave mirror                          (iii) Used by dentists to see enlarged image of teeth.
(e) A concave lens                             (vi) The image is erect and smaller in size than the object.

1. A plane mirror: A plane mirror have a flat reflective surface. The angle of reflection is always equals to the angle of incidence for the light rays which strikes the plane mirror. The angle formed by the incident ray and surface of normal is called the angle of incidence. 

2. A convex mirror: A convex mirror is a diverging mirror in which the reflective surface bulges towards the light Source. The image formed by convex mirrors are smaller than the object but gets larger as they approach.

3. Concave mirror: A concave mirror has a reflective surface that is curved inwards and away from the light source. Concave mirrors are widely used in headlights of auto mobiles motor vehicles, torch lights etc.

4. Convex lens: The convex lens is a lens that converges rays of light that convey parallel to its principal axis which is relatively thick cross the middle and thin at the lower and upper edges. A convex lense in thinner in the middle and thicker towards the edges.

5. Concave lens: A concave lens is a lens that possesses at least one surface that curves inwards. It is a diverging lens, meaning that it spreads out light rays that have been refracted through it. A concave lens is thinner at its edges, and is used to correct short sightedness.

Characteristics of the images formed by a plane mirror:

1. Plane mirror form the image behind the mirror.
2. Plane mirror always makes virtual images.
3. Plane mirror make the exact size images as the size of the object.
4. Plane mirror always forms an erect image.
5. The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of it.
6. The image in a plane mirror is laterally inverted with respect to the object.

A, H, I, M, O, T, U, W and X are the letters that form the same image as the letters is appear same when seen through a plane mirror. These letters are literally symmetrical.

Virtual Image: Virtual image cannot be obtain on screen. A virtual image can be seen only by looking into a mirror. The image of our body formed by a plane mirror cannot be received on a screen, it can be seen only looking into the mirror. The virtual image is just an illusion. So, virtual images are also called unreal images. A virtual image is formed when light rays coming from an object appear to meet after reflection from the mirror. It is possible to obtain a virtual image on the screen because light rays actually do not pass through it. A plane mirror always forms virtual images. Highly polished metal objects also forms virtual images.

We cannot find any image on the white screen which is placed behind the mirror, when an object is placed very close to the concave mirror. A virtual image is formed when reflected rays appear to meet.

 Such images cannot be obtained on screen.
 Plane mirrors, convex mirror and concave lens always forms an virtual image.

Difference between concave lens and convex lens are following:

                                                                                                                                                                                                                    Concave lens

Uses of Convex mirror:
1. Convex mirror are used as rear view mirrors or side view mirrors in vehicles (such as cars, scooters, buses and trucks etc) to see the traffic at the rear (back side) on the road.
2. Big convex mirrors are used as ‘shop security mirrors’.

Uses of Concave mirror:
1. Concave mirrors are used as shaving mirrors to see a large image of the face.
2. Concave mirrors are used by dentist to see the large images of teeth of patients.
3. Doctors use concave mirror reflectors to produce a parallel beam of light for examining the body parts such as eyes, ears, nose and throat.
4. Concave mirrors are used as reflectors in torches, headlights of vehicles (such as car, scooters, buses, trucks, train engines etc) and searchlights to get a strong, straight beam of light.

Concave mirror can form a real image.

A concave mirror or converging mirror contains reflecting surface which is away from the incident ray. Concave mirrors reflect light inwards to one focal point. They are used to focus light. A concave mirror has a reflective surface that is curved inward and away from the light source. Concave mirrors reflect light inward to one focal point. Concave mirror shows different types of images which is depend on the distance between the object and the mirror.

Virtual image is formed by the concave lens because in concave lens the refracted ray never converge. If the rays are not refracted then a virtual image is formed. Concave lens is used to correct short sightedness.

(ii) Concave mirror

A virtual image is larger than the object can be produced by a concave mirror. Concave mirror gives virtual and magnified image of the object. Plane mirror also forms virtual images but the images are same in size as the object.

(iii) 6 m

Initially David is standing in front of the mirror.
The distance between him and his image = 4 m
David moves 1 m towards the mirror
The distance of David from the mirror will be 3 m
Distance of the image from the mirror = 3 m

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

(iii) 4 m/s

The car is approaching the truck with the speed of 2 m/s which shows by the speed of the car 2 m/s. The distance between both of them decreases by double. This is because the truck's image will travel a distance twice the distance covered by the car in same time. Hence, the image of the truck will appear to approach the driver with the speed of: 2 m/s + 2 m/s = 4 m/s.