Spherical Mirrors – Types and Uses

Spherical Mirrors 

 Key Concepts

• Spherical mirrors: Concave and convex
• Image formation by concave and convex mirrors
• Uses of concave and convex mirrors

Introduction

It is known that the plane mirrors form virtual and upright images of an object placed in front of them. Plane mirrors have a flat reflecting surface. On the other hand, surfaces of stainless steel spoon and bowl form an image of an object placed in front of them by reflecting most of the light falling on them. This is because of the smooth nature of their reflecting surface. These surfaces are therefore called curved reflecting surfaces. The most widely used curved reflecting surfaces are spherical mirrors. In this section we will be looking at the spherical mirrors, the kind of images they form at different situations and their uses. 

Explanation

Spherical mirrors: 

The most common curved reflecting surface which is widely used are spherical mirrors. A spherical mirror is usually made by cutting out a piece from a hollow spherical glass structure. One of the surfaces of this piece is then silvered. The inner surface of the piece is called the concave surface and the outer bulged out surface is called the convex surface as shown in the figure below. 

If the outer convex surface is silvered, the inner concave surface is capable of reflecting light. This mirror in which the concave surface is the reflecting surface is called a concave mirror. 

If the inner concave surface is silvered, the outer convex surface is capable of reflecting light. This mirror in which the convex surface is the reflecting surface is called a convex mirror. 

The convex surface of a concave mirror, and the concave surface of a convex mirror are rough surfaces. 

A concave mirror converges a parallel beam of light to a point. In Fig 7.3 the image of the Sun is formed at that point itself. Thus, the image of the Sun is formed by the actual meeting of the reflected rays and is also formed on a screen (paper). Hence, it is a real image. As the Sun’s rays get concentrated at this point, it might give rise to a fire if held steady for a while. 

Image formation by a concave mirror: 

A setup as shown in the figure below using a concave mirror, a lighted candle and a sheet of paper (screen) can be used to study the image formation by a concave mirror. When the lighted candle is placed at some distance and the screen is moved to and fro, a sharp image of the flame is seen on it. 

The properties of the image formed are as follows: 

The image is formed on the screen. This means it is a real image. 

It is also upside down. 

The image is almost the same size as the flame. 

Thus, the image is real and inverted in nature. 

When the candle is moved a little towards the mirror, the screen needs to be moved and adjusted to get a sharp image of the flame. Again, the image is formed on a screen and is also upside down. However, this time it is a lot larger in size as compared to the candle. Thus, the image is real, inverted and magnified. 

When the candle is moved even nearer to the mirror so that it gets too close, its image is no longer obtained on the screen. It can only be seen in the mirror. This time the image is not formed on the screen, is not upside down and is larger than the candle. Thus, the nature of the image is virtual, upright and magnified. Therefore, the concave mirror forms many types of images, such as real and virtual, smaller, larger and the same size as the object. 

Uses of a concave mirror: 

When the object lies too close to a concave mirror, it forms a virtual, upright and magnified image of the object.  

This property of concave mirrors is used in 

1. Shaving mirrors 

2. Makeup mirrors 

3. Dental mirrors. 

As all the above mirrors are required to produce a magnified image of the person in front of it, concave mirrors are used. 

A concave mirror can also direct the light from a source (spreading in all directions) in one direction by reflecting it.  

Because of this property, concave mirrors are used in  

1. Headlights of vehicles 

2. Torches 

Image formation by a convex mirror: 

A setup as shown in the figure below using a convex mirror , a lighted candle and a sheet of paper (screen) can be used to study the image formation by a convex mirror. When the lighted candle is placed at some distance and the screen is moved to and fro, no image is seen on it at any position.  However, the image is seen in the mirror as if it is formed behind the mirror, like that of a plane mirror. This image is not formed on the screen, is not upside down and is smaller in size than the candle. Thus, the nature of the image is virtual, upright and diminished.  

When the candle is moved closer to the mirror its image becomes smaller and smaller. It also continues to be virtual and upright. Therefore, the image formed by a convex mirror is always virtual, upright and diminished, no matter where the object is located. 

Uses of a convex mirror: 

A convex mirror covers a wide area in the image it forms. Because of this property, it is used as: 

1. Rear-view mirror in the vehicles to see the vehicles approaching from behind. 

2. Shop security mirror to watch the activities of the customers. 

3. Road safety mirror at turns on the roads, for a driver to watch the vehicles approaching from the other side.  

Summary

1. Curved reflecting surfaces such as the inner and outer surfaces of a stainless
   steel spoon and bowl can produce an image of an object placed in front of them.
2. The most widely used curved reflecting surfaces are spherical mirrors, which are
   usually made by cutting out a piece from a hollow spherical glass structure and silvering one of its surfaces.
3. A concave mirror is a spherical mirror whose inner surface is the reflecting
   surface.
4. A convex mirror is a spherical mirror whose outer surface is the reflecting
   surface.
5. A concave mirror forms many types of images, such as real, virtual, inverted,
   upright, diminished, magnified and the same size as the object.
6. A convex mirror only forms virtual, upright and diminished image of an object.
7. Concave mirrors are used as shaving, makeup and dental mirrors as they can
   produce a virtual, upright and magnified image of an object placed near to them.
8. Concave mirrors are also used as reflectors in head lights of vehicles and torches.
9. Convex mirrors are used as rear-view, road safety and security mirrors in shops
   as they can produce a virtual, upright and diminished image of a wide area.