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The study of Physics Topics can help us understand and solve real-world problems, from climate change to medical imaging technology.
Some Important Terms Related To Spherical Mirrors
All the mirrors are not straight like plane mirrors. Some of the mirrors are curved mirrors. The common example of a curved mirror is a spherical mirror. A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. The inside surface of a hollow sphere of glass is bent-in’ or ‘concave’ but the outside surface of a hollow sphere of glass is ‘bulging-out’ or ‘convex So the spherical mirrors are of two types:
- Concave mirror, and
- Convex mirror.
We will discuss both the types of spherical mirrors in detail, one by one.
A concave surface means ‘bent-in’ surface. A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface (or bent-in surface). A concave mirror has been shown in Figure(a).
The concave surface of this mirror is on its left side which has been marked A. The reflection of light takes place from this concave surface. The back side of concave mirror has a silver coating which is protected by a layer of red paint. The back side of concave mirror is represented by drawing short, oblique lines.
In Figure(a), the back side of concave mirror has been marked B. Actually, Figure (a) shows a concave mirror as it looks from the side. That is, Figure (a) gives us the side view of a concave mirror. When we look at a concave mirror from the front, it appears like a thin piece of spherical glass whose front surface is shining and bent inward (whereas back surface is covered with a paint and bulging outward). The front view of a concave mirror is shown in Figure (b).
A convex surface means ‘bulging-out’ surface. A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (bulging out surface). A convex mirror is shown in Figure (a).
The convex surface of this mirror is on its left side which has been marked B. The reflection of light takes place from this convex surface. The back side of a convex mirror has a silver coating which is protected by a layer of paint. The back side of convex mirror is represented by drawing short, oblique lines.
In Figure (a), the back side of convex mirror has been marked A. Actually, Figure (a) shows how a convex mirror looks when seen from the side. If we look at a convex mirror from the front, it appears to be a thin piece of spherical glass whose front shining surface is bulging outward (but the back surface covered with paint is bent inward). The front view of a convex mirror is shown in Figure (b).
From the above discussion we conclude that a mirror whose reflecting surface is ‘bent inward’ is a concave mirror whereas a mirror whose reflecting surface is ‘bulging outward’ is a convex mirror. A shining steel spoon represents both, a concave mirror as well as a convex mirror. The front side (or inner side) of a spoon is bent inward, so the front side of a shining spoon represents a concave mirror (see Figure).
The back side (or outer side) of a spoon is bulging outward, so the back side of a shining spoon represents a convex mirror (see Figure). If we look into a shining steel spoon, we will find that the front and back of a spoon give different types of images of our face. This is because the front and back of a shiny spoon represent two different types of mirrors (concave mirror and convex mirror). Please note that just like plane mirror, the spherical mirrors (like concave mirror and convex mirror) also work on the reflection of light from their surfaces.
Why are Concave and Convex Mirrors Called Spherical Mirrors
Concave mirror and convex mirror are called spherical mirrors because they can be considered to be small parts of a hollow sphere of glass. This will become more clear from the following activity.
Activity 2
In this activity we will give the example of a ‘hollow sphere of rubber’ which is a ‘hollow rubber ball’ because it is easy to cut a rubber ball than a glass ball. Take a hollow rubber ball and cut a small portion of it with a knife very carefully (see Figure).
The ‘inner surface’ of the small part of the cut ball is ‘concave Such a surface makes a concave mirror. The ‘outer surface’ of the small part of the cut ball is convex Such a surface makes a convex mirror. This activity shows how the real concave mirror and convex mirror can be considered to be small pieces taken from large, hollow sphere of glass.
Concave Mirror is a Converging Mirror
Light rays coming from a distant object are parallel to one another. Suppose a beam of parallel rays of light falls on a concave mirror from the left side (as shown in Figure). We will see that after reflection from the concave mirror, the parallel rays of light are getting closer together (or coming together).
When the rays of light come closer, we say that the rays of light are converging. The concave mirror reflects the parallel rays of light in such a way that, after reflection, all the rays converge (or meet) at one point called ‘focus in front of the mirror (see Figure).
The focus of a concave mirror is a point in front of the mirror at which parallel rays of light converge after reflection from the concave mirror. Focus (also called principal focus) is denoted by the letter F. The distance from the focus up to the concave mirror is called focal length of concave mirror (see Figure).
The focal length is denoted by the letter f The focus of a concave mirror is in front of it. The focus of a concave mirror is ‘real’ because light rays actually pass through it, Since a concave mirror converges a beam of parallel light rays, therefore, a concave mirror is also known as a converging mirror.
Convex Mirror is a Diverging Mirror
Suppose a beam of parallel rays of light falls on a convex mirror from the left side (as shown in Figure). We will see that after reflection from convex mirror, the parallel rays of light are spreading out (or going away from one another). When the parallel rays of light spread out, we say that the rays of light are diverging. We can now say that a beam of parallel light rays diverges (spreads out) after reflection from a convex mirror (see Figure).
Since the parallel rays of light diverge (spread out), they cannot meet at a point in front of the convex mirror. To a person looking into the convex mirror, the reflected rays appear to be coming (or spreading) from a single point F behind the convex mirror (see Figure). The point F is called focus of convex mirror.
The focus of a convex mirror is a point behind the mirror from which the parallel rays of light falling on convex mirror appear to diverge after reflection from convex mirror. The distance from focus up to the convex mirror is called focal length of convex mirror (see Figure). The focus of a convex mirror is behind it.
The focus of a convex mirror is ‘virtual’ because light rays do not actually pass through it. Since a convex mirror diverges a parallel beam of light rays, therefore, a convex mirror is known as a diverging mirror.