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Many modern technologies, such as computers and smartphones, are built on the principles of Physics Topics such as quantum mechanics and information theory.
What are the Types of Spherical Lenses
A lens is a piece of transparent glass bound by two spherical surfaces. Lenses play a very important role in our everyday life. The magnifying glass which is used to see very small things clearly is a type of lens. Lenses are used in making spectacles, cameras, microscopes, telescopes, film projectors and binoculars, etc. We have already studied spherical mirrors (like concave mirror and convex mirror).
Mirrors are not transparent, so light rays cannot pass through mirrors. Mirrors work on the reflection of light from their surface. On the other hand, lenses are transparent, so light can pass through lenses. When light rays pass through a lens, they refract (or bend) and change the direction. So, lenses work on the refraction of light passing through them. Lenses are of two types :
- Convex lens, and
- Concave lens.
We will now discuss both the types of spherical lenses in detail, one by one.
The lens which is thicker in the middle than at the edges, is called a convex lens [see Figure Convex (a)], In other words, a convex lens is thick in the middle but thinner at the edges. Actually, a convex lens is a spherical lens whose two surfaces are convex (or bulging out). Figure (a) shows a convex lens in which the two surfaces A and B are ‘convex’ or ‘bulging out’ in the middle (which makes it thicker in the middle).
Figure (a) shows a convex lens as it looks from the side. That is, Figure (a) gives us the side view of a convex lens. When we look at a convex lens from the front, it looks like a piece of transparent spherical glass (round glass) [see Figure (b)], We can feel the bulge in the middle of a convex lens by touching it.
The lens which is thinner in the middle than at the edges is called a concave lens [see Figure (a)]. In other words, a concave lens is thin in the middle but thicker at the edges. Actually, a concave lens is a spherical lens whose two surfaces are ‘concave’ (or bent inward). Figure (a) shows a concave lens in which the two surfaces C and D are ‘concave’ or ‘bent inward’ in the middle.
Figure (a) shows a concave lens as it looks from the side. That is, Figure (a) gives us the side view of a concave lens. When we look at a concave lens from the front, it looks like a piece of transparent spherical glass (round glass) having a ‘depression’ at the centre [see Figure (b)], We can feel the ‘depression’ at the centre of a concave lens by touching it.
Please note that depending on our convenience, at some places in the book we will use the side view of convex lens and concave lens in our diagrams whereas at other places we will use their front views. Another point to be noted is that lenses are usually made of glass and they are transparent. We can look through the lenses.
Convex Lens is a Converging Lens
Suppose a beam of parallel rays of light falls on a convex lens from the left side (as shown in Figure). We will see that after passing through the convex lens, the beam of parallel rays of light converges to one point called ‘focus’ on the other side of convex lens (see Figure). The focus of a convex lens is a point where parallel rays of light converge after passing through the convex lens.
In Figure, point F is the focus of convex lens. Please note that a convex lens has real focus because light rays actually pass through it. Actually, a convex lens has two foci, one on either side, at the same distance from the centre of the lens (the word ‘foci’ is the plural of focus’).
The distance from focus up to the centre of convex lens is called focal length of convex lens (see Figure). Since a convex lens converges light rays falling on it, therefore, it is also called a converging lens. Thus, a convex lens is a converging lens. In this respect, a convex lens is similar to a concave mirror. This is because a concave mirror also converges rays of light.
Concave Lens is a Diverging Lens
Suppose a beam of parallel rays of light falls on a concave lens from the left side (as shown in Figure). We will see that after passing through the concave lens, the rays of light are diverging (or spreading out) (see Figure). Since the parallel rays of light diverge (spread out), they cannot meet at a point on the right side of the concave lens.
To a person looking into the concave lens from the right side, the refracted rays of light appear to be coming (or spreading) from a single point F on the left side of the concave lens (see Figure).
The point F is the focus of concave lens. The focus of a concave lens is a point from which the parallel rays of light appear to diverge after passing through the concave lens. PJease note that a concave lens has virtual focus because light rays do not actually pass through it.
Actually, a concave lens has two foci, one on either side, at equal distances from the centre of the lens. The distance from focus up to the centre of concave lens is called focal length of concave lens (see Figure). Since a concave lens diverges light rays falling on it, therefore, it is also called a diverging lens.
Thus, a concave lens is a diverging lens. In this respect, a concave lens is similar to a convex mirror. This is because a convex mirror also diverges rays of light.