Types of Spherical Lenses and Their Uses

Introduction:

A magnifying glass is used to see a magnified image of a small print. It is a type of lens. Lenses are transparent objects made of glass or any other transparent material. They are widely used in spectacles, microscopes, and telescopes as they are capable of forming various kinds of images of an object placed in front of them. In this section, we shall cover the types and uses of spherical lenses and image formation by them.

Explanation:

Spherical Lenses and Their Uses:

Spherical lenses are transparent objects made of transparent materials such as glass or clear plastic. They are widely used in spectacles, microscopes, and telescopes.

Spherical lenses are transparent in nature, as they are made up of glass or clear plastic. Hence, they refract light as light can pass through them completely. Light undergoes a change in speed on entering a lens and again on emerging out of it.

Spherical lenses are used in:

1. Telescopes to see distant celestial bodies such as other planets and stars.
2. Microscopes show a highly magnified image of microscopic objects such as microorganisms.
3. Spectacles to correct the vision of people with various eye defects.
4. Peepholes to view the person standing in front of the door.

Fig 5.1: Uses of spherical lenses (Telescope, microscope, spectacles, and peephole)

Types of Spherical Lenses:

Spherical lenses are basically of two types, viz., convex and concave lenses.

• Those lenses that are thicker in the middle compared to the edges are called convex lenses. Both the surfaces of a convex lens are convex, i.e., bulged out.
• Those lenses that are thicker in the edges compared to the middle part are called concave lenses. Both the surfaces of a concave lens are concave, i.e., bulged in.

Convex Lenses:

• When a parallel beam of light falls on a convex lens, it converges to a point after refraction.
• This point F is called the focal point or focus of a convex lens.
• As a parallel beam of light converges to a point after refraction through a convex lens, it is also called a converging lens.

Fig 5.2: Refraction through a convex lens

Concave Lenses:

When a parallel beam of light falls on a concave lens, it diverges in such a way that it appears to diverge from a point (F) after refraction.

This point F is called the focal point or focus of a concave lens.

As a parallel beam of light diverges after refraction through a concave lens, it is also called a diverging lens.

Fig 5.3: Refraction through a concave lens

Note:

It can be noticed from fig 5.2 and 5.3 that,

• The incident rays and the emergent rays are not parallel to each other, unlike refraction through a glass slab.
• The light ray in the middle refers to the normal incidence and hence goes without any deviation from its usual path similar to the refraction through a glass slab.

Fire Using a Convex Lens:

As convex lenses converge, the light that falls on them behind them can be used to light up a fire.

A magnifying glass, being a convex lens, can be used for the above purpose.

When a magnifying glass is placed in the path of the Sun’s rays, a piece of paper or a dried leaf is placed behind it and the distance between the lens and the paper is adjusted to get a bright spot on the paper; the paper catches fire after some time.

This is because the heat contained in the sun’s rays gets concentrated at the bright spot, which heats it sufficiently to give rise to a fire.

The bright spot is, in fact, the image of the Sun produced by the convex lens.

It is a real image as it is formed on paper, which acts as a screen.

Fig 5.4: Fire using a convex lens

As concave lenses do not converge but diverge the light that falls on them behind them. They cannot be used to light up a fire.

The bright spot that was obtained while using a convex lens is also not obtained in this case, as the light rays are not converged to a single point.

Image Formation by Spherical Lenses:

The spherical lenses can form images just like spherical mirrors. The properties of the image would depend upon the type of lens used and the distance between the lens and the object.

Image Formation by a Convex Lens:

A setup, as shown in the figure below, where there is a convex lens mounted on a stand, a lighted candle, and a sheet of paper (screen) is required to study the image formation by a convex lens.

When the lighted candle is placed at some distance and the screen is moved to and fro behind the lens, a sharp image of the flame is seen on it.

Fig 5.5: Real, inverted, and the diminished image formed by a convex lens

• The image is formed on the screen. This means it is a real image.
• It is also upside down.
• The image is smaller in size as compared to the flame.
• Thus, the image is real, inverted, and diminished 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.

Fig 5.6: Real, inverted, and magnified image formed by a convex lens

• 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 convex lens.

Fig 5.7: Virtual, upright, and magnified image formed by a convex lens

• This image is not formed on the screen; it is not upside down and is larger than the candle.
• Thus, the nature of the image is virtual, upright, and magnified.

Therefore, the convex lens forms many types of images, such as real and virtual, smaller, larger, and also of the same size as the object.

Magnifying Glass- an Application of Convex Lenses:

When the object lies too close to a convex lens, it forms a virtual, upright, and magnified image of the object.  This property of convex lenses is used in magnifying glasses. As a magnifying glass is taken near small printed text it produces a virtual, upright, and magnified image on the same side of the text. This magnified image can be seen from the observer’s side on looking into the magnifying glass.

Fig 5.8: Magnifying glass

Image Formation by a Concave Lens:

A setup, as shown in the figure below, where there is a concave lens with a stand, a lighted candle, and a sheet of paper (screen) is required to study the image formation by a concave lens.

When a 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 lens as if it is formed behind the lens.

Fig 5.9: Virtual, upright, and the diminished image formed by a concave lens

• This image is not formed on the screen; it 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 lens its image becomes smaller and smaller.
• It also continues to be virtual and upright.

Therefore, the image formed by a concave lens is always virtual, upright, and diminished, no matter where the object is placed.

Summary

• Lenses are transparent objects made of transparent materials such as glass and clear plastic. They are widely used in spectacles, microscopes, and telescopes.
• Spherical lenses are basically of two types, viz., convex and concave lenses.
• The lenses which are thicker in the middle compared to the edges are called convex lenses. Both the surfaces of a convex lens are convex.
• The lenses which are thicker in the edges compared to the middle part are called concave lenses. Both the surfaces of a concave lens are concave.
• When a parallel beam of light falls on a convex lens, it converges to a point after refraction.
• When a parallel beam of light falls on a concave lens, it diverges in such a way that it appears to diverge from a point after refraction.
• As convex lenses converge, the light that falls on them behind them can be used to light up a fire. Concave lenses do not converge but diverge the light that falls on them behind them. They cannot be used to light up a fire.
• A convex lens forms many types of images, such as real, virtual, inverted, upright, diminished, magnified, and the same size as the object.
• A concave lens only forms a virtual, upright, and diminished image of an object.