ICSE Class 8 Physics Chapter 03 Lenses

3 Lenses

Lens

A lens is a transparent medium, usually made up of glass or plastic. Generally, both of its sides are curved so as to achieve the required bending of rays of light.

Convex Lens (Converging Lens)

A convex lens converges a beam of parallel rays of light after refraction through it. Such a lens is thickest at the middle and gradually becomes thinner towards its edges. There are three types of convex lenses.

(a) Biconvex or double convex lens or, simply, a convex lens which has both the surfaces convex.

(b) Plano-convex lens which has one surface convex and the other plane.

(c) Concavo-convex lens which has one surface convex and the other concave.

Fig. 3.1: Three types of convex lenses

Concave Lens (Diverging Lens)

A concave lens diverges a beam of parallel rays of light after refraction through it. Such a lens is thinnest at the middle and gradually becomes thicker towards its edges. There are three types of concave lenses.

(a) Biconcave or double concave lens or simply concave lens which has both the surfaces concave.

(b) Plano-concave lens which has one surface concave and the other plane.

(c) Convexo-concave which has one surface concave and the other convex.

Fig. 3.2 Three types of concave lenses

Some Terms Related To Lenses

Each of a convex (double convex or converging) lens and a concave (double concave or diverging) lens has both of its surfaces spherical as shown below:

Fig. 3.3 Concave lens

1. Centre of curvature (C)

It is the centre of the sphere of which the surface of the lens is a part. In a lens, there are two spherical surfaces, therefore there are two centres of curvature of a lens. In Fig. 3.3, C₁ and C₂ are the centres of curvature.

2. Radius of curvature (R)

It is the radius of the sphere of which the surface of the lens is a part. Since there are two spherical surfaces of the lens, we get two radii of curvature. In Fig. 3.3, OC₁ and OC₂ are the two radii of curvature. These radii of curvature of a lens may or may not be equal; but in this chapter, we shall consider both the radii to be equal.

3. Principal Axis

It is a straight line passing through the two centres of curvature of the lens. In Fig. 3.3, the straight line AB is the principal axis. Any ray passing through the lens along the principal axis remains undeviated.

4. Optical Centre (O)

It is the point on the principal axis inside the lens, through which when a ray of light passes does not deviate. In Fig. 3.3, point O is the optical centre of the lens. Any ray passing through the optical centre of the lens remains undeviated.

5. Principal Focus (F)

In case of a convex lens, the rays of light incident parallel to the principal axis meet after refraction, at a point on the other side of the lens, on the principal axis. This point is called the principal focus and is denoted by letter F. A convex lens has two principal foci F₁ and F₂ as rays can be refracted from either side.

Fig. 3.4

It is dangerous to look through a convex lens at the sun or a bright light. You should also be careful not to focus sunlight with a convex lens on any part of your body.

In case of a concave lens, the rays of light incident parallel to the principal axis appear to diverge, after refraction, from a point on the same side of the lens on the principal axis. This point is called the principal focus and is denoted by letter F. A concave lens also has two principal foci F₁ and F₂.

Fig. 3.5

6. Focal length (f)

The focal length of a lens is the distance between its optical centre and its principal focus.

Teacher's Note

Lenses are fundamental optical components used in everyday devices like eyeglasses, cameras, and magnifying glasses to correct vision and capture images.

Representation of lens as a combination of different sections of prism

A lens may be considered to be made up of a combination of different sections of prism.

Converging lens and Diverging lens shown in diagram.

Do You Know?

Concave lenses work to make something look smaller, so they're not quite as common as convex lenses. Spectacles have one convex surface and one concave surface. It can bend the light just the right amount before it gets to your eyes.

Lenses have "aberrations", which means that they don't focus light of different colours or light passing through different portions of a lens at one point. A combination of a convex lens and a concave lens of different materials can approximately get rid of the "chromatic aberration" or the problem of focusing light of different colours differently. Thus concave - convex combinations are used in many high quality telescopes and binoculars so that the colour defect of images can be eliminated.

Real and Virtual Images

The image which can be obtained on a screen is called a real image.

A real image is formed by the actual intersection of the refracting or reflecting rays.

The image which cannot be taken on the screen, is called a virtual image.

A virtual image is formed when the refracting or reflecting rays do not actually intersect but appear to intersect when they are produced backwards.

Difference between real image and virtual image

Image Formed by Lenses

To find out the position, nature, size and whether the image is erect or inverted, we need to draw at least two rays starting from the object and travelling through the lens. While drawing the ray diagrams, we must keep in mind the following important rules:

Rule 1

A light ray incident parallel to the principal axis converges at the focal point in case of convex lens and appears to diverge from the focal point in case of a concave lens (Fig. 3.6).

Fig. 3.6

Rule 2

A ray passing through the optical centre of a lens remains undeviated irrespective of its inclination on the principal axis (Fig. 3.7).

Fig. 3.7

Rule 3

A ray of light which comes through focus (in case of convex lens) or appears to come towards focus (in case of a concave lens) becomes parallel to the principal axis after passing through the lens (Fig. 3.8).

Fig. 3.8

Teacher's Note

Ray diagrams help us predict where images will form when light passes through lenses, similar to how photographers predict where subjects will appear in their viewfinders.

Activity 1

Take a convex lens and fix it vertical on a stand. Place it on a table. Place a lighted candle at a distance of about 50 cm from the lens. Try to obtain the image of the candle on a paper screen placed on the other side of the lens. You may have to move the screen towards or away from the lens to get a sharp image of the flame. What kind of image did you see? Is it magnified or diminished?

Now vary the distance of the candle from the lens. Try to obtain the image of the candle flame every time on the paper screen by moving it. Record your observations. Did you see for any position of the object an image was erect and magnified. Could this image be obtained on a screen? Is the image real or virtual? This is how a convex lens is used as a magnifying glass.

In a similar fashion study the images formed by a concave lens. You will find that the image formed by a concave lens is always virtual, erect and smaller in size than the object. It cannot be obtained on a screen.

To find the focal length of a convex lens, make a screen of a white blank paper on a piece of smooth cardboard. Stand the screen verticals near a window. Adjust the position of a convex lens in front of the screen by moving it to and fro until a sharp inverted image of any distant building or a tree is formed on the screen.

Now measure the distance between the screen and the lens when a sharp image of the object is formed on the screen. This distance is equal to the focal length of the lens used.

Fig. 3.11

Formation of Image by a Convex Lens

We now draw the ray diagrams to get the position, size and nature of the image formed by a convex lens for various positions of the object.

1. Object at infinity

Rays coming from an object at infinity, can be taken parallel to each other. These rays, after refraction form an image at focus as shown below.

Image formed is:

(a) at F

(b) Real

(c) Inverted

(d) Highly diminished

Fig. 3.9 (a)

2. Object beyond 2F

One ray from the object AB can be taken parallel to the principal axis and the other through the principal optical centre. The two rays, after refraction, intersect to form image between F and 2F on the other side of the lens as shown below.

Image formed is:

(a) Between F and 2F

(b) Real

(c) Inverted

(d) Smaller (or diminished)

Fig. 3.9 (b)

3. Object at 2F

One ray from the object AB can be taken parallel to the principal axis and the other through the optical centre. The two rays, after refraction intersect to form image at 2F on the other side of the lens as shown below.

Image formed is:

(a) at 2F

(b) Real

(c) Inverted

(d) Same size

Fig. 3.9 (c)

Teacher's Note

The position of the object relative to a lens determines the size and type of image produced, similar to how the distance you hold a magnifying glass from a page affects what you see.

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