Maharashtra Board Class 10 Science Part 1 Chapter 7 Lenses PDF Download

Chapter 7. Lenses

You must have seen lenses used in day to day life. Some examples are: the lenses used by old persons for reading, lens embedded in the front door of the house, the lens which the watch maker attaches to his eye etc.

Lenses are used in spectacles. They are also used in telescopes as you have learnt in the previous standard.

What is a Lens?

A lens is a transparent medium bound by two surfaces. The lens which has two spherical surfaces which are puffed up outwards is called a convex or double convex lens. This lens is thicker near the centre as compared to the edges. The lens with both surfaces spherical on the inside is called a concave or double concave lens. This lens is thinner at the centre as compared to its edges.

Different types of lenses are shown in figure 7.2. A ray of light gets refracted twice while passing through a lens, once while entering the lens and once while emerging from the lens. The direction of the ray changes because of these refractions. Both the surfaces of most lenses are parts of a sphere.

The cross-sections of convex and concave lenses are shown in parts a and b of figure 7.3. The surface marked as 1 is part of sphere S1 while surface 2 is part of sphere S2.

Teacher's Note

A convex lens is like a magnifying glass used by grandparents to read small letters. A concave lens is like the peephole in your front door that makes the outside view smaller.

Exam Trick

Remember: Convex = Curved OUT = Converges light. Concave = Curved IN = Diverges light. Think of the shape to remember what happens!

Points to Remember

A convex lens is thicker in the middle and brings light rays together.
A concave lens is thinner in the middle and spreads light rays apart.
Light bends twice in a lens - once when entering and once when leaving.
Both surfaces of a lens are parts of spheres.
Lenses are used in spectacles, cameras, and magnifying glasses.

Terms Related To Lenses

Centre of curvature (C): The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses. A lens with both surfaces spherical, has two centres of curvature C1 and C2.

Radius of curvature (R): The radii (R1 and R2) of the spheres whose parts form surfaces of the lenses are called the radii of curvature of the lens.

Principal axis: The imaginary line passing through both centres of curvature is called the principal axis of the lens.

Optical centre (O): The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre of a lens. In figure 7.4, rays P1Q1, P2Q2 passing through O are going along a straight line. Thus O is the optical centre of the lens.

Principal focus (F): When light rays parallel to the principal axis are incident on a convex lens, they converge to a point on the principal axis. This point is called the principal focus of the lens. As shown in figure 7.5a F1 and F2 are the principal foci of the convex lens.

Light rays parallel to the principal axis falling on a convex lens come together i.e. get focused at a point on the principal axis. So this type of lens is called a converging lens.

Rays travelling parallel to the principal axis of a concave lens diverge after refraction in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the concave lens. As shown in figure 7.5b F1 and F2 are the principal foci of the concave lens.

Light rays parallel to the principal axis falling on a concave lens go away from one another (diverge) after refraction. So this type of lens is called a divergent lens.

Focal length (f): The distance between the optical centre and principal focus of a lens is called its focal length.

Teacher's Note

The optical centre is like the middle of a magnifying glass - light passes straight through without bending. The principal focus is where all the light rays meet after passing through the lens, just like how the sun can burn paper through a lens.

Exam Trick

Remember: Focal length = Distance from optical centre to focus. For convex lens, focal length is positive. For concave lens, focal length is negative. Just like your positive and negative marks in school!

Points to Remember

Centre of curvature is the center of the sphere of which the lens surface is a part.
Optical centre is the middle point of the lens where light passes straight.
Principal focus is where parallel rays meet after passing through a convex lens.
Focal length is the distance from optical centre to principal focus.
A convex lens has positive focal length and a concave lens has negative focal length.

Ray Diagram Rules For Convex Lenses

You have learnt the rules for drawing ray diagrams for spherical mirrors. Similarly, one can obtain the images formed by lenses with the help of ray diagrams. One can obtain the position, size and nature of the images with the help of these diagrams.

One can use following three rules to draw ray diagrams of images obtained by convex lenses.

Rule 1: When the incident ray is parallel to the principal axis, the refracted ray passes through the principal focus.

Rule 2: When the incident ray passes through the principal focus, the refracted ray is parallel to the principal axis.

Rule 3: When the incident ray passes through the optical centre of the lens, it passes without changing its direction.

Teacher's Note

These three rules help us draw ray diagrams like following a recipe. Just draw the rays following these rules and you can find where the image forms, just like tracing light with a torch in a dark room.

Exam Trick

Remember the three rules by their numbers: Rule 1 - Parallel to focus. Rule 2 - Focus to parallel. Rule 3 - Through centre, no change. Draw these three rays from the object and where they meet is the image location!

Points to Remember

Ray diagrams help us find position, size and nature of images formed by lenses.
Rule 1 shows how a parallel ray bends through the lens.
Rule 2 shows how a ray through focus bends to become parallel.
Rule 3 shows that a ray through optical centre does not bend.
We need at least two rays to find where the image forms.

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