Refraction of Light by Lenses

 Lens is a transparent or a translucent medium that alters the direction of light

passing through it

Types of Lenses

• Convex lenses

• Concave lenses

Convex (Converging Lenses)

• In a convex lens (sometimes called a positive lens), the glass (or plastic)

surfaces bulge outwards in the center giving the classic lentil-like shape.

• A convex lens is also called a converging lens because it makes parallel light

rays passing through it bend inward and meet (converge) at a spot just beyond

the lens known as the focal point.

• A convex lens is thicker at its centre than at its edge.

• Convex lenses converge light.

• A common application of convex lenses is that they are used in optical devices

such as microscopes, telescopes, eyeglasses, magnifying glasses, etc.

• Convex lenses can be biconvex, plano –convex or converging meniscus

Concave (Diverging Lenses)

• A concave lens is thicker at the edges than at the center. Every concave lens

causes all rays to diverge. Rays that approach the lens parallel to the principal

axis refract as if they came from the focal point.

• Concave lenses include biconcave, plano-concave and diverging meniscus

Terms used on thin Lenses

Consider the diagrams below

• Optical centre:

Is the geometric centre of a lens.

• Centre of curvature, C: Is the geometric centre of the sphere of

which the lens surface is a part of.

• Principal axis: Is an imaginary line which passes through the optical centre

of the lens at a right angle to the lens.

• Radius of curvature, R: Is the distance between optical centre and the

centre of curvature.

• Principal focus (focal point), F: Is a point through which all rays travelling

close and parallel to the principal axis pass through the lens.

• Aperture: Is the width of the lens, from one edge to another

• Focal length, f: Is the distance between the optical centre and the principal

focus

Construction of ray diagrams

The following are the procedures used to locate image in a lens

➢ Choose a suitable scale

➢ Draw a principal axis and the lens

➢ Draw object in the position

➢ Measure the height and the distance of the image

➢ Convert the measurements into actual units using the chosen scale

Drawing ray diagrams for converging lens

Three principal rays can be drawn to construct ray diagrams for convex

lenses, as shown in Figure below.

1. A ray parallel to the principal axis is refracted through the opposite focal point.

2. A ray passing through the optical center of the lens does not refract.

3. Aray passing through the focal point on the same side of the lens as the object

is refracted parallel to the principal axis.

Drawing Ray diagrams for concave lens

Three principal rays can be drawn to construct ray diagrams for diverging lenses.

1. A ray parallel to the principal axis refracts so that its extension (the dashed line)

passes through the focal point F in Figure below.

2. A ray heading towards focal point F′ refracts parallel to the principal axis, as in

Figure below.

3. A ray passes through the center of the lens with no apparent refraction.

NB: The intersection of the rays for both convex and concave lenses shows where

the image is formed.

Worked Examples;

1. An object 10 cm tall stands vertically on the principal axis of a convex lens of

focal length 10 cm and at a distance of 17 cm from the lens. By means of

accurate graphical construction find the position, size and nature of the image

formed Solution:

(a) Position of the image = 24 x 1 = 24 cm from the lens

(b) Size of the image = (14 x 1) = 14 cm tall

(c) Nature of the image: The image is (i) Real and (ii) inverted