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FIND OUT MORE. Eye 140 • Light 178–179

When you look through a magnifying glass, or take a

picture with a camera, you are using a lens. A lens is

a polished piece of glass or transparent plastic with

curved surfaces. There are two main

shapes of lens,. CONVEX and

. CONCAVE.

WHAT IS THE DIFFERENCE BETWEEN

A MICROSCOPE AND A TELESCOPE?

A microscope makes a tiny, nearby object look much

bigger. A telescope makes a large, distant object or

scene appear much closer and brighter. In both

instruments, light from the object passes through two

or more lenses to form an image. The shapes of the

lenses and the distances between them alter the image

that is produced.

WHAT DOES A LENS DO?

A lens changes the direction of light waves by

refraction. It may form an image of a scene or

an object. The image might be smaller (as in a

camera), or larger (as in a microscope). Because a

lens is curved, light rays strike different parts of its

surface and are bent by different amounts. Depending

on the lens shape, this either diverges (spreads out) or

converges (concentrates) a beam of light.

HOW DOES A MAGNIFYING GLASS WORK?

A magnifying glass makes an object look bigger.

Holding it close to an object makes a virtual image

of the object form on the same side of the glass as the

object. When you look through the magnifying glass,

this virtual object seems to be larger than the real one.

The thicker the lens, the larger the virtual image.

This type of lens is thicker in the centre than at the edge. Parallel

rays of light entering it on one side will converge (meet) at

a particular spot on the other side of the lens. Magnifying

glasses and microscopes use convex lenses.

HOW DO CONCAVE LENSES HELP YOU SEE?

If you are short-sighted, your eye lens focuses a scene

just in front of the retina in your eye and the image

you see is blurred. A concave lens spreads out the

light rays before they enter the eye, so that they are

focused on the retina and the image is sharp.

A concave lens is thinner at the centre than at the edge. Parallel

light rays passing into one side of the lens diverge (spread out) as

they emerge from the other side.

OPTICAL MICROSCOPE 3

Optical microscopes use light to magnify an image up

to 2,000 times. Light from a brightly lit specimen (object)

is captured and concentrated by a powerful

objective lens to produce a magnified image. The

eyepiece lens may then magnify the image further.

4 CONVERGING LENSES

Diverging (spreading) light rays

are made parallel by the first

convex lens. The second convex

lens converges, or angles, the

parallel rays so that they meet or

focus at a certain point – the

focal point.

CONVEX LENS

CONCAVE LENS

1 MAGNIFIED IMAGES

Optical microscopes use beams of

light, but electron microscopes

use beams of electrons to give

much greater magnification. The

image of this silverfish has been

magnified thousands of times.

DIVERGING LENSES 3

Diverging light rays are made

parallel by a convex lens. But

when the parallel light rays pass

through a concave lens, they

diverge (spread out) again.

Lenses

Focusing knob

adjusts the distance

between the

specimen and the

objective lens

Objective lenses

of different powers

view the specimen at

different magnifications

Convex lens

Image of

bacteria

magnified

400 times their

real size

Specimen

(object to be

viewed) is

mounted on a

glass slide

Mirror

reflects light on to and

through the specimen

Eyepiece lens

views the

magnified

image

Focal point

Concave lens

Converging

rays of light

Diverging rays

of light

r.

age

lenses