Images Formed By Lenses

IMAGES FORMED BY LENSES

You might have seen a magnifying glass. It is used to read very small print (Fig. 15.21). You might have also used it to observe the body parts of a cockroach or an earthworm. The magnifying glass is actually a type of a lens. Lenses are widely used in spectacles, telescopes and microscopes. Try to add a few more uses of lenses to this list.

Get some lenses. Touch and feel them. Can you find some difference just by touching? Those lenses which feel thicker in the middle than at the edges are convex lenses [Fig. 15.22 (a)]. Those 

which feel thinner in the middle that at the edges are concave lenses [Fig. 15.22 (b)]. Notice that the lenses are transparent and light can pass through them

Activity 15.9

Take a convex lens or magnifying glass. Put it in the path of sunrays. Place a sheet of paper as shown (Fig. 15.23). Adjust the distance between the lens and the paper till you get a bright spot on the paper. Hold the lens and the paper in this position for a few minutes. Does the paper begin to burn?

Now replace the convex lens with a concave lens. Do you see a bright spot

on the paper this time, too? Why are you not getting a bright spot this time? We have seen in the case  of  mirrors that for different positions of the object the nature and size of the image change.Is it true for lenses also?Let us find out.

Activity 15.10

Take a convex lens and fix it on a stand as you did with the concave mirror. Place it on a table. Place a lighted candle at a distance of about 50 cm from the lens [Fig. 15.25 (a)]. Try to obtain the image

A convex lens converges (bends inward) the light generally falling on it [Fig. 15.24 (a)]. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens [Fig. 15.24 (b)].

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 get? Is it real or virtual?

Now vary the distance of the candle from the lens [Fig. 15.25 (b)]. Try to obtain the image of the candle flame every time on the paper screen by moving it. Record your observations as you did in Activity 15.7 for the concave mirror.

Did you get in any position of the object an image which was erect and magnified (Fig. 15.26). 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 (Fig. 15.27).