magnification

Let us discuss the size of the image formed by a lens.

Substituting the values as per sign convention in the above equation, we get

• Is the focal length same for each set of values? You might have noticed that irrespective of object distance and image distance, you will get same focal length.

If you do not get the same value of focal length, there may be some experimental error while doing the experiment.

In such a case, find the average of all the values. This will be equal to the focal length of the lens.

An electric lamp and a screen are placed on the table, in a line at a distance of 1m.

In what positions of convex lens of focal length of f = 21 cm will the image of lamp be sharp?

The distance between the lamp and screen is 100cm and ‘x’ be the distance between lamp and lens.

From figure E-6, we have u = -x and v = 100-x, f = 21

By substituting these in lens formula, We get

After solving this equation, we get

It is a quadratic equation.

Hence we get two solutions.

The solutions of the above equation are