x2 -70x-30x+2100=0 B x (x-70)-30(x-70) = 0 B (x-70)(x30) = 0
We can place the lens at a distance of 70cm or 30cm from the bulb, we get sharp image. _______________
Take the same lens that was used in activity 2. Note the average focal length of the lens that was calculated in the activity. Take a cylindrical vessel such as glass tumbler. Its height must be much greater than the focal length of the lens. (We require the vessel which has a length (depth) nearly equal to four times of the focal length of lens). Keep a black stone inside the vessel at its bottom. Now pour water into the vessel up to a height such that the height of the water level from the top of the stone is greater than focal length of lens. Now dip the lens horizontally using a circular lens holder as shown in the figure (21) above the stone. Set the distance between stone and lens that is equal to or less than focal length of lens measured in activity 2. Now look at the stone through the lens. (Do this in open ground)
You can see the image of the stone if the distance between lens and stone is less than the focal length of the lens (in air). Now increase the distance between lens and stone until you cannot see the image of the stone.
You have dipped the lens to a certain height which is greater than the focal length of lens in air. But you can see the image (when lens is raised further, you could not see the image). This shows that the focal length of lens has increased in water. Thus we conclude that the focal length of lens depends upon the surrounding medium in which it is kept.