Question

how do you find the Rough focal length of a convex lens? Is the same method applicable to concave lens ?​

Answer 1

Answer:

The rough focal length of a convex lens is obtained by forming sharp image of a very distant object on a screen. The distance of the screen from the lens gives us the rough focal length of the lens.

Answer 2

We can find the rough focal length of a convex lens by focusing the light from a faraway object on a screen and measuring the distance between the lens to the screen.

• We know that a convex lens is a converging lens i.e. parallel light rays striking the lens are refracted to converge at the focus.
• So, this property can be used to find the focal length of the lens by the above method.
• A concave lens on the other hand is a diverging lens i.e parallel light rays striking the lens are refracted to diverge.
• The diverging rays seem to come from the focus on the opposite side of the lens.
• Hence, this method cannot be used to determine the focal length of the concave lens.