Question

Materials required for finding the focal length of concave mirror

Answer 1

Image distance and object distance (v and u)

Answer 2

Lens in Contact Method

Materials required:

Illuminated wire gauze

Lens stand

Screen

Metre scale

A convex lens of short focal length

Concave lens

Real Lab Procedure:

Keep the given convex lens of known focal length, f1, in contact with the concave lens whose focal length to be determined. This forms the combination of lenses.

It is placed between the illuminated wire gauze and the screen at a fixed distance away from the wire gauze. It is taken as u cm.

The position of the screen is adjusted to get a clear image of the wire gauze on the screen.

Measure the distance between the combination of lenses and the screen. It is taken as v cm.

Calculate the focal length of the combination using the formula:

                                                                                  

Repeat the experiment by placing the combination of lenses at different distances away from the wire gauze.

Then calculate the mean value of F.

Using the values of f1 and F, the focal length of the given concave lens can be calculated using the formula:

                                                                                 

Observations:

SI No.Distance between the combination of lenses and

Focal Length

                 

Object u (cm)Image v (cm)1   2   3   4   5   

Calculations:

Focal length of the combination lens, Mean F = …………cm

Focal length of convex lens, f1 = ……………cm

Therefore, the focal length of the given concave lens,  

                                                                  

Results:

Focal length of the given concave lens by lens in contact method = …………cm