How do you find the focal length of a lens experimentally? (AS1)

• The rays of light coming from a distant object are considered parallel to each other.

• Parallel rays after refraction from lens converges/diverges to focus of the lens.

Aim: To find the focal length of a lens

Apparatus required: convex lens, metre scale, a white screen, lens holder, a distant object

Procedure:

• Set up the apparatus near a window so to locate a distant object .

• Mount the convex lens on the lens holder.

• Now place the metre scale on the table and keep the lens at a distance of 5 cm mark on the metre scale such that the mid point of lens holder coincides with 5 cm mark.

• Place the white screen behind the lens such that the image of the object is formed on the screen.

• Now move the screen until you get a sharp, diminished & inverted image of the distant object.

• Mark the point on the scale where you get the desired image. The difference between the lens mark & the screen mark will give the focal length.

• Now place the lens on a new position that is 10 cm and find the focal length.

• Repeat the above steps by keeping the lens at 10 cm, 15 cm & so on to get more observations.

Observation: