The object is located at a distance of 30 cm from the collecting lens, and its image

The object is located at a distance of 30 cm from the collecting lens, and its image is at a distance of 60 cm from the lens. Calculate the focal length of the lens.

Thin Lens Formula:

1 / d + 1 / f = 1 / F,

where d is the distance from the object to the lens,

f is the distance from the lens to the image,

F is the focal length.

For real objects, both the image d and f should be taken with a “+” sign.

1 / 0.3 m + 1 / 0.6 m = 1 / F;

(2 + 1) / 0.6 = 1 / F;

F = 0.6 / 3 = 0.2 m.

Answer: 0.2 m (20 cm).