An object and a screen are separated by 20.00 cm.A convex lens is placed between them,5.00 cm from the object.In this position it causes a sharp image of the object to form on the screen.What is the focal length of the lens?

Question

Quizplus · Accepted Answer

Using the lens formula:

1/f = 1/v - 1/u

Where,

f = focal length of the lens, 
u = object distance = -5.00 cm (negative sign indicates that the object is placed to the left of the lens), and 
v = image distance = 20.00 cm (positive sign indicates that the image is formed on the opposite side of the lens as compared to the object).

Substituting these values in the above equation, we get:

1/f = 1/20 - (-1/5)

Simplifying, we get:

1/f = 1/20 + 1/5

1/f = 1/4

f = 4 cm

Therefore, the focal length of the lens is 4 cm or 3.75 cm (rounded to two significant figures).

Hence, the correct choice is (C).

An Object and a Screen Are Separated by 20