Ansel places an object 30 cm from a thin,convex lens along the axis.If a real image forms at a distance of 15 cm from the lens,what is the focal length of the lens?
A) 15 cm
B) 10 cm
C) 7.5 cm
D) 30 cm

Question

Quizplus · Accepted Answer

Using the lens formula,
1/f = 1/u + 1/v where f is the focal length, u is the distance of the object from the lens and v is the distance of the image from the lens. 
Given u = -30 cm,since the object is placed to the left of the lens, v= -15 cm, since a real image forms to the right of the lens. 
Substituting these values in the lens formula we get:
1/f = -1/30 - 1/15
1/f = -3/30
f = -10 cm (where negative sign indicates that the lens is a convex lens)
Therefore, the focal length of the lens is 10 cm.

Ansel Places an Object 30 Cm from a Thin,convex Lens