an object is placed 15.0 cm to the left of a double-convex lens of focal lenght 20.0 cm.?

Question

The image of this object is located:

a) 8.57 cm to the left of the lens
b) 30.0 cm to the left of the lens
c) 60.0 cm to the right of the lens
d) 60.0 cm to the left of the lens
e) 8.57 cm to the right of the lens

Answer 1

The object is placed inside the focal point of the lens (u = 15 cm < f = 20 cm). Hence the image formed is virtual, erect and on the the same side of the lens as the object.

Use the standard formula to get v = - 60 cm. (It is negative because the image is virtual).

The answer is

d) 60.0 cm to the left of the lens.

Answer 2

Convention all distances are positive at the right of lenses , negative to the left . Convex lens focal length is positive p distance lens object , p' distance lens, image


1/p' = 1/p +D where D =1/f = 1/0.2 =5 p = 1/1.5 =-6.67

So 1/p' = 5-6.67 = -1.67 p' =-0.6

The image has the same sens that object and 4 times as big

so the good answer is d)