Two converging lenses,the first with focal length 25 cm and the second with focal length 20 cm,are separated by 30 cm.A 4.0 cm tall object is placed 50 cm in front of the first lens.Where is the image caused by the first lens? Give the most inclusive answer.
A) 50 cm after the first lens
B) 20 cm after the second lens
C) both 50 cm after the first lens and 20 cm after the second lens
D) 50 cm after the second lens
E) 20 cm before the second lens

Question

Quizplus · Accepted Answer

To find the location of the image caused by the first lens, we can use the thin lens equation:
1/f = 1/do + 1/di
where f is the focal length, do is the object distance, and di is the image distance.

Using the given values, we can solve for di:
1/25 = 1/50 + 1/di
di = 33.3 cm

Therefore, the image caused by the first lens is 33.3 cm in front of the second lens. To find the final image location, we can use the thin lens equation again with the second lens:
1/20 = 1/33.3 + 1/di2
Solving for di2, we get:
di2 = 20 cm

So the final image is located 20 cm after the second lens. However, there is also an image formed by the first lens at a distance of 50 cm in front of it. This image is virtual, upright, and smaller than the object. Therefore, the most inclusive answer is C, which includes both image locations.

Two Converging Lenses,the First with Focal Length 25 Cm and the Second