By what factor will the size of an object placed 10cm from a convex lens be increased if the image is seen on a screen placed 25cm from the lens?
- 15.0
-
2.5
- 1.5
- 0.4
The correct answer is: B
Explanation
Form the lens formula = 1/U + 1/V = 1/f=> 1/10 + 1/25 = 1/f;
∴ (5 + 2) / 50 = 1/f
∴ f = 50/7cm
since the image is observed on a screen, it is a real image. Thus magnification
M = f/U-f
| = | __50/7 |
| 10 - 50/7 |
| ∴ M = | 50/7 |
| 20/7 |
= 50/7 x 7/20 = 2.5