# Light and lenses

1. Dec 11, 2007

### dranseth

1. The problem statement, all variables and given/known data
1)A philatelist uses a magnifying glass with a focal length of 10cm. In order to examine a rare postage stamp, he holds the lens 4 cm from his eyes. If the distance for the most comfortable viewing of the image is 30cm from his eyes, how far from the lens must the postage stamp be placed? What will the magnification be?

2)A lens has a focal length of +20cm and a magnification of 4. How far apart are the object and the image?

2. Relevant equations

1/2) 1/f=1/Do+1/Di
Mag = hi/ho = -Di/Do

3. The attempt at a solution

I don't even know where to begin with the first question. I'm having difficulty interpreting it. Any help would be greatly appreciated.

2. Dec 11, 2007

### rl.bhat

If the distance for the most comfortable viewing of the image is 30cm from his eyes, and the lens is at 4 cm from the eye, you have to keep the stamp at such distance from the lens that it forms the inage at 30cm - 4 cm = 26 cm from the lens. Now find the object distance.