# Homework Help: Optics HELP!

1. Nov 9, 2004

### SpatialVacancy

Hello,

I have just finished what seems like an infinite amount of questions for this lab report. 2 of them I am just not sure how to proceed. Any help would be appreciated.

Question 1:
In a film projection apparatus, it is desired to produce pictures 12 ft wide on a screen 50 ft from the lens. The size of the picture on the film is 1 in. wide. What must be the focal length of the projection lens used?

Question 2:
The magnifying power of an astronomical telescope 24 in. long is equal to 15. Determine the focal length of the objective lens and that of the eyepiece. Assume the telescope is focused on a distant object.

Thank you.

2. Nov 9, 2004

### Pseudo Statistic

1) Because 1/u + 1/v = 1/f, from what I understand u = 50ft and v = 1 inch (Could be wrong..)

2) Guessing here, but Power of lens = 1/focal length, thus 15 = 1/f, 15f = 1, f = 1/15

3. Nov 9, 2004

### SpatialVacancy

Im not sure that is right. How does it take into consideration the other parameters of the problem?

4. Nov 10, 2004

### Pseudo Statistic

That's exactly where my uncertainty comes from..
I'll have to get back to you on this one.