What is the Unknown Distance in a Lens Equation Problem?

[petern]

Homework Statement

http://img368.imageshack.us/img368/9489/physicszv8.jpg

Homework Equations

(1/f) = (1/p) + (1/q)

The Attempt at a Solution

I tried solving this but I don't know what the original P was so I can't solve for q. How can I solve this?

 

[Kurdt]

The focal length is 5cm and the object distance is 100cm. You only have one unknown variable, so it should be easy to solve.

 

[tiny-tim]

I tried solving this but I don't know what the original P was

Hi peter!

In diagram (a), the light rays were parallel, "from a very distant object".

So the original p was infinity.

And 1/infinity = … ?