Calculating Refractive Power of Two Lenses Combination

[snnicols]

hey- I've been working on this one for quite some time now and just can't seem to get it:
Two converging lenses with focal lengths 10 cm and 20 cm are placed 30 cm apart. Rays from a very distant object are impinged on the lens system parallel to the principal axis.
What is the refractive power of the combination of these two lenses?

it offers this help:
Look at your ray diagram. Suppose you replaced the two-lens combination with one lens that focused the incoming parallel rays the same distance away. The focal length f of the replacement lens is how far away it focuses incoming parallel rays, and the refractive power is 1/f !

i have tried everything i know how to with the given numbers...
thank you for your help!
-shawna

 

[Data]

First, you should probably try to find where the image appears. Since the object is "very distant," you can approximate that the incident rays are parallel. Use the image formed by the first lens as the object position for the second lens and apply the Gaussian lens formula (assuming thin lenses) to find the final image position. If you can't figure out how to get the refractive power from there, post your work and we'll give some more help.

 

[snnicols]

but what am i supposed to use for the object distance??

i don't even know where to start, really...

using: 1/p+1/q=1/f i don't know how to find any of the variables, for object distance, image distance, and then the focal length...
i guess i don't understand what you are saying, sorry.

 

[Data]

Well, you have two lenses.

Say the parallel rays are incident on the lens of focal length 10cm from the left. This just produces an image 10cm from the right of that lens, and thus 20cm to the left of the second lens. Use the 20cm as the "object" distance for the second lens (note that the rays from the first lens converge to the focal point and then diverge again before they hit the second lens, so it's exactly like you had just placed an object at the first lens' focal point), and just apply the Gaussian formula as you would for any thin lens to see where the final image appears (you are given the focal length of the second lens).

 

[snnicols]

i got the answer, Data, thank you...
0.
those tricky profs sometimes, I'm sure they get a kick outta kids like me stressing out about this.
thank you again!
-shawna

 

[Data]

Very good :)