Hello everyone, i'm confused on this issue. How can I tell if the focal length is going to be postive or negative when 2 lens are combined like the ones below:(adsbygoogle = window.adsbygoogle || []).push({});

Here is the directions:

You grind the lenses shown in Figure 34-49 from flat glass disks (n = 1.5) using a machine that can grind a radius of curvature of either 15 cm or 30 cm. In a lens where either radius is appropriate, you select the 15 cm radius. Then you hold each lens in sunshine to form an image of the Sun.

What is the focal length f for each lens?

for some reason i'm getting this wrong, all i'm doing is taking the center of radius and dividing it by 2 to get the focal length. But I'm not getting that right either. I don't see where i'm messing up. THe professor just said, take the given cirvature of raidus which is 15, divide it by 2 and thats ur answer, then you have to decide if its positve or negative. I tried both postive and negative both wrong. Any ideas?

In my notes i have it saying:

Focal length of a convex lens is +

Focal Length of a concave lens is -

so if i had a bi convex, is that like a convex and a concave put together, there for, a end result of a negative focal length becuase a postive times a negative is -?

Here is my work:

http://img146.imageshack.us/img146/5817/lastscan5bq.jpg [Broken]

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# Homework Help: Can someone explain to me how i can figure out the sign of the focal length with lens

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