Posted by Anonymous on .
1) A convex lens with a focal lenght of 4.0cm is placed 20.0cm from a concave lens with a focal lenght of 5.0cm. Find the position of the image when the object is placed 12.0c, in front of the convex lens.
2) A double concave lens has a refractive index of 1.5 radii of curvature of 10.0cm and 15cm. Determine its focal length.
Thank you for helping in advance!

Physics: Lenses 
drwls,
1) Use the position of the image formed by the first lens as the location of the object for the second lens.
For the distance of the image from the first lens, di, use
1/12 + 1/di = 1/4
1/di = 2/12
di = 6
That means the first image is 20  6 = 14 cm in front of the concave lens, which has a negative focal length. For the location of the final image, di', use
1/14 + 1/di' = 1/5
2) For this problem, use the "lens maker's formula". If it is not in your textbook, you can find it at
http://scienceworld.wolfram.com/physics/LensMakersFormula.html
Both of the "1/R" terms will be negative. The focal length will be negative. 
Physics: Lenses 
Anonymous,
Thank you drwls for your help!