My teacher assigned us 2 problems to do that were the exact same. One he wanted us to use the thin lenses equation, and the other he wanted us to use think lens equation. i need help becuase I dont know which one I am doing wrong and my answer is not lining up as it should, since they are the same problem, it should be the same solution. I think I am messing up on the thin lense beucase he worked out part of it out when he wanted us to use think lense 1. The problem statement, all variables and given/known data Find the effective focal length of the two lens system. The focal length of the first lens is 20 cm and the second lens is -20 cm with the seperation between the two lens being 10 cm. Locate the principal planes. Locate the image of an object that is 1 m in front of the first lens. Express the location relative to the location of the second lens. f1is 20cm f2is -20 and the distance between the two is 10 cm the object is 1m(100 cm) in front of the first lens 2. Relevant equations for thick lense the equation is : (1/so) + (1/si) = 1/f and f of both lens is found by ff = ((1/f1) + (1/f2) - (d/f1f2))-1 for the thin lens it is si2 = ((1/f2) - (1/so2))-1 3. The attempt at a solution For the thick lens I found h1 = -(f1f2)d/f2 = -20 h2 = -(f1f2)d/f1 = -20 so = d - (h1 = 80 and that ff = 40 from the above equation so i had (1/80) + (1/si) = 1/40 solved for si = 80 However for thin lens I found So1 = 100cm f1 = 20 cm Si1 = ((1/f1) - (1/so1))-1 = 25 then so2 = d - si1 = 10 cm - 25 cm = -15 cm finally si2 = ((1/f2)-(1/so2))-1 = 60 obviously 60 does not equally 80 and I have no idea what I did wrong. I think that what I got for so for the thick lens is wrong because when i switch it to 120 i get 60 . However my professor said that so was 80.