# Light rays, convex lens and focal length

• Apr 17th 2009, 11:38 PM
crackle
Light rays, convex lens and focal length
Hi guys,

I am not sure which category this best fits into so apologies in advance if it's not correct.

I am part way through a semester at uni and the subject is aimed at late high school/entry level knowledge but it has been years since I've looked at this. Would love you advice on a, b and c ? (Happy) I

http://img8.imageshack.us/img8/9959/mathsq.jpg

For a, I have a focal length of 4cm, what are others opinions?

For b, the S2 should be larger than f but I am not able to get a logical answer - I can only get an answer of 0.3999 or 3999/10000 = 1/S2 ..?

Hope to hear from you guys soon (Nerd)

Thanks!
• Apr 18th 2009, 12:12 AM
crackle
Quote:

Originally Posted by crackle
For b, the S2 should be larger than f but I am not able to get a logical answer - I can only get an answer of 0.3999 or 3999/10000 = 1/S2 ..?

Not sure if it's ok to add something but I will lol.

For b, this is what I've worked out:
100m = 100 000mm, 2.5cm = 25mm
So:
1/100 000 + 1/S = 1/25 - Find S
1/S = 1/25 - 1/100 000
1/S = 4000/100 000 - 1/100 000
1/S = 3999/100 000
S = 1 / (3999/100 000)
S = 25.006mm

Sub into original equation:
1/100 000 + 1/25.006 = 0.04mm and original is 1/25 which is 0.04mm

Does this look right?