Lenses in physics !?

Question

So first of all look at this img :
http://aycu35.webshots.com/image/31034/2003792274542358389_rs.jpg

OK now as u can see , there are 2 different lenses in the picture one is biconvex and the other one is biconcave . Since u already know that lets get to the point .

Since i am a novice in this catagory , and the teacher promiced to give an A+ to whoever solved this 1st , i beg you to help me find the FOCAL point in both cases .


Thank you all , cheers if u solved it . An img response with a little explaning will do very good .

Answer 1

In a simple lens, all the rays from a parallel beam come together on the focal plane. The focal point of the lens is where the optical axis hits the focal plane. You have only one ray, so you need a second ray, parallel to the first, to see where they intersect. One thing you can count on with a simple lens is that a ray passing through the center is undeflected. So draw a parallel to the incoming ray through the center of the lens. Where that line intersects with the exit ray defines the focal plane. drop a perpendicular from there to the optical axis and that's your focal point.