# Convex lenses

1. Apr 24, 2010

### choco_moo

1. The problem statement, all variables and given/known data
1. A small bulb is placed in front of a convex lens. It is placed at one focal point of the lens. Draw at least five rays from the bulb that pass through the lens.

Where is the lens located in this case? Explain. (Hint: How are the rays that have passed through the lens oriented? From where do these rays appear to have come?)

2. Relevant equations
none

3. The attempt at a solution
Sorry I can't show the picture but since the bulb is at the focal point, doesn't that mean it is right on the optical axis? I really don't know how to draw 5 rays from that.

2. Apr 24, 2010

### Antiphon

It's on the axis. The rays begin at the lamp and go in different directions.

3. Apr 24, 2010

### ehild

Yes, the bulb is on the optical axis. So just draw a lens with its optical axis and focal points and put the bulb at one of them. Consider the bulb a point-like light source.

Remember that you can collect the sun rays with a convex lens, and ignite a piece of paper. The rays of sun come from so far that they are parallel. Rays incident parallel with the optical axis are collected in the focus at the other side of the lens. Draw 5 such rays.

You know that the direction of light rays is reversible. If you turn the direction of the sunrays over, they will follow the same path as they arrived, going back towards the sun.

Starting rays from the focus, in what direction will they go after crossing the lens?

ehild

Last edited: Jun 29, 2010