# Ray tracing rules

1. May 6, 2010

### kusiobache

Can anyone brief of on the different rules for doing ray tracing for both concave and convex lenses, as well as both concave and convex mirrors?? I understand the concepts but i can't seem to keep ray tracing in my head from the way our textbook explains it...

2. May 6, 2010

### cesiumfrog

The ray right along the axis is trivial. Likewise, other rays passing the exact center of the lens are also trivial.

It's easy to draw the ray that is initially parallel to the axis. Likewise, the ray that is initially directed at the appropriate focal point.

(In all cases do sketch the virtual rays, by extending the final rays in the directions they seemed as if to come from.)

By now you've drawn three different rays from the top of the object, and you've noticed that the three corresponding final rays emanate from another single point. (You've also drawn one ray from the base of the object).

The key is that whenever multiple rays from the same point on an object are focused back as if they came from a common point in space, then ALL rays from that point on the object (even ones at odd angles) will also be focussed as if they came from that common point. And moreover, all rays from any other point in the same plane as that first object point will be focussed to another common point in the same plane as the first image point, and in corresponding proportion. That means, if you've figured out where the the (perhaps virtual) image of the top of the object is, then you know the sign and scale of the entire image. (This key point is derived from a mathematical proof that they may have neglected to teach to you.)

If you're still unsure of how to interpret your results.. add a pinhole camera (a very small aperture with a screen a short way behind it) at the side of your page, and figure out exactly what colour of light ray will be illuminating each different part of its screen. Compare what this "eye" sees to what it would see if the lens/mirror were not there.

Last edited: May 6, 2010