This wonderful simulation from Fu-Kwun Hwang is a beautiful way to explore the mixing of coloured light and shadows.

Instructions

This Java applet lets you play with coloured light and experiment with colour mixing and shadow.

Click different color checkbox to turn on that light source. (Default: Red light)

Click "show trace" button to display light paths.

There is a screen on the left. An object is located between light source and the screen. This object causes areas of the screen to be in shadow.

Left click the object and drag the mouse to move the object.

Right click the object and drag it up/down to change the length of the object.

If you click the mouse button twice continuously, it is toggle between an object or a slit.

(Use similar method to change the width/location for the slit)

1. Point source:

Click the light source and drag it to different location.
2. Rod source:
change its location:
Click the left mouse button inside the region of the light source and drag it.
change its length:
Click the right mouse button inside the region of the light source and drag it down.

Have fun ! Play with it and try with different cases!

There are more of Fu-Kwung's brilliant applets at http://www.phy.ntnu.edu.tw/ntnujava