A thin beam of light is often called a ray.
When we stand in the sunlight, some of the light is stopped while other rays pass on in a straight-line path.
A shadow is formed where light ray can not reached.
This java applet let you play with shadow and image.
To make it more fun, there are 3 different colors (Red/Green/Blue) light source.
You can also study the mixing of colored light.

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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.

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)

You can select point light source or rod shape light source from the list.

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!