The Shadow Knows!

Look at your computer monitor.

Now, hold a pencil vertically, close to your face, to one side of the monitor. Suddenly snap it across to the other side of the monitor. Keep it vertical all the way, and move it as fast as you can.

Why do you see a series of pencil shadows?

Your computer monitor is emitting light.

Normally, when you hold an object in front of a light, you see its entire shadow. (But don't believe us! Try it!) The light from your computer screen is going on and off 60 times a second or more. So each time the light flashes back on, you see another pencil shadow.

How many pencil shadows do you see?

That depends. The faster you move the pencil across the screen, the fewer times your monitor light flashes on and off behind it - which means fewer shadows.

What gives? If the pencil's vertical, why are its shadows slanted?

Most monitors don't simply flash on and off. In fact almost all of your screen is dark almost all of the time (unless you are using a flat-screen like those on a laptop).

The monitor paints pictures using just one horizontal line at a time (wanna see a scan line?), starting with the top line and quickly scanning to the bottom. (Can you change the direction your monitor scans?) You only see a bit of the pencil silhouetted in front of the moving line. And since you move the pencil across the screen as the line of light moves down, you see the vertical pencil's shadow stretched out to a diagonal.

Which way do the diagonal shadows slant?

Assuming your computer paints pictures from top to bottom:

• If you move your pencil from left to right across your screen, you see shadows that slant from top left to bottom right.
• If you move your pencil from right to left, you see shadows that slant from top right to bottom left.

While you're at it, why not throw your pencil a curve?