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An Interactive Müller-Lyer Illusion Demonstration

The top horizontal line is called the standard line; the bottom line is called the comparison line. Your task is to adjust the bottom comparison line so that it is the same length as the top standard line. You can change the length of the comparison line by holding down (or clicking) the "Increase line" or "Decrease line" buttons. When you think the two lines are equal, click on the "Equal?" button. If the two lines are equal, the bottom comparison line will turn green. If they are not equal, it will turn red. So:

1st)  adjust the length of the comparison line by using the "Increase line" and "Decrease line" buttons.
2nd) when you think the two lines are equal, click on the "Equal?" button.
3rd) if you fail, click on either end of the standard line and the comparison line to remove the arrow heads and try again. Click again to restore the arrow heads. (you can also use your mouse to drag the comparison line closer to the standard line).

If you don't see anything but black, press this button

Holding or clicking these buttons will change the length of the comparison line

Holding or clicking these buttons will change the angle of the comparison line's arrow heads

This button will test if the standard & comparison lines are equal. If they are, the bottom line will turn green. Else it will turn red.

This button will reset the comparison line to new state

NOW PLAY WITH THE ILLUSION!

As you can see, many factors affect the strength of this illusion. By playing with this demonstration, we can see that the illusion distorts our perception of the length of the comparison line. But if we only rely on such simple, casual observation, it is very difficult to know, exactly, how much our perception is distorted, and how the strength of the distortion will vary under different conditions. To answer these questions, we need to use experimental psychological techniques. We therefore now invite you to participate in a short 10-15 minute on-line experiment.