An optical illusion is something that you see that is deceptive or misleading.
Most optical illusions occur because the brain incorrectly "reads" what
the eyes have seen. The three optical illusions shown here are examples
of distortion illusions. They get this name because the image tends to
distort the size or shape of an object.
The first illusion we will look at is called the Poggendorff Illusion. It
was first proposed by Johann Poggendorff in 1860. Look at the diagonal
blue lines on the top and bottom of the rectangle. Is it possible that
the rectangle is covering a straight line?
The answer is yes. Get a ruler and place it over the blue diagonal lines. You will see that they could be the same line! This illusion makes it appear that the two blue diagonal lines are offset (not directly lined up).
The next illusion is called the Ponzo Illusion and was first proposed by Mario Ponzo in 1913. Which of the two horizontal lines is the longest?
Actually both of the horizontal lines are exactly the same length! Get your ruler and measure them. In the Ponzo Illusion the line on top appears longer even though it is exactly the same size.
The next illusion is my favorite distortion illusion. It is called the Muller-Lyer Illusion and was first proposed in 1889. Look at the following two figures. Which vertical line is longer, the one in the figure on the left or the one in the figure on the right?
Believe it or not, both of the vertical line are exactly the same length! This is a fantastic illusion because the vertical line in the figure on the left clearly appears shorter than the vertical line in the figure on the right. If you still don't believe it, get your ruler and measure for yourself (I did when I first saw this illusion).