Through our eyes we see light, and the way light interacts with the objects around us tells us a great deal about our surroundings. What we see also depends on how images are sensed by the chemical reactions in our eyes, and how they are interpreted by our brains.

Read on for some examples of visual and optical illusions:

 


TWO ILLUSIONS: Optical and Visual

 

Excerpted from the 2016 "Once Upon a Christmas Cheery, In the Lab of Shakhashiri"


REFRACTION


A pencil propped between the bottom and side
of an aquarium partially filled with water.


The path followed by a beam of light can change direction when it passes from one transparent medium into another. When a beam of light enters a flat slab of clear, transparent glass at an angle, the beam of light changes direction as it enters the glass. The beam is bent so that the angle is smaller in the glass than in the air. This change in direction of light when it passes from one transparent medium into another is called refraction.

You can easily observe refraction if you partially immerse a pencil in water so some of it is in the water and some in the air (see photo above). The pencil seems to bend where it enters the water. As it is lowered or raised in the water, the place where it seems to bend stays at the surface of the water.

Other phenomena caused by refraction include "heat waves" rising from the highway on a hot day, desert mirages, or the shimmering you observe when one liquid is poured into a different liquid. In these cases the changes of medium are from warmer to cooler air or from one liquid to another. All these refraction phenomena are a result of the different speeds of light in the different media: air and water, cooler and warmer air, and different liquids, for example.

Try this at home with your own pencil, and a glass of water!


THE SPINNING DANCER: A Visual Illusion

 


Image by Nobuyuki Kayahara, Procreo Flash Design Laboratory

What direction is the dancer spinning, clockwise or counter-clockwise?

Does she change as you continue to observe her?


THE PERCEPTION OF BRIGHTNESS IS RELATIVE

Our eyes are sensitive to very small differences in brightness. Our perception of the brightness of a particular area depends on the relative brightness of its surroundings. Our perception is also influenced by what we expect (from experience) to see.

Study the checkerboard image above.

Look at the square in the center of the checkerboard and the second square from the right in the front row of the checkerboard.

Here is an image that indicates these squares:

Are they the same shade of gray?

Even though it may not appear so, they are the same. Perhaps this animation will help:

Can you see that they're the same shade now?

 

You can make your own version to show your friends and family. Click on the large checkerboard above to open a printable version of the checkerboard. It is sized so that you can print it at home on a piece of regular copy paper. Once you've printed it, take another blank sheet of paper and lay it over the top, so you can trace the two squares. Cut those squares out of the blank sheet, and lay it back over the checkerboard, as is shown in the animation above.
Unless otherwise noted, the material above was adapted from
Chemical Demonstrations: A Handbook for Teachers of Chemistry
Volume 5
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