Illusions

Illusions

Is Seeing Believing?

Introduction

It is often said that seeing is believing, but can we really trust what our eyes are telling us? In this resource, students explore optical illusions and how our brain strives to make sense of the information it receives.

Objectives

• Understand that visual information is processed by the brain.
• Explain that the brain bases its interpretations on past experiences.
• Describe how optical illusions occur.

Background

An optical illusion occurs when the brain perceives something that does not reflect reality. In fact, the word illusion comes from the Latin verb illudere or "to mock.

Optical illusions can result in three ways:

1. Your brain makes faulty assumptions based on patterns you have experienced in the past (cognitive illusion),
2. Your eyes have structural quirks that misinterpret visual cues (physical illusion), or
3. Your eyes or brain react to something they've been observing for a long time (physiological illusion).

Your eyes are optical instruments, like a camera, microscope or telescope, but they adjust automatically. Your eyes can tell different colours apart, they can adapt very quickly to variations in the amount of light they're receiving, and they can focus themselves automatically.

The iris controls the quantity of light entering the eye. The lens focuses the light to form an upside-down image on the retina. Right at the beginning of your development as a baby, your brain learned to turn the reversed image right-side up.

The back wall of the human eye (retina) is lined with two kinds of receptors for distinguishing light: there are 12 million rod cells and 7 million cone cells. Rods respond to all wavelengths of visible light in low light conditions. They allow us to see objects when the lights are dim. Cones, on the other hand, respond to certain wavelengths, sending colour information to the brain.Describe how optical illusions occur.

There are 3 different types of cone receptors for coloured light: one is most sensitive to red light, one to green light, and one to blue light. With these three colour receptors we are able to perceive more than a million different shades of colour. Cones are less sensitive than rods, requiring higher levels of light. This is why it is difficult to make out the colour of objects in low light conditions.

Ghostly coloured or squirmy after-images can appear when rod or cone cells get tired. An image that your eye collects is just the first step in the creation of meaning. Where the information from your senses is unclear, incomplete or seemingly backwards, your brain works to understand the world by filling in gaps, ignoring data or making decisions, usually based on your previous experience. This interpretation may not reflect reality: it can result in an illusion.

Although this resource emphasizes optical illusions, both tactile and auditory illusions also result from the brain's faulty interpretation of data received from these senses. To reinforce this concept, tactile and auditory illusions have been included as activities and extensions.

Teacher's Tip:

We suggest that you rotate between the different types of illusions, ideally setting up stations around the classroom for students to visit in turn.

Entire Lesson

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Other Resources

Science World Resources | Full Lesson | Colour and Light
Science World at TELUS World of Science | Puzzles and Illusions