Fool Your Vision to Find Out How It Works

We see with our eyes and our brain, as shown in Figure 1. Our eyes register incoming light; electrical pulses transport the information to the brain, which processes it and informs us of what we see.

Diagram showing the parts of an eyeball and brain that are used to see an object. Light from a tree on the left enter the eye through the cornea and move past the iris and lens. The lens focuses the light onto the back wall of the inner eye where the macula and retina process the light and send signals to the brain through the optic nerve at the back of the eyeball. The optic nerve connects to the visual cortex at the back of the brain where the signals sents by the optic nerve is interpreted as a tree by the brain.

Figure 1. Anatomy of the human visual system. Note the eye and brain are not depicted to scale.

Vision starts with visible light—electromagnetic waves with wavelengths between 400 and 700 nanometers—reflected by an object and falling into our eye. These waves are the visual stimulus picked up in the area at the back of our eye (the retina) by light-sensitive cells (the sensory receptors) that send electrical signals to the brain when triggered. There are two types of sensory receptors for vision: rods and cones. Rods are very sensitive to light and mainly register movement, shape, and light intensity changes, while cones are responsible for our color vision and suited for detail; they need bright light. We are also equipped with different types of brain cells that process visual information, each of which conduct their own special tasks.

Humans can perceive a continuous spectrum of colors (or wavelengths) because we are equipped with three types of color-sensitive cone cells, each responding to a spectrum of wavelengths centered around a certain color (roughly red, green, or blue). Once light reaches the eye, each type of cone cell is stimulated differently by different wavelengths. The variance in signals from each type of cone cell allows the brain to perceive a continuous range of colors. This requires blending colors in the brain to obtain the different hues.

You might be familiar with mixing primary colors of paint to create a huge range of colors. A similar thing can be done with light. Subtractive light mixing applies filters to a beam of light and combines colors like paint colors mix. You can also overlap (or add together) two or more beams of different-colored light to create new colors, a principle that is used in most electronic visual displays. The commonly used primary colors for additive light mixing are red, green, and blue. These are exactly the color ranges that your three different cone types are most sensitive to. Figure 2 shows how the primary colors of light (red, green, and blue) combine to create secondary colors.

Figure 2. Combining colored light. Note how the three primary colors combined yield white light.

Cone cells, like many sensory receptor cells, can show fatigue after being exposed to a stimulus for an extended time, making them temporarily unresponsive. This can give rise to an afterimage—a perceived image that lingers after the real stimulus has disappeared. You might recall looking at a bright light, then seeing a dark, fading spot in your central vision once you have turned away. This was an afterimage. Light-sensitive cells in your central vision were fatigued and temporarily unresponsive. Afterimages have the same size and shape as the original image, but are in the complementary (opposite) color.

Humans have binocular vision, which means we use two eyes together to create one image. Each eye registers a slightly different image on the back of the eye as shown in Figure 3. The information of the two images is sent to the brain where it is processed. In a fraction of a second, our brain combines the information coming from the left and right eyes and brings one cohesive three-dimensional image to our awareness.

Figure 3. With binocular vision, each eye registers a slightly different image, allowing us to see in three dimensions.

Our visual system has evolved to serve us well. It is good and fast because it makes some intuitive assumptions. These also makes the brain vulnerable to being tricked. For example, the brain assumes that your eyes are focused on the same object. This is almost always true, but what if it is not? When these suppositions fail, we can get visual perceptions that differ from what is really there. We call these optical illusions.

In this lesson plan, you and your students will explore how our visual system works with the help of two fascinating optical illusions. Students will come up with their own explanations for these optical phenomena, present them, and discuss their validity.