Fool Your Vision to Find Out How It Works
OverviewHow is it possible that our eyes can see things that are not really there? In this fun lesson plan, your students will explore how our vision works with the help of two short experiments that involve some fascinating optical illusions. Let your students discuss why they see a hole in their hand, or why they see colors that were never there, and let them construct their own explanations.
- Understand the main components of the visual system and how they work together.
- Formulate and defend a hypothesis based on experimental evidence for illusions created by fatigued sensory cells or for optical illusions created when the two eyes look at a different object.
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- MS-LS1-8. Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.
Science & Engineering Practices
Planning and Carrying Out Investigations. Conduct an investigation to produce data to serve as the basis for evidence that meet the goals of an investigation.
Engaging in Argument from Evidence. Use argument supported by evidence to support or refute an explanation or a model for a phenomenon.
Disciplinary Core Ideas
LS1.D: Information Processing. Each sense receptor responds to different inputs (electromagnetic, mechanical, chemical), transmitting them as signals that travel along nerve cells to the brain. The signals are then processed in the brain, resulting in immediate behaviors or memories.
Cause and Effect. Cause and effect relationships may be used to predict phenomena in natural systems.
Systems and System Models. Systems may interact with other systems; they may have sub-systems and be a part of larger complex systems.
Materials per group of two students:
- Computer with internet access, or a color printout of the student version of Figure 4. As an alternative, a projector and white surface or smart board can also be used to project the image for the entire class.
- Stopwatch or clock that counts seconds
- Colored pencils (at least yellow, light blue or cyan and purple or magenta) or a basic computer graphics program
- White paper, 8.5 by 11 inches
- Clear tape
Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
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.