Our ability to see is fascinating, yet something that we usually take for granted. Young children spontaneously experiment with the way they see — looking at objects with one eye closed, spinning a sparkler to see a ring of light, or looking through a grandparent’s spectacles. 

Middle school students ask questions like, ​‘Why do we need two eyes? ’ and ​‘Why does a ceiling fan moving at full speed look blurry? ‘. These early experiments and questions invite an exploration of human vision. 

This article (and the accompanying activity sheets) looks at some aspects of human sight that students can discover through simple, yet interesting, activities.

Depth perception

The left and right eye are in different locations, about 6.5 cm apart. Each eye receives a slightly different two-dimensional image onto its retina. Both these images are sent to the brain. The brain uses both to generate a single three- dimensional interpretation. 

This results in our capacity for stereopsis or depth perception as demonstrated in Activity I (see Activity Sheet: Are two eyes better than one?).¹

The ability to perceive depth is one of the advantages of having binocular vision (using two eyes to see one image).

The blind spot

We tend to believe that what we perceive is exactly what our eyes see. However, this isn’t always true and parts of what we see are ​‘made up’ by the brain!

Each eye has a blind spot — a small part of the visual field of each eye where there are no photoreceptors (rods or cones) on the retina (see Fig. 1).

This is the region where the optic nerve enters the eye. As demonstrated in Activity II (see Activity Sheet: Finding your blind spot), you are unable to see the black dot when the paper is held at a particular distance from you because the image of the dot falls on the blind spot on your retina.¹

Why don’t we notice the blind spot more often? This is because the brain compensates for the blind spot by adding in the missing information.

This phenomena is called visual filling in.² Notice that in Activity III (see Activity Sheet: Filling in), your brain fills in the image even when your other eye is closed. This is an example of how your brain doesn’t rely only on the images it receives from the retina. It ​‘predicts’ what it is likely to see. The predictions that the brain makes are so convincing that we often ​‘see’ things that don’t exist! This is the basis for many optical illusions.

Moving images

Optical illusions can also arise because of another physiological limitation of the visual system — the speed at which it can process images that fall on the retina.

One application of this is in the Thaumatrope — a popular optical toy in the 19^th century. As seen in Activity IV (see Activity Sheet: Using a Thaumatrope), the two images on either side of the disk appear to merge into a single image when the disk is twirled quickly.³ This is because of the time that our visual system takes to process the images that fall on the retina.

The visual system retains an image for about a 15^th of a second. When a different image falls on the retina within this time period, the two images are perceived as a single one. This phenomena is called the persistence of vision.

When our visual system sees a series of 10 – 12 distinct images in a second, they are perceived as a single continuous moving image. This optical illusion is called beta movement, and is the basis for animations and films. The Phenakistoscope, that you build in Activity V (see Activity Sheet: Using a Phenakistoscope), was the first popular animation device to be built.

Invented in 1832, this device was an early precursor to modern day animations (see Fig. 2)

A flip book is another simple toy that you can make based on the same principle (see Fig. 3)⁴

Optical illusions

Optical illusions are perplexing, fascinating, and fun. They occur when what we perceive appears different from what is actually present, because of the interaction between the images the eye receives and the interpretations that the brain makes (see Fig. 4).

To conclude

There are hundreds of optical illusions like the ones in the activities discussed here. Each of these illusions reveal different aspects of human vision and its limitations (see Box 1). Many of them are the subject of ongoing scientific research in human physiology, psychology and neuroscience.

Key Takeaways

• Sight is inherently intriguing to young children, and they often spontaneously experiment with the way they see.
• What we see is a result of interactions between our eyes and our brain.
• We can explore vision to introduce students to related scientific concepts and their practical implications.
• Our ability to perceive depth is a result of our brain’s ability to combine two-dimensional images from both our eyes.
• Each of our eyes has a blind spot where the optic nerve enters it. We don’t notice the blind spot because our brain fills in the missing information.
• When two or more images fall on the retina within the 15^th of a second, they are perceived as single image.
• Optical illusions occur when our brain’s perception of images is different from what is actually present.

References:

1. ‘The Blind Spot’. Chudler, Eric H., Neuroscience for Kids — Vision Exp. URL: http://​fac​ul​ty​.wash​ing​ton​.edu/​c​h​u​d​l​e​r​/​c​h​v​i​s​i​o​n​.html.
2. ‘Filling-In’. Wikipedia, Wikimedia Foundation. 18 Apr., 2018. URL: https://​en​.wikipedia​.org/​w​i​k​i​/​F​i​l​l​i​ng-in.
3. ‘The Historians Toy box — Children’s toys from the past you can make yourself’. Provenzo Jr, Eugene F, et al. Prentice Hall, 1979. Retrieved from the URL: https://​archive​.org/​d​e​t​a​i​l​s​/​T​h​e​H​i​s​t​o​r​i​a​n​s​T​o​y​B​o​x​/​p​a​g​e/n55.
4. ‘How to make a flipbook’. Andymation, YouTube. Uploaded on 19 Jan., 2018. URL: https://​www​.youtube​.com/​w​a​t​c​h​?​v​=​U​n​-​B​d​B​SOGKY.

About the author

Kavita Krishna graduated from IIT Madras and worked as an engineer for 10 years. She retrained as a teacher and has over 15 years of experience in teaching, curriculum development, and teacher education in rural and urban schools. She can be reached at kavitak2006@​gmail.​com.