Backgrounders

How We See

Eyeglasses and a visual test chart

Eyeglasses and a visual test chart (DDurrich, iStockphoto)

Eyeglasses and a visual test chart

Eyeglasses and a visual test chart (DDurrich, iStockphoto)

Biology,  Physics
Let's Talk Science
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Learn about how human vision works as well as some common types of vision problems.

Vision is the sense we know the most about, but how do we see? Most people would say, “That’s easy, our eyes see for us!” In truth, it is more complicated than that. It involves our brains as much as our eyes - if not more!. Let’s take a look at how this works from the beginning.

 

Cones help us see colour. They are found mainly in the fovea, which is at the center of the retina. The fovea is the area of the retina where an image falls when the viewer is looking directly at something. Rods are very important for seeing movement. Unlike cones, rods only transmit information to the brain in black and white. Rods tend to be found more at the edge of the retina. So, if you are trying to figure out if a car is red or blue, you use your cones. When you want to catch a baseball, you use your rods. This is not to say that you only ever just use rods or cones. Most of the time you are using both!

All of the information from the rods and cones leaves each eye through the optic nerves. These never cross over at the optic chiasm. This is so that both sides of the brain get information from each eye.

From the optic chiasm, the information passes through the lateral geniculate nucleus, or LGN. Then it goes to the primary visual cortex (also known as V1). This part of the brain is found in the occipital lobe.

Sight and brain pathway
Sight and brain pathway (Source: Let’s Talk Science using an image by VectorMine via iStockphoto).
Image - Text Version

Shown is a colour illustration of a human head, showing how the eye and brain see a green apple. 

The head is shown from the side, looking toward the left edge of the illustration. A green apple is left of the head, in front of the eye. 

Two dotted, diagonal, straight lines lead from the top and bottom of the apple, into the eye. This is a pink sphere with clear, round, raised structures at the front, labelled "Eye."

The lines cross over each other in the middle of the pink sphere. A smaller, upside down image of the apple appears on the back inside wall of the eye. A tube leading from the back of the eye to the brain is labelled "Optic Nerve." A small, pale blue area halfway along the nerve is labelled "Optic Chiasm."  

The brain is depicted as a pink, lumpy, oval structure. The visible side is labelled "Left Hemisphere." A line at the top edge leads to a label that reads, "Right Hemisphere (opposite side)."

The optic nerve leads to a pale blue circle near the centre of the brain. This is labelled "Lateral Geniculate Nucleus." From here, white curving lines fan out toward a region in the back upper part of the brain. 

This area is coloured in pale blue. The top left part of it is labelled "Occipital Lobe." The lower right part is labelled "Visual Cortex."

Did you know?

The left side of the brain is called the left hemispheres and the right side of the brain is called the right hemisphere.

So, why do we need a brain to see? For one thing, when an image in the form of light hits the lens, it is flipped upside down. It is also reversed from left to right. Information in this form wouldn’t be very helpful to us!

One of the first things the brain does is to take the information sent from the eye and flip it right side up and right to left. The occipital lobe in the brain can then process the corrected information. Eventually, this information gets to a part of the brain where you become consciously aware of it and you can see! You have now experienced perception! This may not seem exciting because you do this all the time, but it really is quite amazing. How we see is very complicated, and scientists still have much to learn.

When people have normal vision, an image is formed on the back of the retina, near the optic nerve. For some people, the shape of their eye can cause the image to appear in the wrong place. This can make the image blurry.

Eye with normal vision
Eye with normal vision (Let’s Talk Science using an image by ttsz via iStockphoto).
Image - Text Version

Shown is a colour illustration of an image of an apple formed in an eye with normal vision.

The eye is shown from the side, as if cut in half. It is shown as a pink sphere, with the cornea, pupil and lens labelled on the left. The front of the eye is facing left, where a green apple is in front of the eye.

Straight, blue, diagonal lines lead from the top and bottom of the apple, getting closer as they move towards the eye. These lines travel through the cornea, pupil and lens. They cross over each other in the centre of the pink sphere of the eye.  

The crossed lines move apart again until they hit the retina at the back of the eye. Here, another, smaller green apple is shown upside down.

Individuals with myopia are commonly known as short-sighted or near-sighted.

This means they are able to focus on objects that are close up and not on objects that are far away. Myopia happens when the light that enters the eye lands in front of the retina instead of where it should.

Myopia can occur if the eye is too long front to back. We call this axial myopia. It can also occur if the cornea or lens are the wrong shape. Both of these things can cause the image to be focussed in front of the retina. This is why distant objects appear blurry.

Diagram of myopia and lens correction with a biconcave lens/Diagramme de la correction de la myopie par une lentille biconcave
Diagram of myopia and lens correction with a biconcave lens (Let’s Talk Science using an image by ttsz via iStockphoto).
Image - Text Version

Shown are two colour illustrations of an image of an apple formed in an eye with myopia, and and the same eye through a biconcave lens.

In both illustrations, the eye is shown from the side, as if cut in half. It is shown as a pink sphere, with the cornea, pupil and lens labelled on the left. In front of the eye is a green apple.

Straight, blue, diagonal lines lead from the top and bottom of the apple, getting closer as they move towards the eye. These lines travel through the cornea, pupil and lens. 

In the top illustration, the lines cross over each other near the front, inside the pink sphere of the eye.

The crossed lines move apart again until they meet an image of a smaller, upside down green apple. This image is not on the retina, but in the middle of the pink sphere, near the back. 

In the bottom illustration is a blue lens with two edges that curve inwards. It is placed between the apple and the eye. Here, the diagonal blue lines leading from the apple cross over each other in the centre of the pink sphere. The image of the smaller, upside down apple is formed on the retina, on the back wall inside the eye.

If a person is found to have myopia, an optometrist can recommend getting corrective lenses. The shape of the lenses depends on the person’s degree of myopia.

Myopia is corrected using a biconcave lens. When the concave lens is placed in front of the eye, it causes the light to diverge or spread out. This pushes the focal point further back onto the retina. By refocusing the image on the retina, the person is able to see clearly.

Individuals with hyperopia are commonly known as far-sighted.

This means they are able to focus on objects that are far away and not on objects that are close up. Myopia happens when the light that enters the eye lands behind the retina instead of where it should.

Hyperopia can occur if the eye is too short front to back. It can also occur if the cornea or lens are the wrong shape. This is why distant objects appear blurry.

In extreme cases, distant objects can also appear blurry.

Diagram of hyperopia and lens correction using a biconvex lens/Diagramme de l’hypermétropie et de la correction avec une lentille biconvexe
Diagram of hyperopia and lens correction using a biconvex lens (Let’s Talk Science using an image by ttsz via iStockphoto).
Image - Text Version

Shown are two colour illustrations of an image of an apple formed in an eye with hyperopia, and and in the same eye through a biconvex lens.

In both illustrations, the eye is shown from the side, as if cut in half. It is shown as a pink sphere, with the cornea, pupil and lens labelled on the left. In front of the eye is a green apple.

Straight, blue, diagonal lines lead from the top and bottom of the apple, getting closer as they move towards the eye. These lines travel through the cornea, pupil and lens.

In the top illustration, the lines cross over each other near the back, inside the pink sphere of the eye.

The crossed lines move apart again until they meet an image of a smaller, upside down green apple. This image is not on the retina, but beyond the back wall of the eye, on the white background of the illustration. 

In the bottom illustration is a blue lens with two edges that curve outwards. It is placed between the apple and the eye. Here, the diagonal blue lines leading from the apple cross over each other in the centre of the pink sphere of the eye. The image of the smaller, upside down apple is formed on the retina, on the back wall inside the eye.

Hyperopia can be corrected using refractive biconvex lens. When the convex lens is placed in front of the eye, it causes the light to converge or come together. This pulls the focal point forward onto the retina. By refocusing the image on the retina, the person is able to see clearly.

As people get older, their eyes get weaker. This makes them less able to focus on nearby objects, like small print and details. The name for this is Presbyopia. It usually starts after 50 years of age. Presbyopia happens when the lens becomes less flexible. Presbyopia tends to get worse over time, but it can be corrected with convex lenses.

Instead of wearing eye glasses, people can correct both myopia and hyperopia by wearing contact lenses. Contact lenses are thin lenses that are placed directly on the eye. When fitted properly, contact lenses can be very comfortable to wear.

Some contact lenses are thrown away and replaced after one use. Others can be cleaned and stored in a special solution overnight before being worn the next day. Typically these reusable contact lenses need to be thrown away after two weeks or a month, depending on what they are made of.

A person putting in a contact lens/Une personne qui met une lentille cornéenne
A person putting in a contact lens (Source: איתן טל [CC BY] via Wikimedia Commons).
Image - Text Version

Shown is a person holding a clear, round lens on one fingertip, close to their right eye.

The lens is in the foreground of the photograph. It looks like a tiny clear glass cereal bowl. It is small enough to sit on the tip of the person's finger. 

In the background, the right side of the person's face takes up more than half the image. Their eye is open and they look towards the right edge of the photograph. They are holding the lens  up, just in front of their eye.

There are a number of different reasons why people choose to wear contact lenses instead of glasses. This includes doing physical activities, such as playing sports, when glasses can get damaged. It also includes not liking the appearance of glasses, and not wanting to limit peripheral vision. Contact lenses are a good alternative to glasses for many people, although often contact lenses cannot totally substitute for glasses. Most people choose to correct their vision with a combination of both types of lenses.

Learn More

The Visual System: How Your Eyes Work (2016)

This video by the National Eye Institute (2:20 min.) explains how your eyes and your brain work together to gather information and make sense of it.

How do glasses help us see? This video by TED-Ed (4:23 min.) explores the history of glasses and how they use refraction to correct vision problems.

References

Bianco, C. (n.d.). How vision works. HowStuffWorks.

The Children's University of Manchester. (n.d.). How the eye works. University of Manchester.

IslandRetina. (n.d.). How vision works.

Related Topics

Human Eye
Vision
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