Image formation in the eyes

The human eye, a masterpiece of biological engineering, is the gateway to our visual world. This intricate organ allows us to perceive the beauty and complexity of the world around us. In this article, we'll delve into the simple physics behind image formation.

Key Components of the Eye

• Cornea: This is the doom-shaped surface at the front of our eyes.

• Iris: The colored part of the eye that surrounds the pupil. It also controls the amount of light that enters.

• Pupil: the black hole at the center of the iris and the light passes through it.

• Lens: A transparent structure behind the iris that helps to focus light on the retina.

• Retina: The light-sensitive layer at the back of the eye that contains photoreceptor cells that convert the light into electrical signals.

Image Formation

Here’s how images are formed in the brain. 

• Light Entry: Light rays enter the eye through the cornea, the transparent outer layer that is shaped like a dome.

• Pupil Adjustment: The iris, the colored part of the eye, controls the amount of light entering by adjusting the pupil’s size.

• Lens Accommodation: The lens focuses the light rays onto the retina to form a real, inverted image on the retina.

When a person observes a near object like their digital screen, the ciliary muscles surrounding the lens decrease in size, so the ligaments pulling the lens relax, leading the lens to look rounded. This adjustment enables the person to gain greater focus by refracting light. Note that this process is a reflex response not controlled by the brain.

• Retinal Signal Processing: The retina contains specialized cells called photoreceptors. These cells convert light energy into electrical signals.

• Neural Transmission: The optic nerve carries these electrical signals from the retina to the brain.

• Brain Interpretation: The brain processes and interprets the signals as an upright image. You may wonder why the image was upside down. The reason is the cornea bends light when it enters the eye so the brain turns the image up when it processes the information.

Some Common Refractive Errors Corrected by Glasses:

• Nearsightedness (Myopia): People with this condition can see close objects but find it difficult to see far-away objects. The image is formed in front of the retina so it doesn’t get translated into signals to be processed by the brain. Concave lenses in glasses help to diverge light rays before they enter the cornea, allowing them to focus properly on the retina by going through the eye’s lens.

• Farsightedness (Hypermetropia ): People with this condition can see far-away objects clearly but have difficulty seeing close objects. The image is formed behind the retina.  Convex lenses in glasses help to converge light rays before they enter the eye, allowing them to focus properly on the retina.

The human eye is a marvel of biological engineering, capable of capturing and processing visual information with remarkable precision. By understanding the interplay of light, lenses, and neural pathways, we gain a deeper appreciation for the complexities of vision.

References

National Eye Institution. Parts of the Eye (2019) [online] Available at: https://www.nei.nih.gov/sites/default/files/2019-06/parts-of-the-eye.pdf (Accessed: 01 December 2024).

National Eye Institute (2022). How the Eyes Work | National Eye Institute. [online] Nih.gov. Available at: https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work. (Accessed: 01 December 2024). 

Carr, B.J. and Stell, W.K. (2017). Figure 7. [The effect of ciliary muscle...]. [online] www.ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK470669/figure/myopia.F7/.  (Accessed: 01 December 2024). 

AccuVision - The Eye Clinic. (2022)  Understanding Eye Accommodation: What Does It Mean? [online] Available at: https://www.accuvision.co.uk/glossary/eye-accommodation/.  (Accessed: 01 December 2024). 

Sightsavers. (2017). How do the eyes work? Parts of the eye | Sightsavers. [online] Available at: https://www.sightsavers.org/eye-conditions/how-the-eye-works/.

www.wasatchview.com. (2012) Wasatch View Eye Care | Eye Doctor in South Jordan & Lehi. [online] Available at: https://www.wasatchview.com/refractive-conditions.