How do your eyes form images?

You've just read the title of this post, your eyes, splendid byproducts of millions of years of evolution, often taken for granted, were able to distinguish the strokes and curves of the letters and their spacial orientation, allowing you to read and make sense of the sentence. However, did you ever ponder how your eyes actually form these images, for example how they perceive this giant eyeball? This statue is located in Dallas, if you're interested.

As you may already know, you can perceive objects, because the light reaching them bounces back and enters through your pupils. A first step to understand how this amazing organ - your eyes, work, is to understand light itself.

Light is a form of energy that we call electromagnetic radiation. Electromagnetic radiation has different wavelengths, ranging from radio waves which are the longest wavelengths, to the shortest gamma rays. Most of these wavelengths are imperceptible to the human eye, but there is an array of wavelength that is visible, which we call visible light, ranging from 400 to 700nm. For sake of simplification, we'll call light when referring to visible light.

Light that bounces off an object, emanates in all directions of space, from every point of this object. To form an image, light has to be sorted. This is accomplished by the pinhole of our eyes, called pupil. The pupil is small enough to filter a few rays of light from each point of the object, producing an inverted image. However, the aperture of the eye alone would at best produce a blurred image. In fact, other translucent components of the eye (the aqueous and vitreous humor, as well as the lens inside the eye), being denser than air, will bend the light angle, to produce a clearer picture. We can thus say that the combination of the small aperture of the eye and its translucent components, are responsible for a clear picture.

This is not the whole story however. To perceive it as an image, light can't simply be filtered and projected into the eye, it needs an interface to be sensed. This interface, is called a photoreceptor, which is a neuron located at the back of the eye and it contains photopigments. We're now at the chemical part of forming images. An extremely sensitive light pigment or photopigment called rhodopsin, possesses a double bond in the dark, making it static or non-rotational. When light hits this photopigment, the double bond disappears momentarily and the opsin part of the rhodopsin molecule changes conformation and activates.

Opsin, which is an enzyme, is responsible for a cascade of enzymatic reactions in the photoreceptor, leading to the closure of gates called sodium ion channels on the neural membrane of the photoreceptor. Sodium ions (Na+) are positively charged, which means there's an increase in positive charges being locked out of the neuron. Meanwhile, the interior becomes more negative and this increase in negativity, changes the voltage enough (between the interior and exterior of the membrane), resulting in an action potential that travels to the optic nerve and to the brain. The brain then processes this signal and renders the final picture, which is the one that you're seeing right now.

Sodium ion channels open in the dark and close in the light

- Lifeyard

Sources

- Brain and Space, a course from the Duke university