The eye is the organ responsible for sight. It converts light into electrical signals and sends them to the brain. As mammals, our eyes are placed at the front of the head to allow binocular vision; this gives us depth of vision. This section will delve into the mechanism of light perception and the Physics of the eye.
Physics of the eye: Vision
The eye is a spherical structure. It can perceive images thanks to the refraction that occurs within the inhomogeneous liquid that fills it. This liquid is called vitreous humor. The light arrives at the cornea, the most external part of the eye, and this focuses it towards the pupil, whose aperture is regulated by the iris. The iris is also the part that gives our eyes their color. The stroma can be from light brown to black and even green, blue or hazel, thanks to the phenomenon of light scattering on this surface.
Behind the pupil is the lens that projects the light to the retina on the opposite side of the eye. The optic nerve there captures the image, transforms it into an electrical signal and sends it to the brain.
Figure 1.- Structure of the eye. Source: Wikipedia (CC by 3.0)
There are many types of eyes, subdivided into two categories, compound and non-compound. These have further subcategories:
- Non-compound eyes, characterized by having pit, spherical lenses, multiple lenses, refractive cornea, reflector eyes.
- Compound eyes. Typical of arthropods like insects and crustaceans. Mantis, flies, lobsters, bees and so on have this kind of eye structure. characterized by having apposition, superposition, parabolic superposition, others.
Figure 2.- Detail of the compound eye of a fly. Source: USGS, public domain.
Physics of the eye: The eye as a lens
To understand the main mechanism that makes the eye work, we can examine how a lens works and what it is. A lens is a transparent interface that allows visible light to pass through, ideally without any reflections (loss in the incident energy).
Not all lenses are transparent, some are semi-transparent, and others are opaque. A lens can be concave or convex.
Figure .3 - light rays (in blue) passing through a lens. The light is focused at a point called focal point, in red. In the eye, the same mechanism occurs, and the focal point corresponds to the retina, Toma - StudySmarter Originals
Physics of the eye: Light and color processing
Visible light has two qualitative aspects, intensity and color. Inside the retina, there are photosensitive cells called rod cells and cone cells or just cones. Cone cells are situated in the center of the retina and have less sensitivity to light than rod cells.
There are three types of cone cells in the eye, and each type can catch a different range in the light's spectrum. L-cones are those associated with lower wavelengths; hence the name, particularly with the color blue; M-cones are associated with green and S-cones with red. M and S stand for middle and small, respectively, referring to the wavelength they can sense.
Rod cells are on the outer edges of the retina. There are twenty times more rods than cones, and their sensitivity is a five hundred to a thousand times more than that of the cones. They are responsible for night vision and peripheral vision, but they are not involved in the perception of colors. This explains not only our trichromatic vision but also why we cannot distinguish colors at night.
Figure 4.- Spectrum of trichromatic vision, the peaks correspond to the maximum values o sensitivity o the cone cell in the human eye. Source BenRG, public domain.
Physics of the eye: Defects of vision
The eye is a sphere to catch the light from a wide angle, but a spherical lens will alter an image (Figure 5). The inhomogeneous liquid corrects this inside the eye because it has a refractive index higher than the air. This avoids image alterations.
Figure 5.- A drop of water alter light after passing through it. Source: Nan Fry (CC by-SA 2.0 ).
Sometimes floaters form in this liquid, little clumps of viscous material inside the eye. They often appear within the range of vision but are harmless.
Alterations of the structure can cause defects in vision. For example, myopia occurs when the focus of the eye's lens is in front of the retina instead of on it. These distant objects to be blurry causes. When the focus of the lens is behind the retina, we talk of hypermetropia.
How can we correct defects of vision?
Many things can go wrong with vision. Corrections range from using glasses to surgical operations depending on which part of the eye is compromised or not working properly.
Physics of the eye: Glasses and lenses
When the problem is an alteration of focus within the eye, it is possible to correct it with appropriate lenses. This works by modifying the angle of incidence of the light to correct the focus. For instance, a person with myopia needs lenses that change the angle of the light to focus further away. If this is combined with the shortened focus of the eye, the result is the correct alignment of the image on the retina.
Figure.6 - Correction of hypermetropia. The lens alters the incoming light in a way that the eye's lens focuses on the retina, Camacho - StudySmarter Originals
Physics of the Eye (A2 Only) - Key takeaways
- Eyes are structures that allow us to perceive images. Eyes focus the light that approaches the cornea to reach a photosensible part of the eye called the retina.
- The retina has two kinds of cells called cones and rods. These are responsible respectively for color vision and night vision. Cones are specialized in detecting colors but are less sensitive to light, and rods are more sensitive to light but unable to distinguish between colors.
- The eye is spherical, and this allows a wide range of sight. It also has an inhomogeneous liquid inside it, which makes corrections using refraction.
- It is possible to use lenses to correct focus-related sight problems. These modify the light approaching the eye in order to achieve the correct incidence on the retina.
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