The Human Eye

The human eye is the organ which gives us the sense of sight. We use our eyes in almost every activity we perform, whether reading, working, watching television, writing a letter, driving a car, and in countless other ways. The eye allows us to see and interpret the shapes, colours, and dimensions of objects in the world by processing the light they reflect or emit.(i)

Figure 1.1 displays the anatomy of the human eye, whilst Table 1.1 describes its anatomical functions.

Fig 1.1 - The Human Eye
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Table 1.1 - Anatomy of the Eye
Structure
Description
Function
Conjunctiva
The conjunctiva is a thin, clear layer of skin covering the front of the eye, including the sclera and the inside of the eyelids.
The conjunctiva keeps bacteria and foreign material from getting behind the eye. It also lubricates and nourishes the eye.
Cornea
The cornea is a clear layer at the front and centre of the eye. The cornea is located just in front of the iris, which is the coloured part of your eye.
The main purpose of the cornea is to help focus light as it enters the eye.
Sclera
The sclera is a leather-like tissue which extends around the entire eye except at the front where it is continuous with the cornea.
The sclera surrounds the eye giving it its shape and protecting the internal structures of the eye. The sclera is also attached to the extraocular muscles, which helps in focusing.
Choroid
The choroid is a thin vascular layer between the sclera and the retina.
The choroid supplies blood to the retina and conducts arteries and nerves to other structures in the eye.
Retina
The retina is the inner layer of the eye which contains the two types of photoreceptors (i.e. rods and cones – further explored later).
Its primary function s to convert light into electrochemical signals so that the Central Nervous System is able to interpret and process the image.
Iris
The iris is a ring shaped tissue with a central opening, which is called the pupil. It is the coloured part of the eye.
The iris regulates the amount of light that enters the eye. It does this by either constricting (making it smaller) or by dilating (making it larger) the pupil.
Lens
The lens is transparent biconvex disc made of protein that is located just behind the iris and the pupil.
Its primary function is to focus light onto the retina as it passes through.
Aqueous and Vitreous Humour
The aqueous humour is a liquid which circulates the area between the cornea and the lens. The vitreous humour fills the vitreous cavity (area behind the lens and in front of the retina)
The aqueous humour helps maintain a constant inside the eyes. The vitreous humour helps maintain the shape of the eye. They also assist the lens by acting as a refracting medium for light.
Ciliary Body
The ciliary body is a structure that extends from cornea and the sclera and attaches to both the iris and the lens
The ciliary body is responsible for accomodation. Accomodation is the changing shape of the lens to focus at different distances. As the ciliary body relaxes the lens becomes round to focus at closer objects. When the ciliary muscle contract the lens becomes flatter for distant objects.
Optic nerve
The optic nerve, a bundle of over 1 million nerve fibers connected to the brain.
It is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information on an image for processing by the brain.

The Electromagnetic Spectrum

The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. Light is a particular type of electromagnetic radiation that can be seen and sensed by the human eye, but this energy exists at a wide range of wavelengths.

Fig 1.2 - The Electromagnetic Spectrum
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Light waves form part of the electromagnetic spectrum in the mid range of 400-700 nanometres in wavelength. This part of the spectrum is known as the visible light spectrum and comprises all the different clours of the rainbow from red to violet. The sun and stars emit much of their radiation in this part of the spectrum.

Table 1.2 - Range of wavelengths detected by different organisms
Type of organism
Organism
Section of the Electromagnetic Spectrum
Wavelength Detected (nanometers)
Vertebrate
Human
Visible
700-400

Rattlesnake
Infa-red and visible
850-480

Japanese Dance Fish
Ultra-violet and visible
as low as 360
Invertebrate
Honeybee
Ultra-violet and visible
700-300

Mantis shrimp
Ultra-violet and visible
640-400

Different groups of vertebrates and invertebrates line in different niches and these produce different obstacles for the organism. These niches may come in the form of electrmagnetic fields. Some animals have ingenious adaptations that use electricity and magnetism for survival.

For example: The ability to sense infrared thermal radiation evolved independently in several different families of snakes. Essentially, it allows these animals to detect radiant heat at wavelengths between 5 and 30nm. The pit organ in snakes is used extensively to detect and target warm-blooded prey such as rodents and birds. This particular adaptation is due to the environment they live in, which demands more than vision to provide them information for the presence of prey.

Fig. 1.3 - Pit organs in snakes
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Another example is found in bees. Bee eyes sense polariztion of visible light in the sky and also have a high sensitivity to UV light. Bees seem to detect blue light (max at 420nm) best however they are also able to detect ultraviolet colours beyond the blue light which humans are able to detect. Flowers often have sections which absorb or reflect UV light. UV fluorescence may be a common trait to most flowers, but might be of temporary occurrence for parts of the flower. Anthers, style, and pollen grains occasionally are seen to fluoresce. This information allow bees to effectively distinguish flowers which have pollen and those which do not. Figure 1.4 displays flowers in UV light, the markings indicate the presence of pollen within the flowers.

Fig. 1.4 - Flowers exposed to UV light
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Refraction of Light


Refraction occurs when a light ray changes mediums. Light travelling from air going into water would be an example. The speed of the light ray changes upon changing mediums and in almost every case changes the direction of the light ray.

Fig. 1.5 - Refraction of light (air to glass)
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Fig. 1.6 - Light entering the eye
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Refraction also occurs when light passes from air into the denser material of the cornea. The cornea has a density close to that of water, thus acting as a refractive medium fas light enters the eye. The light is then refracted again by the aqeuous and vitreous humour thought only having a minimal refractive effect. As it passes through the denser lens, extra refraction is provided so that it is focused onto the retina.

Accommodation is the changing shape of the lens to focus at different distances. If objects are close, the lens becomes rounder, if it is farther the lens flattens. Accomodation is achieved by the contraction and relaxation of the ciliary muscles of the ciliary body. For the proper focusing to occur on the retina, accomodation is necessary. Figure 1.7 displays the shape of the lens when focusing on nearby or distant objects.

Fig. 1.7 - Shape of the lens
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As the lens is flattened, the refractive power of the lens is decreased as light from distant objects reaches the eyes in almost parallel rays. This means that little refraction is needed for focusing the image onto the retina. However when objects are close, the ciliary muscles contract as light rays from nearby objects diverge as they reach the eye. By contracting the muscles the lens bulges or becomes much rounder, this is so that the focal length is shortened and the image is properly focused onto the the retina.
The video below explains the refraction of light as it enters the eye and the process of accomodation in greater detail.

Fig. 1.8 - How light enters the eye and accommodation


Myopia and Hyperopia

Fig 1.9 - Myopia and Hyperopia
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Nearsightedness, or myopia, as it is medically termed, is a vision condition in which close objects are seen clearly, but objects farther away appear blurred. Nearsightedness occurs if the eyeball is elongated or the cornea has too much curvature. As a result of this the image is focused before the retina thus a blurred image is processed by the brain. Causes of myopia are known to be visual stress or is hereditary however there are many ways which myopia is corrected. One way myopia can be corrected is with concave lenses for distance viewing, they cause light rays to diverge slightly before entering the eye. With this the focal length is increased so that the image is focused onto the surface of the retina. Contact lenses may also be used, having a similar purpose to lenses on the glasses.
Another way myopia can be corrected is through eye surgery. Refractive surgery can reduce or even eliminate your need for glasses or contacts. The most common procedures are performed with an excimer laser (An instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures.).
In Photorefractive Keratectomy (PRK) the laser removes a layer of corneal tissue, which flattens the cornea and allows light rays to focus closer to or even on the retina.
In laser-assisted in situ keratomileusis (LASIK) a flap is cut through the top of the cornea, a laser removes some corneal tissue, and then the flap is dropped back into place. see Figure 1.11

Fig. 1.10 - LASIK
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Farsightedness, or hyperopia, as it is medically termed, is a vision condition in which distant objects are usually seen clearly, but close ones do not come into proper focus. This is due to when light rays entering the eye focus behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal. Similarly to myopia, cerrective lenses may be prescribed to the patient or corrective eye surgery may be performed.
Convergent lenses must be prescribed to hyperopic patients, this is so that the light rays from nearby objects would converge before entering the eye and thus focusing onto the surface of the retina. PRK and LASIK can also be used on hyperopic patients. CK can also be performed, i.e. Conductive keratoplasty (CK) is a non-laser refractive eye surgery designed to correct mild hyperopia and help people who are middle-aged and older reduce their need for glasses after they become presbyopic.
In this procedure the eye surgeon uses a hand-held instrument with a tiny probe (smaller than a human hair) to apply low-level, radio frequency (RF) energy to specific spots that form a circular pattern on the outer part of the cornea.
Connective tissue then shrinks where the RF energy was applied, causing the circular band to act like a belt that "tightens" and steepens the cornea. This change in the curvature of the eye's surface affects the way light rays enter the eye to bring near vision back into focus.

Fig. 1.11 - LASIK eye surgery


Depth Perception


When the eyes face forward, each eye sees an image of an obeject in the light path. The two images are fused into on image in the cerebral cortex of the brain. This fusion into on image is related to the perception of depth.
Depth perception is the ability to judge the distance of an object from our eyes, it is the sense of depth that occurs when objects are viewed with binocular vision (The ability to maintain visual focus on an object with both eyes, creating a single visual image). The eyes are separated horizontally, enabling humans to have stereoscopic vision (the ability to see things in 3D). When an object is a slightly different distance from each eye, it is imaged by each eye at a different distance from the fovea. This gives the perception of depth as this image is fused and seen to be a different distance from the eye to another object that is closer to the eye.

Photoreceptor Cells


Photoreceptor cells are are highly specialized and differentiated neurons with stacks of photosensitive disks that contain rhodopsin and other quantitatively minorproteins in their outer segments.They generate impulses which travel back along various neuron layers to the optic nerve and then to the brain. There are two types of photoreceptor cells, these are rods and cones. These cells absorb light energy and convert light into electrochemical signals that the brain can interpret. See Fig. 1.12 for the structure of a rod and cone cell.
Fig 1.12 - Rods and Cones
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As the light hits the surface of the retina, the light energy travels through the retinal layers, i.e. through the ganglion cells, bipolar cells and towards the photorecpetor cells. Here rhodopsins and photopsins converts light energy into electrochemical signals (which consists of waves of sodium and potassium ions). It then travels back through the bipolar cells, to the ganglion cells then towards the optic nerve where it is then transmitted to the brain for processing and interpretation. See Fig 1.13 for Retinal Layers

Fig. 1.13 - Retinal layers
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Rods and Cones

Firstly rods and cones are modified neurons and are no distributed around the retina uniformly. Figure 1.14 indicates rod and cone density around the fovea.

Fig. 1.14 - distribution of rods and cones in the retina
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Rods are elongated rod-shaped cells that are sensitive to low levels of light and are unable to discriminate between colours. The image formed with the information provided by rod-cells lacks in detail due to it being insensitive to high intensities of light. There are approximately 125 million rods within the retina which are linked in groups of single neurons and are mainly found around the periphery of the retina. And as Figure 1.14 indicates, they are absent from the fovea. When the pupil is dilated more rods are exposed, therefore they are much more suited to night vision. Thee main purpose of rods is for the detection of light and shadow contrasts, primarily for peripheral vision.

As Figure 1.12 shows, cones are conical cells. They contain pigments sensitive to high intensities of light in three different forms so that they are able to distinguish between colours. They have extensive connections with the brain and produce a more detailed image. There is a large number of cones in the fovea as indicated in Figure 1.14 and approximately 6-7 million cones in each retina, this is primarily for maximum visual acuity. When the pupil contracts it will be mainly cones that are exposed and activated. In poor light, humans are unable to distinguish colours, this is due to cones requiring light of high intensity.

Rhodopsins and Photopsins

Rhodopsin is the light absobing pigments in the rods which are cimposed of a derivative of vitamin A called retinal, which is bonded to a protein called opsin. The retinal is the light-absobing part of the molecule whereas opsins are of different types which can affect the light absobing ability of the retinal. When light enters a rod cell, it splits rhodopsin molecules into its opsin and retinal. Activated pigment causes a change in electrical charges of the membrane of the rod. The two products slowly recombine, ready to be split again by more light. This is known as the visual cycle. Rods only contain one type of rhodopsin, and these are sensitive to blue-green light.

Cones contain three different photopigments. The trichromatic theory of color vision suggests that each is sensitive to a different range of wavelengths, corresponding to the three primary colors of red (long wave), green (medium) and blue (short). The sensitivity of these is broad enough to allow them to cover the full spectrum of visible light- overlap in some detected colors. The three photopsins are called erythrolabe , chlorolabe , and cyanolabe . Table 1.3 indicates the wavelengths each photopsin detects.

Table 1.3 - Types of Photopsins
Type of cone
Colour/Region
Wavelength
Peak range
Cyanolabe
Blue
400–500 nm
420–440 nm
Chlorlabe
Green
450–630 nm
534–545 nm
Erythrolabe
Red
500–700 nm
564–580 nm

Colour Blindness


Colour blindness occurs when individuals are unable to distinguish certain colours and is caused by a sex-linked genetic deficiency. There are three types, the most common is red-green colour blindness which affects 8% of males and less than 1% of females. Individuals who suffer from red-green colour blindness have either the red cones or the green cones absent and as a result cannot distinguish between the colour red and/or green. In blue-yellow colour blindness there is an absence of blue cones, making the colours blue and yellow difficult to see or are confused with one another.

Fig. 1.15 - Colour blindness (ehowhealth)