By Habre Nadim, Marianna DeMartini and Razan HannaBy Habre Nadim, Marianna DeMartini and Razan Hanna

École La Dauversière, Montreal, June 2000École La Dauversière, Montreal, June 2000

Content validation and linguistic revision : Stéphane Lamarche

Science animée, 2000

Content validation and linguistic revision : Stéphane Lamarche

Science animée, 2000

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Translated from French by Nigel WardTranslated from French by Nigel Ward

Structure of the eye

Visual perception

Circuit of the eye to the brain

Vision defects

Perception by the eye

Seeing Colours

The eyeball possesses three membranes.

-the sclera: the most external membrane, white, fibrous and very resistant, protects and gives shape to the eye. The front part of the sclera has a hole in which the transparent, dome-shaped cornea is located. -the choroid: intermediate membrane, pigmented, richly vascularised (many blood vessels). It is this membrane which nourishes the eye. The ciliary muscle and the iris are located in a hole in the front of the choroid.-the retina: the most internal membrane, it covers the back third of the eye. It supports the light-sensitive nerve cells. The light rays converge on the retina where the images are formed.

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the structures

Blind Spot

A white, fibrous membrane which surrounds the eyeball. The front part of the sclera has a hole in which the transparent, dome-shaped cornea is located.

click to return to the eye.

The eye’s first lens, deprived of blood vessels in order to ensure its transparency to light.

the yellow mark represents the part concerned.

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A liquid that fills the anterior (front) chamber of the eye, between the cornea and the iris.

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A thin (less than 1 mm) richly vascularised (many blood vessels) layer or membrane between the sclera and the retina. It is this membrane which nourishes the eye.

the yellow mark represents the part concerned.

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A ring of muscle tissue surrounding the crystalline lens. When the ciliary muscle contracts it squeezes the lens, making it rounder and allowing the eye to focus on objects that are close.

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A ring of smooth, coloured muscle whose constriction and dilation controls the size of the pupil which is its central opening.

the yellow mark represents the part concerned.

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The central opening in the iris. The pupil widens in dim light to let more light through.

the mark en yellow represents the part concerned.

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The eye’s internal lens. By adjusting its curvature, accommodation (focusing) is possible. The eye also possesses an external lens, the cornea.

The yellow mark represents the part concerned.

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Membrane sensitive to light covering the inner surface of the back of the eye, which converts the optical images into nerve signals which the optic nerve carries to the brain.

the mark yellow represents the part concerned.

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A yellow patch situated at the centre of the back part of the retina, on the optical axis. It is a non-vascularised zone (no blood vessels), of 3 to 4 mm of diameter, at the centre of which is the fovea. It is on the fovea that the sharpest images are formed. The majority of the cones are concentrated at this spot. The cones are the nerve cells responsible for colour vision and for the sharpness of images. The rods are the neurons which allow you to see when the light is dim but only in shades of gray.

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Centre of the macula, characterised by a high concentration of cones. The cones are the cells responsible for the vision of details and colours.

the yellow mark represents the part concerned.

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Nerve of vision (one for each eye). Each one has about 1 million fibres carrying visual information from the retina to the brain.

the yellow mark represents the part concerned.

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A zone on the retina where the nerve fibres from the 800 000 light-sensitive cells (rods and cones) join together to form the optic nerve. This zone has no light-sensitive cells so it is called the ‘blind spot’.

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A viscous gel occupying the principal cavity of the eye, between the crystalline lens and the retina. ‘Vitreous’ means non-crystalline.

The yellow mark represents the part concerned.

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Perception of a distant object:

When one looks at a distant object, the ciliary muscles relax causing the lens to become flatter and thinner. Light rays passing through the lens are refracted (bent) only slightly.

Image

Click here to see how the

eye focuses on a close object

The ciliary muscles are relaxed and the crystalline lens is relatively flat.

Looking at a distant object

Perception of a close object:

To enable us to see objects that are close, the ciliary muscles contract, squeezing the crystalline lens and making it more rounded.

The closest point that the object can be without appearing blurred corresponds to the maximum curvature of the lens.

Image

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Looking at a close object

The ciliary muscles contract, squeezing the lens and making it more rounded.

Short sight:

Short sight is due to the eyeball being too long. The image forms in front of the retina instead of on it.

Solutions:

Concave glasses or contact lenses can be used to correct the focus.

Image of the short sight

to long sight

to correctshort sight:

light rays

cornea

crystalline lens

concave lens to correct short sight

Long sight:

Long sight is due to the eyeball being too short. The image forms behind the retina instead of on it.

Solutions:

Convex glasses or contact lenses make it possible to correct the focus.

Image of long sight

Use a convex lens to correct long sight.

Correction of the anomalies of the eye

In ‘radial keratotomy’, it is possible of make incisions in the cornea with a scalpel or a laser beam in order to modify the refraction of light rays.

The laser can also be used to reduce the thickness of the cornea to change the shape of the eyeball.

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The ‘purest’ colours are the colours you see in a rainbow (the ones you see on the back of this slide). We often say there are seven colours in the rainbow: red, orange, yellow, green, blue indigo and violet (“Richard of York gave battle in vain!”) but in fact there are many more shades than that. Also, many other colours exist that do not appear in a rainbow (gray, brown etc.).

Your eyes can distinguish more than a million different colours but actually contain only three types of colour-sensitive cells. How can that be?

Recall that CONE CELLS are the cells on the retina that are responsible for sharp colour vision in bright light (as opposed to the ROD cells which give less sharp greyscale vision in dim light). There are in fact three kinds of cone cells: one kind detects mainly short wavelength light (the blue end of the spectrum), one detects intermediate wavelengths (the green part of the spectrum) and one detects longer wavelengths (the red end of the spectrum).

We can perceive more than a million different colours because each colour stimulates the three types of cones in a different way. Pure yellow light, for example, stimulates the red and green detectors. This means that by combining red, green and blue light together in different combinations we can ‘trick’ our eye into seeing any colour that it is capable of seeing. For example, if the eye receives a combination of red and green light then the eye will see yellow – it is impossible for the eye to tell the difference between this ‘compound yellow’ and the pure yellow found in a rainbow, for example, since they both stimulate the red and green receptors in the same way.

By adding pairs of primary colours together we obtain the SECONDARY COLOURS yellow, cyan and magenta. Pairs of colours on opposite sides of the picture, such as yellow and blue, are known as COMPLEMENTARY COLOURS. If you are looking at this on a computer screen then look very closely and you will see that the screen consists of red, green and blue dots only – if you look at the yellow part of this picture you will see that the red and green

Since humans eyes have cells that respond to red, green and blue light, scientists call these colours the PRIMARY COLOURS (in art class you may have been given a different set of primary colours but that is because art class uses paint pigments which absorb (subtract) colours whereas scientists are more interested in adding coloured lights together). The image below shows what you would see if you projected circular red, blue and green light beams onto a white screen such that they overlap.

dots are glowing in that area – this combination gives yellow. Any colour you have ever seen can be reproduced by mixing red, blue and green in the right proportions.

This apple appears red. Here’s why…

Light beam

Explanation

When you look at an apple that appears red, is it really the apple that is red, or is rather the light coming from the apple to your eye which is red? If it is the apple that is red then would it still be a red apple in a completely dark room? Or in a room lit only by a blue light?

Best answer: When we say that an apple is red we mean that when illuminated by white light (a mixture of all colours) it will reflect only red light.

We understand that if the apple is illuminated by light that is not white then the apple may no longer appear red. For example, the same apple illuminated by green light, will probably appear black (the apple can only reflect red light but the green light does not contain any red light).

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Skin of the apple

the colours of white light

Absorption of all other colours of light

Reflection of the light red

The reflected rays (red) arrive in our eyes.

This is how we perceive the colours of all objects.

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The light passes through the transparent parts of the eye and is captured by the light-sensitive cells of the retina. It is transformed into nerve signals which are carried by the optic nerve to the visual zone of the brain which interprets the signals. Most of the signals from the right eye are sent to the left side of the brain for processing and most signals from the left eye are sent to the right side of the brain.

left eyeright eye

left visual cortex

right visual cortex

optic voie

nerve signals

optic chiasm (crossing point)

optic nerve

retina

Field of binocular vision Field of binocular vision

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General Information:

- Marieb N., Elaine and Laurendeau, Guy. Anatomie and physiologie humaines, Quebec, Éditions du Renouveau pédagogique Inc, 1993, 1014 pp.

- Mader S., Sylvia, Biologie, Ottawa, Éditions Reynald Goulet Inc., 1987, 767pp.

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- Union des opticiens de France. (Page consulted 16 April 2000). La vision sous tous ses angles, [En ligne]. Adresse URL : www.udo.org/