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The Eye

 

biology

visual angle

angular magnification

near point

short sight

long sight

 

 

 

Eye Biology

 

 

diagram of the human eye

 

 

retina - a light sensitive region on the rear of the inner surface of the eyeball.

 

accommodation - the ability of the eye to produce a focussed image on the retina. This is done by altering the shape of the eye-lens by muscle in the shape of a ring, called the ciliary muscle. The ciliary ligament transfers force between the muscle and the lens.

 

iris - a ring of muscle controlling the amount of light entering the eye.

 

lens - made of clear cartilage. In old age the lens can become opague. In a simple procedure it can be replaced with a plastic lens.

 

cornea - front part of the eye. Most of the deviation of light coming from an object occurs at the air/cornea boundary. Old age/disease can cause the cornea to become fogged, eventually causing blindness. Transplants from cadavers can remedy this.

 

pupil - the space inside the iris. This appears black because it leads into the eyeball. Inside the eyeball is caverness and dimly lit from light entering.

 

humours - the aqueous and vitreous humours are clear liquids within the eye with similar refractive indices.

 

aqueous humour (1.33 )


vitreous humour (1.34 )


eye lens (1.41 )


cornea ( 1.38 )

 

 

 

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Visual Angle

 

 

the eye - visual angle

 

 

The size of objects perceived by the eye depends on the size of their image on the retina.

Suppose an object subtends an angle θ at the eye, producing an image of height Hi on the retina.

 

If We is the width of the eyeball between lens and retina, then for small angles, to a good approximation:

 

the eye - visual angle

 

 

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Angular Magnification (magnifying power)

 

This is used in optical instruments - microscopes, telescopes.

Angular magnification is the ratio of the angle subtended at the eye by the image (βo), to the angle subtended by the object (αo).

 

 

the eye - angular magnification

 

 

magnification in terms of subtended angle

 

 

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Near Point

 

The near point (D) is the closest distance infront of the eye where a positioned object is in focus. It is sometimes referred to as 'the least distance of distinct vision'.

 

 

 

the eye - near point

 

 

The near point varies from person to person, but a good average is 25 cm.

 

 

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Short Sight (Myopia)

 

As the name implies, a person with 'short sight' can see objects close up, but not in the distance.

 

Short sight is a result of an abnormally long eyeball or the eye lens being too strong. The consequence is that the image of a distant object is formed infront of the retina. The resulting image actually reaching the retina is therefore out of focus.

 

 

the eye - short sight

 

 

Correction of short sight is by concave lens. A concave lens is a diverging lens. So the effect is to push the image back towards the surface of the retina, where it will be perceived as in focus.

 

 

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Long Sight (Hypermetropia)

 

As the name implies, a person with 'long sight' can see objects far away , but not close up.

Long sight is a result of an abnormally short eyeball or the eye lens being too weak. The consequence is that the image of a distant object is formed behind the retina. The resulting image perceived on the retina is therefore out of focus.

 

 

the eye - long sight

 

 

Correction of long sight is by convex lens. A convex lens is a converging lens. So the effect is to pull the image back from behind the eyeball towards the surface of the retina, where it will be perceived as in focus.

 

 

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