Unit 5: Option B - Medical physics: Physics of the EyE

Chapter 1 of Unit 5: Option B - Medical Physics. Physics of the Eye. from the A2 revision guide


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The structure of the eye

1) The cornea is a transparent window, and has a high refractive index. It does most of the eyes focusing.

2) The aqueous humour is a watery substance between the pupil and the lens.

3) The iris(the coloured part of your eye) consists of radial and circular muscles that control the size of the pupil(the hole in the centre) this regulates the intensity of light that can enter the eye.

4) The lens is controlled by the ciliary muscles and acts as a fine focus. When the ciliary muscle contracts, tension is released and the lens becomes more spherical. when the muscle relaxes, the suspensory ligaments pull the lens into a flatter shape.

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Physics of the Eye: The structure of the eye Cont.

5) The vitreous humour is a jelly like substance and is what keeps the eyes shape.

6) Images are formed on the retina, which contains two types of light-sensitive cells, rods and cones.

7) The yellow spot, is the most sensitive part of the retina, in the centre of this is the fovea. This part of the retina contains the highest concentration of cones.

8) Finally, the optic nerve. This is the part of the eye that carries signals from the rods and cones to your brain. The brain then receives an upside image of what you see and flips it around.

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Physics of the Eye: The eye is an optical refracti

1) The far point of the eye, is the distance that the eye can focus on comfortably. For people with normal sight, this distance is infinity. When the eye is focused on this point, they're "accommodated".

2) The near point, is the closest distance that the eye can focus on. For young people this is about 9cm.

3) The cornea and aqueous humour act as a fixed converging lens with a power of about 41D(dioptres). The lens itself has a power of about 18D unaccommodated, however by changing its shape this can increase to up to about 29D in young people, this would happen when trying to focus on objects at the near point.

4) By thinking of the eye as a single converging lens you can add together to the power of the cornea, aqueous humour and the lens to give a total power of 59D at the far point.

5) When looking at a nearer object, the eye's power increases as the lens changes shape and the focal length decreases. However the distance between the lens and the image(on the retina), v, stays the same.

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Physics of the Eye: Rods and Cones

1) The back of the retina is covered in cells called rods and cones. light travels through the retina to these cells.

2) The cells contain a chemical pigment that bleaches when light reaches them. This reaction causes the cell to send a signal to the brain via the optic nerve.

3) Enzymes created from vitamin A in the blood causes the cells to "unbleach" or reset.

4) There is only one type of rod, but there are three types of cones. One type is sensitive to red light, another to green light and the last to blue light, each cone is able to absorb a range of wavelengths.

6) The eye is less responsive to blue light, than to red or green so it appears dimmer.

7) The brain receives signals from all three types of cones and interprets there "weighted relative strengths" as colour.

8) Any colour can be produced by combining different intensities of red, green and blue light.

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Physics of the Eye: Spatial resolution

1) In order to be able to tell two objects apart there has to be at least one rod or code between the light your eye receives from each object.

2) Spatial resolution is best at the "yellow spot" of the eye. This is the part of the retina that is most sensitive to light where the cones are very densely packed, also each cone has its own nerve fibre.

3) There are no rods in this section of eye, this means that in dim light resolution is best slightly off the eyes direct line of sight where the rods are more densely packed.

4) Cones do not work in dim light.

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Physics of the Eye: Persistence of vision

1) Nerve impulses from the eye take about a fifth of a second to decay. This means that a flashing light with a frequency of 5Hz or more appears to be continuous.

2) For lights with a higher intensity a higher frequency is required as it causes more nerve cells to trigger.

3) Cinema and Tv screens rely of persistence of vision to give the illusion of smooth movements, while in fact the screen is continuously flashing.

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Defects of Vision: Real and Virtual

Lenses produce both real, and virtual images. You need to follow the "real is positive, virtual is negative" rule.

1) A converging lens produces a real image, so it has a positive focal length.

2) A diverging lens produces a virtual image, so it has a negative focal length.

1/f = 1/u + 1/v

F = focal length

U = Distance between the object and the lens

V = distance between the lens and the image

The linear magnification of a lens, m = size of image / size of object. and m = v / u

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Defects of Vision: Myopia

1) Myopic(Short-sighted) people have a far point closer than infinity. This means they can not focus on distance objects.

2) Myopia occurs either when, the cornea and lens are too powerful, or the eyeball is too long. This causes the image of the object to form in front of the retina instead of on it.

4) A lens of negative power( A diverging lens) is used to correct this.


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Defects of Vision: Hypermetropia

1) Hypermetropic (Long-sighted) people are unable to focus on near objects. This happens when their near point is further away that normal ( Around 25cm+).

2) Hypermetropic occurs either because the cornea and lens are too weak, or the eyeball is too short. This causes the images of near objects to form behind the retina.

3) A lens of positive power( A converging lens) is used to correct this.


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Defects of Vision: What lens?


For people with myopia the lens needed depends on their far point.

lens power needed = 1 / f(far point)

EG. for someone with a far point of 5m

f = -5 (as a negative power is needed)

1 / -5 = = -0.2D


The lens power needed to correct hypermetropia is calculated with the lens equation.

1 / f = (1 / u) + (1 / v)

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Defects of Vision: What lens? Cont.


The lens power needed to correct hypermetropia is calculated with the lens equation.

1 / f = (1 / u) + (1 / v)

EG. For someone with a near point of 5m

u = 0.25m ( An "acceptable" near point is 25cm)

v = -5m

1 / f = ( 1 / 0.25 ) - ( 1 / 5 )

1 / f = 3.8

lens power = +3.8D

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Chris Morris


If your going to be printing these notes, i suggest you also print this image:


or see the image in your A2 revision guide on pg. 70.

charlotte ryan


This really helped! Graciassssssss

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