Medical Physics Tutorial 1 - The Eye


Structure of the Eye

Sensitivity of the Eye

Depth of Field


Structure of the Eye

The eye is a sophisticated organ.  Without it we cannot do many things that we take for granted.  Without your eyes you cannot read this text here.  We can mimic the action of the eye with a TV camera.


Question 1

List three different ways in which the eye is like a TV camera.  How is the eye superior to a TV camera?




The eye has a structure like this:


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In the eye:

The eye has a system of muscles called ciliary muscles that alter the shape of the lens.  If you look at a close-up object the lens fattens, becoming optically stronger.  If you look at a distance object the muscles relax and the lens goes thinner.


Light from an object enters the eye through the cornea. The curved cornea and the lens produce an image on the retina.   There are several boundaries at which refraction takes place.  However the main refraction takes place at the air cornea boundary, not the lens, which does the fine focusing.  The lens does the fine focusing.


Behind the cornea is a watery liquid called the aqueous humour.  Between the lens and the retina is the vitreous humour, a clear jelly like material that helps to keep the eye in shape.


The image is upside down on the retina as shown:


The image is made on the yellow spot, which is where there is the greatest concentration of cone-shaped light receptor cells (cones).  


The retina looks like this:



Notice that the rods and cones are at the back of the retina.  There are sound biological and evolutionary reasons, but we won't worry about that here.



Sensitivity of the Eye

It said that the eye can respond to a single photon.  It can respond to a difference in light intensity of 109 times.  There are two mechanisms:


In the dark the iris is dilated.  Photosensitive chemicals build up the rod-shaped cells on the retina, a process that can take up to 30 minutes and is called adaptation.  Rods also share nerve cells so smaller intensities cause a signal.  However the visual acuity (how accurately you can see) is reduced.  Also the rods are spread either side of the yellow spot.  Therefore the image is not well focused.


The rods are most sensitive to light of wavelength about 510 nm (green light), while the sensitivity of cones as a whole is about 560 nm (yellow light).  This can be shown in the graph below:



In low light levels, when only the rods are operative, we see in monochrome (black and white).  A certain level of light is needed before colour vision is possible


Question 2

"Bad light stopped play".  Cricket games in the UK are often abandoned if the weather gets gloomy.  Can you explain why?



In bright light the iris dilates.  The image is focused onto the yellow spot where there are large numbers of cones.  There are three kinds of cones:

The colour that is seen depends on the proportion in which each type of cone is stimulated.  The most sensitive cones are the green ones, while blue cones are fairly insensitive.  The graph shows the way in the cones respond to different wavelengths.



On the retina the spacing of cones is about 3 mm apart, which gives an angle of resolution (which represents the visual acuity) of about 0.008o which is quite small.  In the rod rich area of the retina the acuity is about 1/20th of the acuity of the yellow spot, about 0.16o


Nerve ends respond to changes in light level.   The eye is never still; it is constantly scanning so that new nerves are stimulated.  However the nerve endings take a definite time to respond.  Flickering lights fuse into a continuous light.  This is why you don't see the flickering of a TV set.  The delay period is called persistence of vision.  Movies are filmed at 24 frames a second; the eye can fill in the gaps between the action of each individual photograph.


A frog's eyes are fixed.  It cannot see the insect in front of it until it moves.  Then it can aim with good accuracy.


A flash light for a camera lasts for less than 1 ms.  We see it for longer due to the persistence of vision.


Question 3

 Explain why yellow is a good colour for a high-visibility jacket.




Depth of Field

If you take a picture with a camera, a large aperture gives a reduced depth of field.  So if you focus on a near object, far objects are out of focus.  If you reduce the aperture, you can bring the far objects into sharper focus without changing the position of the lens.  The downside is that detail can be lost.


It's the same with the eye.  If you are focused on a near object in bright light, you will be able to see far objects reasonably clearly.  If you are focused on the same object at the same distance in dim light, far objects are blurred.  The depth of field is reduced.


Question 4

Explain how the eye changes the depth of field