Triple Physics Topic 4 - The Eye and Correction of Defects of Vision
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.
|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:
Author not known
the tough outer sclera has a transparent region at the front
called the cornea;
the muscular iris controls the size of the pupil and hence the
amount of light reaching the retina;
lens is held in position by suspensory ligaments and ciliary
the retina contains the receptor cells which are sensitive to light.
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 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.
The lens in the eye is a convex or converging lens. This means that the lens makes rays of light come together, or converge.
The rays parallel to the principal axis are converged onto the principal focus. The focal length is the distance between the lens axis and the principal focus (strictly speaking, the focal plane).
Thicker lenses bend light more, and are therefore described as more powerful. Powerful lenses have short focal lengths. The power of a lens is measured in dioptres (D) and is given by the formula:
Power = 1
focal length (m)
|The optician's prescription for a lens is +0.2 D What is the focal length in metres?|
The principal focus of a convex lens is called real. The images made by convex lenses are in most cases real. This means that the image can be projected onto a screen. We will see later how images are made with ray diagrams.
Most refraction in the eye is at the cornea, not the lens. The lens makes small adjustments to the rays of light so that it is focused on the retina.
Defects of Vision
Short sight is usually caused by the converging lens in the eye being too strong, which means it brings the rays together in front of the retina, so that the image on the retina is not focused properly, and is blurred.
Diverging lenses are used in glasses that correct short sight.
Converging lenses are used to correct long sight, where people cannot make the lens fat enough to see close-up objects clearly.
The converging lens brings the rays in so that the image is focused correctly on the retina.
|What kind of lens would an optometrist prescribe to a short-sighted patient? Explain your answer.|
The magnifying glass produces a virtual and magnified image:
magnification = height of image
height of object
For a convex lens, the nature of the image depends on where the object is:
|Infinity||F||Diminished||Inverted||Real||Image on a film (at infinity)|
|>2F||Between F and 2F||
Image on a film
|Between 2F and F||>2F||
|<F||<F (on same side)||Magnified||Upright||Virtual||Magnifying glass|
Near Point and Far Point
Optometrists (opticians) measure the near point and far point of their patients as a normal part of the eye test. The normal eye has a far point of infinity. This means that when the eye is fully relaxed, it can see objects at infinity, in other words, a very long way away. When an object is brought up close to the eye, the eye accommodates by making the lens thicker, hence more powerful. It can focus on objects about 20 cm away. This is called the near point.
People who have short sight can often focus on objects that are closer than 20 cm. Their near-point is about 10 cm. However their far point is about 2 metres. It can be even less in people with more severe short-sight.
For long-sighted people the near point is about 0.5 m, so they cannot read a book comfortably. Their far-point is infinity, so they only need glasses for reading.
For many middle-aged people, the eye does not accommodate so well, and the near point moves away. They have to buy reading glasses.
Other people can become short sighted by doing a lot of close-up work, or microscopy.
As short-sighted people grow older, they too may need reading glasses as well as distance glasses. Varifocal lenses can be made. The top of the lens allows the eye to see objects at infinity; the bottom allows them to read.
|A patient has a near point of 15 cm and a far point of 4.5 m. Explain whether the patient is long or short-sighted.|
When lenses are made for glasses, they need to be made of light-weight material. People would not want their glasses to be to heavy. And very thick glasses look a bit strange as well. So we can use material of a higher refractive index to make glasses bend the light more. The focal length of a lens is determined by:
the refractive index of the material from which the lens is made, and
the curvature of the two surfaces of the lens.
A 5 D lens would have a focal length of 20 cm. We could make a 5 D lens in two ways:
The material of lower refractive index is said to be optically less dense. An optically dense material has a high refractive index.
Optical density has nothing whatever to do with physical density.
It is better to make lenses as thin as possible, as thick lenses can cause distortion.
Comparing the eye with the camera
The human eye is a high-tech piece of optical equipment that can only be rivalled by equipment that costs tens of thousands of pounds. The eye can be compared to a camera.
Both have one lens to focus the light;
Both have light-proof box;
Both have an iris diaphragm (the iris in the eye) to adjust the light level.
The retina and CCD (charge coupled device) pick up the light.
Both have a real and inverted image.
There are many differences, for example:
The camera lens has to be moved to focus onto close-up object, while the eye lens changes thickness.
The camera has a shutter to give a pulse of light onto the charged coupled device.
The eye sends analogue signals to the brain, while the charge-coupled device sends digital signals to the computer.
|Do the interactive matching exercise that gets you to match up features of the camera or the eye.|
Older cameras use film that had chemicals that react when they are exposed to light.