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.

 

Question 1

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

Answer

 

 

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 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)

 

Question 2

The optician's prescription for a lens is +0.2 D  What is the focal length in metres?

Answer

 

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.

 

 

Question 3

What kind of lens would an optometrist prescribe to a short-sighted patient?  Explain your answer.

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:

 

Object at

Image at

Size

Orientation

Nature

Use

Infinity F Diminished Inverted Real Image on a film (at infinity)
>2F Between F and 2F

Diminished

Inverted

Real

Image on a film

(close up)

2F 2F

Same size

Inverted

Real

Photocopier

Between 2F and F >2F

Magnified

Inverted

Real

Projector

F Infinity Magnified Inverted Real Spot light
<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.

 

Question 4

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.

Answer

 

 

 

Making Lenses

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:

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.

 

 

Similarities include:

 

There are many differences, for example:

Question 5

Do the interactive matching exercise that gets you to match up features of the camera or the eye.

Question

 

Older cameras use film that had chemicals that react when they are exposed to light.