# How does the world look without glasses?

##### Demonstration

Mimicking the behaviour of short and long sight and its correction.

#### Apparatus and materials

For each student

Spherical - 7D lens, 2

Spherical + 7D lens, 2

#### Procedure

a To demonstrate short sight you might say to students:

Hold the + 7D lenses in front of your eyes (in front of your spectacles if you usually wear them). Look at the view through the window. That is what a short-sighted person sees.

Now add spectacles to correct your temporary short sight. Do this by holding - 7D lenses in front of the lenses you are already holding.

b To demonstrate long sight you might say:

Hold - 7D lenses in front of your eyes. Try to read a newspaper or book. Add spectacles to correct your temporary long sight. These should be + 7D lenses, held in front of the others.

#### Teaching notes

1 The 'average-eye' range is from infinity to 25 cm or, in terms of power of the incident fan of rays, from 0 to 4D. The extra + 7 D lens brings that range in closer so that it runs from (0 + 7) D to (4 + 7) D or from 7D to 11D. These modified eyes are short-sighted with range from 14 cm to 9 cm.

2 To imitate long sight, the modifying lenses must be negative - the opposite of a real long-sighted person's correcting spectacles. If the student holds negative lenses in front of their eyes (or their own glasses) they will push this range outward.

The choice of power for those negative lenses depends on the pupil's own range, and therefore on their age. At age 14 the range will be much greater than the 4 D 'average-eye' range from 25 cm to infinity. We want the modifying lens to push the pupil's near point out beyond a usual reading distance, say to 50 cm.

3 The student who understands the idea of this quick experiment may say that the easiest way is to borrow someone else's spectacles and hold them in front. If those spectacles are for short sight you will see what a person with long sight sees. If they are for longsight you will see what a person with short sight sees.

This experiment was safety-checked in January 2007