The range of accommodation of the eye

Class practical

Estimating the near point and far point for the human eye.

Apparatus and Materials

None required.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Ask students to look out through an open window (preferably not through a pane of glass) and decide whether they can see things that are far away very sharply. If they can, they can say that their eyes see things as far as 'infinity'.
2. Students with spectacles may wish to take them off. If they find they cannot see things very far away, they should find the farthest distance at which they can see an object comfortably and clearly by looking at objects at various distances.
3. Now each student should look at a book held at arm's length, and bring it closer and closer to the head until the print can no longer be seen comfortably and clearly. With young eyes the range of accommodation is very large, and many students will be able to focus sharply on objects only a few centimetres from their eyes. This will involve uncomfortable squinting, which can be avoided by holding a hand over one eye.
4. Tell students that older people do not have such a large range. They cannot see a printed page clearly when it is held so close. Holding a book so close may also be uncomfortable for their arms, is likely to be poorly lighted, and ordinary printed type will look very large. Books are printed with type that will look a comfortable size when held about 25 cm away; and at that distance the reader's arms are comfortable and the lighting can be arranged fairly easily. An 'average eye' likes to have things 25 cm away, or farther, for comfortable vision.
5. Ask students to look at a book, read a few words, and then to look quickly at a far wall and then back at the book. It may be possible to feel the eye lens being squeezed and let go.

Teaching Notes

• Young people have a tremendous range of accommodation. It will be confusing to attempt to persuade them that 25 centimetres is the closest distance of comfortable vision as is normally quoted. Accept the actual range each student finds. The range of accommodation for a class will vary, especially for those students who normally wear spectacles, and for the old teacher.

• The graph shows the range of accommodation of a human eye, plotted against age. The range is in dioptres, reciprocal metres (metre ^-1 ). People of a given age differ widely in the placing of their near point, but their range [l/ (near distance) - (l/ (far point distance)] is much the same for most of them.
• At age 40 the range is about 4 D. So we may think of an average eye as belonging to a person of age 40 with a near point at 25 cm and a far point at infinity. Another 40-year old whose near and far points are different can bring their range to that of an average eye with a single pair of spectacles.
• Spectacles are designed to form virtual images so that they effectively move the object to a comfortable image point and then the eye looks at the image. Spectacles add just the right amount of lens power to the wearer's eyes to make the combination of spectacle plus eye, equal to an average eye.

This experiment was safety-tested in January 2007