Power of accommodation

The idea

The ability of the eye lens to adjust its focal length is called accommodation; the focal length cannot be decreased below a certain minimum limit.

Reading very close

You hold a printed page right up to your nose. The words look blurred, and your eyes feel tired.

What does this suggest about the eye lens?

• It cannot get thick enough to focus so close
• It has stopped working for good
• The page has changed shape

Bring the book closer

Slide the book towards the eye. The numbers come from the same simple model as before. The book is always drawn in the same place, so the picture is not to scale.

An adjustable lens with a limit

The eye lens can adjust its focal length to focus on things at different distances. This is called accommodation. But the focal length cannot become smaller than a minimum, so very close things look blurred.

The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length of the eye lens cannot be decreased below a certain minimum limit.

Notes

Accommodation is the ability of the eye lens to adjust its focal length; the focal length cannot be decreased below a certain minimum limit.

Check yourself

What is the ability of the eye lens to adjust its focal length called?

What may happen when a printed page is held very close to the eyes?

The focal length of the eye lens…

In the picture, the book is brought closer than 25 cm. Where does the image fall?

• Dispersion. Dispersion is the splitting of light into its component colours.
• Accommodation — correct. Yes!
• Refraction. Refraction is the bending of light. The ability of the eye lens to adjust its focal length is called accommodation.
• The image becomes clearer because the lens becomes as thick as needed. The focal length of the eye lens cannot be decreased below a certain minimum limit.
• Nothing happens, whatever the distance. Holding a page very close to the eyes may give a blurred image or strain in the eye.
• The image may be blurred, or the eye may feel strain — correct. Yes!
• cannot be decreased below a certain minimum limit — correct. Yes!
• can be decreased as much as is needed. There is a minimum limit below which the focal length of the eye lens cannot decrease.
• cannot be changed at all. The curvature of the eye lens can be modified by the ciliary muscles, which changes its focal length.
• On the retina, so it is sharp. That happens at 25 cm or farther, where the lens can still adjust enough.
• Behind the retina, so it is blurred — correct. Yes!
• In front of the retina, so it is sharp. An image that is not on the retina is blurred. Here the lens cannot become thick enough, so the image falls behind the retina.