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Adjusting the Lens of the Eye

by Greg Crowther


Light entering the eye is focused on the back of the retina by the cornea and the lens. While the cornea's focusing power is fixed, we can adjust the thickness and thus the focusing power of the lens, thus letting us focus on either nearby or distant objects. The thickness of the lens is adjusted by contraction and relaxation of the ciliary body, made up of smooth muscle.


If you relax too much, you might get fat.
If your muscles do enough, your belly's flat.
But the story is inverted, my dear friends,
For the ciliary muscles and the lens.

When the muscles relax, the lens gets flat,
And it loses some of its power to refract.
When the muscles contract, the lens gets fat,
And it gains back some of its power to refract.
And that's that!

Other Files

sheet music (with melody play-back)

Lesson Plan

Songs like this one can be used during class meetings and/or in homework assignments. Either way, the song will be most impactful if students DO something with it, as opposed to just listening.

An initial, simple follow-up activity could be to answer the study questions below. A more extensive interaction with the song might entail (A) learning to sing it, using an audio file and/or sheet music as a guide, and/or (B) illustrating it with pictures, bodily poses, and/or bodily movements. The latter activity could begin with students identifying the most important or most challenging content of the song, and deciding how to illustrate that particular content.

Study Questions

(1) Where are the ciliary muscles located?

(2) Do nearsighted people tend to have lenses that are too fat, or too flat?

(3) How do the ciliary muscles differ from the pupillary muscles?

(Answers may be found on the answers page.)