A boy reads a book held 25 cm away, then looks up at a kite 60 m away…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A boy reads a book held away, then looks up at a kite away.
- Which of the two is at his near point?
- What do his ciliary muscles do as he looks up?
- Why can he not read the book at ?
Need a reminder? The key ideaShow
The cornea and the eye lens form a real, inverted, small image on the retina, about behind the lens. The iris sets the size of the pupil. The ciliary muscles change the lens (accommodation). A normal eye sees clearly from its near point, about , to its far point at infinity.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The book: about is the near point of a normal eye.
Step 2. (b) They relax, so the lens becomes thinner, with a longer focal length, for the far kite.
Step 3. (c) At the book is well inside the near point. The muscles cannot make the lens thick enough, so the image on the retina blurs.
Answer: (a) the book. (b) They relax. (c) It is closer than his near point.
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