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A simple model of the eye puts the retina 17 mm behind the lens system…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A simple model of the eye puts the retina $17$ mm behind the lens system. Use $P = \dfrac{1}{u} + \dfrac{1}{0.017}$ , with $u$ in metres and $P$ in dioptres.

What is the power of the eye for a very distant object, where $1/u$ is zero?

What is the power for an object $25$ cm away?

By how many dioptres must the power rise?

Need a reminder? The key ideaShow

Cones work in daylight and give colour vision, and rods work in twilight. A photopigment is opsin plus retinal. Light separates them, the permeability of the membrane changes, and the signal passes to the bipolar cells, the ganglion cells and the optic nerve. In a simple model of the eye, $P = \dfrac{1}{u} + \dfrac{1}{0.017}$ in dioptres, with $u$ in metres.

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.

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