A student walks from bright sunlight into a dark cinema…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student walks from bright sunlight into a dark cinema. At first she sees almost nothing, but after some minutes she sees the seats.
- Which photoreceptors worked in the sunlight, and which work in the dark?
- What happened to the rhodopsin in the sunlight, and what happens to it in the dark?
- What happens to the pupil in the dark?
- Why does she see the seats but not their colours well?
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, in dioptres, with 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.
Step 1. (a) In sunlight, the cones worked. In the dark, the rods work.
Step 2. (b) In sunlight, the light separated the retinal from the opsin of the rhodopsin, so the rods were not useful. In the dark, the retinal and the opsin join again. This takes some minutes.
Step 3. (c) The pupil widens and lets in more light. The widening is not described by NCERT.
Step 4. (d) The rods, which work in the dark, have one kind of pigment, so they give no colour.
Answer: (a) cones in sunlight, rods in the dark; (b) split by light, rebuilt in the dark; (c) it widens; (d) rods do not give colour.
Watch out. The cones are not damaged by the sun. They are simply not sensitive enough for a dark room.
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