Pupil reflex sim
Drag a lamp's dimmer slider to change the light at a model eye, watch the pupil constrict or dilate and see which iris muscle contracts, record the pupil diameter at each setting and plot it against light intensity.
The pupil reflex: in bright light the circular muscles of the iris contract and the radial muscles relax, so the pupil constricts; in dim light the radial muscles contract and the circular muscles relax, so the pupil dilates; antagonistic muscles; receptor and effector; why the reflex is useful · covers the O-Level, IGCSE topic
What the sim shows
The pupil reflex: a model eye, 5 times life size, on a lab bench beside a desk lamp with a dimmer and a light meter. Brighter light: the circular muscles of the iris contract and the radial muscles relax, so the pupil constricts (2.2 mm at 10 000 lux) and less light reaches the retina. Dimmer light: the radial muscles contract and the circular muscles relax, so the pupil dilates (6.5 mm at 0.1 lux) and more light gets in. The two sets of muscle are antagonistic: when one contracts the other relaxes. It is a reflex: light receptors in the retina send impulses along the optic nerve to the brain and back to the iris muscles, automatically. Pupil sizes from a formula after Moon and Spencer (1944), D = 4.9 − 3 tanh(0.4 log₁₀ L), typical, not exact (≈). Simplified on purpose: the light is taken to fill the eye's view evenly, so the scene's brightness in the formula is the light meter's lux divided by π; one typical pupil is used (a formula after Moon and Spencer, 1944); real pupils differ from person to person and get smaller with age; the readings have no measuring error, so the curve of best fit passes through every point; the time the pupil takes to change is illustrative: it starts after about 0.25 s, and constricting is faster than dilating; the iris close-up is a colour key (circular muscles red-brown, radial muscles blue-grey); real iris muscle is not coloured like this; the fibre lengths are worked out from the pupil's size with an illustrative 12 mm iris and a 0.75 mm ring of circular muscle; the model eye is 5 times life size and its cornea is left off so the iris shows clearly; only the pupil's response is shown; how the eye focuses is a separate figure.
- Lit: the dimmer slider you drag
- Close-up: the iris from the front, and its two muscles (colours are a key)
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