Lens Accommodation and Light/Dark Adaptation
Two different adjustments students are always asked to tell apart
Step through the eye adjusting first to distance, then to changing brightness.
For distant vision the ciliary muscles are relaxed, the suspensory ligaments stay taut and the lens is held stretched, thinner and more flattened.
In bright light the pupil constricts, rhodopsin is bleached and the cones do the seeing.
Free play — step through the process at your own pace, and move the slider to see what changes. Switch to when you're ready to test yourself.
What this lesson covers
What you do
- Say what happens to the ciliary muscles when you look up from a book at a distant hill.
- Explain why you cannot make out the colour of flowers on a moonlit night.
- Compare light and dark adaptation on three points: pupil size, pigment regenerated, and which cells become active.
Challenges to clear
- Step through all four stages.
- Reach the stage where the lens is at its fattest.
- Dim the light until the pupil is wide open.
The task
Step through the eye adjusting first to distance, then to changing brightness.
What this lesson covers
- Distant object — For distant vision the ciliary muscles are relaxed, the suspensory ligaments stay taut and the lens is held stretched, thinner and more flattened.
- Near object — For near vision, nearer than 6 metres, the circular ciliary muscles contract and pull the ciliary body forward. Tension on the suspensory ligaments is released and the elastic lens becomes thicker and more convex.
- Into the dark hall — Dark adaptation: rhodopsin or visual purple of the rods, broken down by the earlier bright light, is regenerated, and the pupil dilates so that more light enters.
- Back into sunlight — Light adaptation: the visual purple of the rods is bleached, reducing their sensitivity, the pupil constricts and the eyelids close partly to cut down the light entering.
- Ciliary muscles contract → ligaments slacken → lens fattens → near object in focus
- In dim light the pupil dilates and rhodopsin is rebuilt, so the rods take over.
- In bright light the pupil constricts, rhodopsin is bleached and the cones do the seeing.
Hint
Accommodation changes the LENS; adaptation changes the PUPIL and the visual pigments. Near vision needs a fatter lens, which means contracted ciliary muscles and slack ligaments — the opposite of what most students guess.