Light entering eye: focused on retina - cornea does most refraction. Cornea and vitreous humour have the same refractive index

- lens fine focuses using ciliary muscles:

Distant object: ciliary muscles relax, ligaments tighten > lens becomes flat and thin

Close-up object: ciliary muscles contract, ligaments relax > lens becomes more spherical and fat

Iris controls pupil size:

Dim light: radial muscles contract, circular muscles relax > pupil dilates

Bright light: radial muscles relax, circular muscles contract > pupil constricts

Rods respond to dim light, responsible for peripheral vision. Contain the visual pigment, rhodopsin

Cones respond to bright light, responsible for central and colour vision. Contain the visual pigment, iodopsin

Rods – low visual acuity
Cones – high visual acuity

Trichromatic theory = red, blue and green cones. Each detects different wavelength of light

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A-Level Biology: Control in Animals & Plants