Light intensity / arbitrary units Pupil diameter / mm 10 7.5 50 5.0 100 3.5 200 2.0 A student investigated how the diameter of the pupil in a volunteer's ey...

(a) The table shows a clear negative relationship: as the light intensity increases, the pupil diameter decreases [1]. This is supported by the data - at 10 units the pupil is 7.5 mm, but at 200 units it has fallen to 2.0 mm [1].

(b) The two sets of muscle in the iris are the circular muscles and the radial muscles [2] (one mark each).

(c) To widen the pupil in dim light the two sets act in opposition: the radial muscles contract [1] while the circular muscles relax [1]; because the radial muscles run outwards like the spokes of a wheel, their shortening pulls the iris open so the pupil dilates (becomes wider) [1]; this lets more light enter the eye and reach the retina [1], allowing vision in the dim conditions.

(d) Two muscles that produce opposite effects are described as antagonistic [1].

(e) In very bright light the pupil reflex protects the eye because the pupil becomes smaller (constricts) so that less light enters [1]; this protects the retina / the light-sensitive receptor cells from damage [1] by the very intense light.

(f) One other variable that should be kept constant (any one): the distance of the light source from the eye, the use of the same volunteer / same eye, or the time allowed for the eye to adjust [1].

(g) Each measurement should be repeated several times so that anomalous (odd) results can be spotted and their effect reduced [1], and so a mean can be calculated, making the results more reliable [1].