Human Eye

Two key concepts

  1. accommodation: how different parts of the eye allow light rays to be focused onto the retina from a distant or near object

  2. pupil reflex: how different parts of the eye allow pupil size to change in response to changes in surrounding light intensities

Classwork_IP Bio_Eye_ANS.pdf

Light rays from a distant object

  1. Light rays from a distant object are nearly parallel.

  2. They enter the eye and are refracted by the cornea through the pupil.

  3. The lens does not need to be as thick as light does not need to be bent as much in order to focus it on the retina.

  4. The ciliary muscles relax.

  5. This causes the suspensory ligaments to become taut, which pull on the lens more.

  6. The elastic lens is pulled into a thinner and less convex shape.

  7. Photoreceptors on the retina are stimulated. Nerve impulses are transmitted along sensory neurones in the optic nerve to the brain.

  8. The brain interprets the optical information.

Light rays from a near object

  1. Light rays from a near object are diverging.

  2. They enter the eye and are refracted by the cornea through the pupil.

  3. The lens needs to become thicker and more convex to bend light more sharply in order to focus it on the retina.

  4. The ciliary muscles contract.

  5. This causes the suspensory ligaments to slacken and reducing their pull on the lens.

  6. The elastic lens becomes thicker and more convex.

  7. Photoreceptors on the retina are stimulated. Nerve impulses are transmitted along sensory neurones in the optic nerve to the brain.

  8. The brain interprets the optical information.

iris radial muscles & iris circular muscles

Pupil Reflex

The pupil reflex is an involuntary reflex action where the pupils constrict or dilate in response to changing light intensities.


  • The pupils dilate under low light intensity, for more light to enter the eye for better vision.

  • The pupils constrict when light intensity is high, to restrict light entry to prevent excessive light from damaging the retina.


The size of the pupil is controlled by two sets of antagonistic muscles (effectors)

  • circular muscles of the iris

  • radial muscles of the iris

The pupil of the eye constricts under high light intensities.

The pupil of the eye dilates under low light intensities.


High light intensities

  1. Under high light intensities, the size of the pupil decreases / the pupil constricts. This is an involuntary / reflex action (cranial reflex).

  2. This reduces the amount of light entering the eye and falling onto the retina, prevent damage to the photoreceptors on the retina. (state very clearly that it must fall on the retina)

  3. The effectors of the pupil reflex are the circular and radial muscles of the iris. (*note: you must say circular muscles of the iris, as there are also circular muscles in the alimentary canal which carry out peristalsis)

  4. The circular muscles of the iris contract and the radial muscles of the iris relax, causing the pupil size to decrease.

Low light intensities

  1. Under low light intensities, the size of the pupil increases / the pupil dilates. This is an involuntary / reflex action (cranial reflex).

  2. This increases the amount of light entering the eye and falling onto the retina, allow more light to hit the photoreceptors on the retina. (state very clearly that it must fall on the retina)

  3. The effectors of the pupil reflex are the circular and radial muscles of the iris. (*note: you must say circular muscles of the iris, as there are also circular muscles in the alimentary canal which carry out peristalsis)

  4. The circular muscles of the iris relax and the radial muscles of the iris contract, causing the pupil size to decrease.