Light intensity

• In bright light:

  • Lots of light enter eye and fall on photoreceptors in the retina
  • Photoreceptors are stimulated and produce nerve impulses
  • Nerve impulses travel through the sensory neurones in the optic nerve to the relay neurone in the brain and then to the motor neurone and the effector which are the radial and circular muscles in the iris
  • Circular muscles contract, radial muscles relax
  • Pupil constricts, less light enters the eye
• In dim light:

  • Little amount of light enters the eye and falls on the photoreceptors in the retina
  • Photoreceptors are stimulated and produce nerve impulses
  • Nerve impulses -> sensory neurone(optic nerve) -> relay neurone(brain) -> motor neurone -> effector(radial and circular muscles in iris)
  • Circular muscles relax, Radial muscles contract
  • Pupil dilates, more light enters the eye
• What is pupil reflex?


  a reflex action where pupil becomes larger when the surrounding light intensity is low/smaller when the surrounding light intensity is high