Eyes

• 2 types of retinal cells

  Cone cells - Allow us to see colour

  Rod cells - More sensitive so allows us to see in the dark
• Fovea

  Where light focuses on the retina.

  Has the most cone cells and gives the sharpest image
• Cornea and lens

  Refracts light so that it focuses on the fovea
• Iris reflex

  Automatic reflect that contricts or dilates the pupil in response to light.

  Allows the eye to control the amount of light that reaches the retina
• When dilated the radial muscle contracts and the circular muscle relaxes

  When constricted the radial muscle relaxes and when constricted the circular muscle contracts
• Accommodation

  A reflex that changes the refractive power of the lens to allow us to focus on nearby or distant object
• Thicker lens

  Stronger refraction
• Slim lens

  Weaker refractions
• What happens to the ciliary muscle and suspensory ligaments to make the lens short and fat

  Ciliary muscles contract

  Suspensory ligament will slacken (loosen)
• What changes take place when it focuses on a distant object

  Light will only need to be refracted weakly, so the lens needs to be long and thin.

  To make the lens long and thin the ciliary muscle will relax and the suspensory ligaments will be pulled taut.
• Long sighted

  Eye is unable to focus on nearby objects
• Short sighted

  Unable to focus on distant objects
• What lens is needed for short sightedness

  Concave because it would refract the light outwards to counteract the over-refraction of the eye's lens