6.1 Human Vision

 Light enters the eye through the pupil  The iris (the coloured part of the eye) controls the amount of light entering the eye  In dim light the iris dilates (expands) the pupil to allow more light to enter  In bright light, the iris contracts the pupil to reduce the light entering the eye

 The cornea covers the iris and pupil  Transparent enough to let light pass through yet tough enough to hold the eye together  Surrounding the cornea is an opaque tissue called the sclera (looks white to us)  Behind the pupil is a flexible convex lens that focuses the light onto the retina (screen) on the back of the eye  Light sensitive cells in the retina detect the image  Other cells convert the light rays into electrical signals that are sent to the brain through the optic nerve

 Light passes through a focussing system involving the cornea, the lens and spaces in the eye filled with a watery fluid  Fluid between the lens and the cornea supports both the cornea and the lens and provides nutrients to the cornea which doesn’t have any blood vessels  Fluid behind the lens gives shape to the eye and supports the lens  The cornea refracts incoming light rays so that they converge toward the retina  The lens does the remaining focussing by automatically changing its shape  A thicker lens can focus on near objects and a thinner lens (muscles relaxed) can focus on distant objects

 The image is upside down on your retina but the brain interprets the image as being upright  The area where the optic nerve enters the retina does not have any light-sensing cells and is known as the blind spot  Some cells in your retina detect low levels of light and other cells detect bright light...two basic shapes  Rod cells absorb almost any color of light but is used to determine shades of light and dark and helps us see shapes and movement  Cone cells possess one of three types of pigment (red, green or blue) to allow us to see all the colours

 In normal vision the light rays converge at the retina to produce a sharp image  The lens automatically changes shape to handle distant objects and near ones  People who can see nearby objects clearly but can’t see distant objects are near-sighted  The lens converges the light rays to form an image in front of the retina  A concave lens is used to diverge the parallel rays slightly so that the image forms farther back

 Far-sighted means you can see far objects but not near ones  Near objects converge “behind” the retina  A convex lens is needed for the light rays to focus exactly on the retina  Astigmatism is when your cornea is distorted  Causes an image to focus on more than one point on the retina resulting in blurry vision  Can be corrected with glasses, contact lenses or laser surgery

Blindness can be any vision impairment  In developing countries blindness is most often a result of disease or malnutrition  Snow blindness is caused by overexposure to the glare of sunlight off snow  Night blindness – difficult to see in dim light  Rod cells lose their ability  Colour blindness is the ability to see only in shades of grey although the most common one is where people can’t tell red and green apart