the eye

• Cornea

  clear coating on the front of the eye which bends/refracts light ray onto lens
• Iris

  controls the size of pupil , made up of circular and radial muscles (antagonistic)
• Pupil

  allows light into the eye , dilates in dim light and constricts in bright light
• Lens

  flexible structure which bends/refracts light ray to focus onto the retina
• Retina

  contains photoreceptors known as cone and rod cells which generate an electrical impulse
• Sclera

  thick , tough , outer covering of eyeball
• Optic nerve

  transmits electrical impulses from retina to the brain
• What are rod and cone cells?

  -both photoreceptors

  -when they detect light they generate an electrical impulse

  -the electrical impulse is transmitted along the optic nerve and responds so you can see.
• Rod cells

  Light intensity: dim light

  Vision: black/white

  Distribution: everywhere in the retina to detect in as much light as possible
• Cone cells

  Light intensity: bright light

  Vision: coloured vision

  Distribution: found in small part of retina called foeva because there is enough light entering the eye
• Pupil dilation

  -muscle which contracts is radial

  -muscle which relaxes is circular
• Pupil constriction

  -muscle which contracts causes pupils to constrict

  -muscle which relaxes causes pupils to dilate
• Figure 1 shows a reflex in the iris of the human eye in response to changes in light levels. Describe the changes in the pupil and iris going from A to B from figure 1. Explain how these changes occur.

  -from A to B the pupil is dilating

  -radial muscles in the iris contract and causes pupil to dilate

  -circular muscles in the iris relax

  -light levels have become dimmer
• Describe how light rays are focused to give normal vision (2 marks)

  -cornea will bend/refract light rays so that they focus on the lens

  -lens bends/refracts light rays so that it focuses onto retina
• What does accomodate mean?

  changing focus
• What do the ciliary muscles do for distant objects?

  -ciliary muscles relax and suspensory ligaments contract making the lens pull thin

  -this helps to focus the light onto the retina
• What do ciliary muscles do for close objects?

  -ciliary muscles contract and suspensory ligaments relax allowing the lens to go fatter

  -this helps focus the light onto the retina
• What are cataracts?

  -protein layer building up on inside of the lens making it cloudy , this will give a cloudy vision

  -full vision can be restored by replacing the clouded lens with a plastic lens
• Why is colour blindness more common in males? (3 marks)

  -female chromosomes= XX

  -male chromosomes=XY

  -for a male to have colour blindness , only one X chromosome needs to have a faulty defect (mutation)

  -for females , both X chromosomes need to have a faulty defect
• Defects of cornea

  -if cornea has a defect , light rays will not bend/refract correctly

  -treatment would be laser eye surgery
• What is short-sighted?

  can see close objects not long distances

  -eyeball is too long

  -cornea is too curved

  -light rays bend/refract too much

  -light rays focus before the retina

  -as a result , you are now short-sighted
• What is long-sighted?

  can see long distances not close objects

  -eyeball is too short

  -cornea is not curved enough

  -light rays bend/refract too little

  -light rays focus behind the retina

  -as a result , you are now long-sighted
• Treatment for shortsighted

  -Diverging lens = helps diverge the light rays so that they bend/refract and focus onto the retina
• Treatment for longsighted

  -Converging lens = helps converge the light rays so that they can bend/refract onto the retina
• Explain which lens would correct the eye defect shown in figure 6 (if shortsighted eye shown)

  -figure 6 is showing a short sighted eye

  -the light rays are focusing before the retina

  -lens X the diverging lens would diverge the light rays so the bend/refract and focus on retina