Vision: optics

Question 1

what is the role of the lens

Answer 1

image is inverted: brain must flip image to make sense of it

Question 2

what are diopteres

Answer 2

• the dioptere is a measure of lens focussing power
• power = (1/F)D
• reciprocal of focal length
• 1D means focal point is 1m away
• both curvature and refractive index determine lens strength
• difference in refractive index between medium, combined with lens curvature determines lens strength (as measures in diopteres)

Question 3

what are the optics of the eye

Answer 3

• refraction occurs initially at cornea. refractive index fixed at ~1.34
• further refraction caused by lens. variable refractive index (mean ~1.41)
• cornea and lens have similar refractive indicies
• however, most refraction occurs at the cornea (48D of total 57D)
  
  • provides the interface with the air (low refractive index)

Question 4

what is the accommodation of the lens

Answer 4

• facial length is changed entirely by altering the shape of the lens: accommodation achieved by ciliary muscle
• contraction of ciliary muscle relieves ligament tension on lens
  
  • causes the lens to squash
• this increases lens power and shortens focal length for closer objects

Question 5

how is myopia caused

Answer 5

myopia caused by eye ball being too long, or an overly powerful cornea

Question 6

what is hypermetriopia

Answer 6

• normal sight (near object is clear) (emmetriopia)
• farsighted (eyeball too short) (hypermetriopia)
• farsightedness corrected

Question 7

what is presbyopia

Answer 7

• normal range of lens accommodation ~12D
• lens seizes up with age - no longer bulges when ciliary muscles contract
  
  • near point moves further away

Question 8

what is the pupil

Answer 8

• pupil diameter is first means of adaptation to changing light levels
  
  • varies from -2 to 8mm
  • can alter the amount of light captured by ~16 times
• this accounts for a very small portion of the eye’s total light adaptation abilities

Question 9

what is the pupillary muscle

Answer 9

• pupil diameter controlled by two muscles
  
  • sphincter pupillae
  • dilator

Question 10

what are the benefits of a smaller pupil size

Answer 10

• less light reaching retina
• greater depth of field (more things in focus)
• reduced spherical aberration
• reduced glare (scattering of light)

Question 11

what are the effects of pupil size

Answer 11

• reducing pupil size effectively creates pinhole camera
• infinite depth of field
• compensates for myopia

Question 12

what is the retina

Answer 12

• cannot simply observe another persons retina directly; too much light absorbed
• ophthalmoscope: shines light directly onto subjects retina. adjustable lens used to bring retina into focus
• optician can estimate required spectacle strength based upon the ophthalmoscope lens required to bring the retina into focus
• optic nerve carries all information from retina
• passes through optic disk ~15 degrees nasal
• results in blind spot