COM S3

Question 1

The conditions of refraction

Answer 1

• Refraction of light  bending of light rays
• Occurs when light passes from one medium into another that has different density
• At boundary of two different media  density changes  causes speed of light rays to change
• Light travels slower through water (more dense) than through air (less dense)
 Moving from WATER to AIR – speed of light increases
• Light rays refracted at boundary only  approach at angle
• Rays that enter at 90 = travel in straight line = normal
• Rays that enter at angle  bent away from the normal as speed up and towards normal as they slow down

Question 2

Whats the refractive index?

Answer 2

degree to which object will bend light

Question 3

What is refractive media in the eye?

Answer 3

• Each made of different densities  refractive media
• Light rays refracted at each boundary between different structures
• Cornea  where most of refraction occurs in eye
 Largest change in index of refraction as light leaves air and enters cornea
• Lens  ciliary muscles can change shape of lens – amount of light refracted varies
 Allows focusing on objects at different distances
 Does most changing/

Question 4

Identify accommodation as the focusing on objects at different distances, describe its achievement through the change in curvature of the lens and explain its importance

Answer 4

• Cornea and anterior (front) & posterior (back) surface of lens  refract light entering eye
• Lens  convex lens
• Birds and mammals  lens is focused with ciliary body

Question 5

Accommodation of the eye

Answer 5

• Process  lens changes shape to focus images of objects at different distances onto the retina
• Way in which eye adjusts so light always falls on retina
• Image needs to fall on fovea  greatest density of cones to ensure…
Visual acuity  clearness of vision
• Light rays from distant objects  tend to be parallel and need less refraction to form clear image

Question 6

Changes in curvature of lens

Answer 6

• Focusing on near objects  requires more effort than distanced objects
• Long Distance vision  ciliary muscles = relaxed and lens thinned out
• Short Distance vision  ciliary muscles = contract and lens more rounded
 Bends diverging rays more so that clear image forms on fovea

Question 7

Whats the importance of accommodation of the eye, and the changes in curvature of the lens?

Answer 7

• Focusing = because of accommodation
• Essential for image to be focused  achieve clear vision
• Accommodation  allows humans to see both near and far objects clearly

Question 8

Changes in the eye due to distant and near objects?

Answer 8

• Lens is rounder  focused on near objects

* Lens is flatter  focused on distant objects

Question 9

When looking at something close?

Answer 9

Eye accommodates

Ciliary body squeezes until lens = short and dense & has greater curvature
Maximum > Accommodation & refractive power of lens

Question 10

When looking at something at distance?

Answer 10

Ciliary muscles relax

> Lens becomes longer and thinner

Question 11

Compare the change in the refractive power of the lens from rest to maximum accommodation, in terms of refractive power.

Answer 11

• Refractive power of lens  changed by altering its shape
• Thick lens ( )  able to refract light rays more than a thin lens ()
• When lens contracted  bulges out towards the front of the eye and is most powerful
• When lens relaxed  becomes flatter and is less refractive
• Shape of lens is altered by ciliary muscles  form ring around lens
 Lens attached to ciliary muscles by ring of suspensory ligaments

Question 12

Rest accommodation =

Answer 12

(Long distance)
 Ciliary muscles relaxed
 Pull on suspensory ligaments  pull on lens and keep thin
 Reduced refractive power of lens – so we can focus on objects in middle distance and far away

Question 13

Maximum accommodation =

Answer 13

(Short distance)
 Ciliary muscles tighten & contract inwards towards centre of lens
 Suspensory ligaments become looser
 Allow lens to bulge
 Increases refractive power
 Thicker lens brings divergent rays together quicker & sooner to converge on retina
• Focal point: point of convergence of light rays on retina
• Thicker lens = shorter focal length

Question 14

Model the process of accommodation by passing rays of light through convex lenses of different focal lengths, give some background.

Answer 14

• Double convex lenses of various thicknesses
• Distant objects  light rays travelling in almost parallel manner
• Close objects  light rays diverging from light source
• Image = inverted by brain (normally opposite way):

Question 15

Model the process of accommodation by passing rays of light through convex lenses of different focal lengths, name an aim

Answer 15

To model process of accomodation

Question 16

Model the process of accommodation by passing rays of light through convex lenses of different focal lengths, name a method.

Answer 16

1. Use ray box kit with double convex lenses of various thicknesses
2. Pass triple beam through each lens and plot path of each ray.
3. Measure distance between centre of lens and focal point
4. Repeat with lenses of different thicknesses

Question 17

Model the process of accommodation by passing rays of light through convex lenses of different focal lengths, name a conclusion

Answer 17

Some of the processes of accommodation could be modelled using double convex lenses of different thicknesses

Question 18

Whats Myopia?

Answer 18

• Short-sightedness
• Distance between lens and retina is too great or lens is too strong  image focused in front of retina
• Image appears blurred

Question 19

Whats Hyperopia?

Answer 19

• Far-sightedness
• Distance between lens and retina too short or too weak  image focused behind retina on imaginary spot
• Image appears blurred

Question 20

Name some technologies used to correct the conditions of myopia and hyperopia.

Answer 20

Eyeglasses & Contact lenses

Laser or Lasik Surgery

Question 21

Eyeglasses and Contact lenses do what?

Answer 21

• Artificial lenses used to correct myopia and hyperopia
• Myopia = concave
• Hyperopia = convex

Question 22

Laser or Lasik surgery does what?

Answer 22

• Reshaping the curvature of cornea
• Thin flap of corneal tissue cut out
• Tissue then folded back and laser beam is applied to exposed corneal tissue
• When laser finished  flap returned
• Contact lenses used for one week to hold corneal tissue in place

Question 23

Describe cataracts

Answer 23

• Cataracts  condition where lens grows cloudy and eventually becomes opaque
• Lens of human eye contain fibres made up of proteins and waterclef  when clumps of protein left on eye = cataract
• Opaque areas of the lens prevent light from reaching the retina  person becomes blind

Question 24

Describe the technology that can be used to prevent blindness from cataracts

Answer 24

• Cataract microsurgery:
 Replacing damaged or cloudy lens with intraocular lens
 Tiny incision made where sclera and cornea meet
 Instrument inserted – breaks up cloudy cataract lens and sucking it away leaving only lens capsule
 Then insert artificial intraocular lens
 2-20 minutes

Question 25

Discuss the implications of this technology on society

Answer 25

• Thousands of people who were cataract blind can now see
• Surgery  people can lead more independent lives
• Less public health cost = less community involvement for welfare
• More productive economy
• Many people die within four years of contraction
• Fred Hollows Foundation Australia:
 Work to bring affordable sight-restoring eye surgery to indigenous Australians, particularly in remote and isolated communities

Question 26

Explain how the production of two different images of a view can result in depth perception

Answer 26

• Depth perception: ability to judge distance from our eyes
• Depth perception = depends on binocular vision  fields of vision overlap
• Predatory animals  eyes placed toward front of head gives greater distance and depth perception

Question 27

Binocular Vision and depth of perception?

Answer 27

– When we use both eyes to look at something, the images formed on the retina are different  because eyes are spaced wide apart so position of objects look different from one eye
– Brain can interpret differences in two images from distance
– Beyond 60m  binocular vision does not improve depth perception  difference between images two small
 Rays of light from distant objects less divergent and images fall close to centre of

Question 28

Experience and depth perception?

Answer 28

– We have learned how tall or short certain objects are

– When we look at something  size of image on retina interpreted as being close or distant

Question 29

Movement and depth perception?

Answer 29

– Indication of distance
– Outer edge of retina sensitive in detecting movement
– Dim light  rods stimulated  aware of movement outside corner of eye