From StudyingMed

< AEB
Jump to: navigation, search

Reflection and refraction

  • Light can reflect from a reflective surface
    • The angle of incidence = the angle of reflection
  • Light travels at different speeds through different media
    • The higher the refractive index, the slower it travels
    • If light slow down, it bends towards the normal, if it speeds up, away from the normal
      • Light travels faster in air than in water
    • Refraction is the bending of light rays when it passes from one medium to the other
      • Bending increases based on the refractive index and the size of the angle of incidence from normal
  • Refractive index
    • The ratio of the speed of light in a vacuum to the speed of light in the medium
      • Air = 1, water = 1.33 (25oC)
  • Read wikipedia:Snell's law
  • Read also the lensmaker's equation

Lenses

  • Two types:
    • Convex lenses - converge light to a point
      • If parallel light rays are passed through the lens, the point they pass through is the focus
      • The distance of this from the lens is the focal length
    • Concave lenses – diverge light
  • Power
    • Depends on:
      • Radius of curvature of the lens
      • Refractive index of the medium
    • Power (D) = 1/focal length
      • In concave lenses, the diopter (power) is negative
  • In the eye, the lens changes shape to focus the incoming light onto the retina
  • Read this about lensmaker's eqn and power of lens.



The eye

  • The lens causes focussing of the incoming light onto the retina
    • This causes inversion of the image, so on the retina, the image is inverted
    • The brain, however, doesn’t swap the image the right way up, it doesn’t know any different
  • Anatomy
    • Front part of eye (lens, aperture, etc) is all concerned with forming an optically good image
    • Back part of the eye is about recording the optical information
    • Retina – light causes electrical activity
      • Photoreceptors are at the back
      • Blood vessels are at the front, break through the retina at the blind spot (optic disk; also where the retina
      • Macula lutea (yellow region) with a pit in the middle (fovea)
      • No big blood vessels on central part of the retina (improve acuity)
    • Ciliary body – adjusts lens power
      • To look at close objects, the ciliary muscles contract causing an increased lens radius
    • Pupil – controls light exposure and focus via the pinhole effect
      • Pinhole effect – if light is coming in parallel only, focus is guaranteed
  • The pupil constricts on close focus to reduce peripheral light rays that are hard to focus
  • Power of the lens and cornea depend on the refractive index either side
    • Thus, although the lens has a higher refractive index, the cornea is 2x as important as the lens in focussing because it has a higher refractive index difference
      • This explains why underwater is blurry – lower refractive index difference
    • To focus at infinity (ie, at its most relaxed), the power of the eye is 59D, focal length 17mm
      • 2/3 of this power is due to the air/cornea interface (lens is only about 1/3 of the power of the lens)
    • At rest, we should be able to focus on distant objects (infinity) because rays are coming in parallel
      • Thus if this is case, the eye is emmetropic (i.e. a normal eye where this happens is called emmetropic)
      • To focus on closer objects, we need to contract the ciliary muscles causing a fattening of the lens (accommodation)
  • Lens power can be changed by up to 14D in young people (due to elastic lenses), this decreasing with age
    • Loss of accommodation is called presbyopia
      • Clouding of the lens is called cataracts



Accommodation

  • Distance vision, ciliary muscles are relaxed
    • For close vision, rays are more diverging so ciliary muscles contract to accommodate and create a rounder lens
    • Relaxation of the ciliary muscles pulls the lens flat by tugging on the zonules
  • Illusion
    • There is a limit to the resolution in the retina
      • In the small image (further away) we can’t see the details of the angry face
      • In the other image, the features are so broad so they are not picked up until it is far away
        • Therefore the visual system cannot resolve a) fine detail or b) too-course detail. There is an optimal range of fineness of detail that our visual system operates in.
  • Long and short sightedness
    • Normal eye – light falls directly on the retina
    • Hyperopic (long sighted) eye – light falls behind the retina
      • Eyeball is too short
      • Not many in young people because they can use their lens to compensate
  • Ie, can contract more (hyperopes), can’t relax more (myopes)
    • Myopia (short sighted) eye – light falls in front of the retina
      • Eye ball is too long
      • Thought to be due to the eye lengthening because of blurry vision when reading late into the night
  • Laser surgery makes the radius of curvature of the cornea smaller.



Visual fields and pathways

  • The retina uses several cell types to process visual information (outside to inside):
    • Ganglion cells, amacrine cells, bipolar cells, horizontal cells, and photoreceptors (cones and rods)
  • Pathway
    • Retina --> optic nerve --> optic chiasm --> optic tract --> lateral geniculate nucleus of thalamus --> optic radiations --> primary visual cortex
  • Fovea
    • At the fovea there are more receptors and cortical neurons than other areas
      • This allows focus on the area we are looking at
      • Resolution changes from centre to periphery
    • Has cortical magnification – small area in visual field has a larger area of cortex devoted to its processing
    • The retina is "back to front" so that the rods and cones have a better blood supply
    • Foot of photoreceptor makes a synapse with other cells in the retina (to transmit their information to the optic nerve)
  • Retinal molecule inside opsin; retinal is derived from vitamin A
    • Photon hits retinal, makes it change shape, and it pops out of the opsin
    • This makes the opsin (embedded in cell membrane of photoreceptor cell) change shape
    • The opsins are within disks in the photoreceptor cell, to maximise the chance of a single photon hitting an opsin
    • The opsin activates intracellular pathways (transducin-->phosphodiesterase) that reduce cGMP production (so a single photon can cause a lot of intracellular changes and be detected as AP).
    • Photoreceptors are most active in the dark, because there is lots of cGMP, allowing Na influx, then Ca influx further down, NT release and postsynaptic AP production
    • Overall: light gates the activity, but does it in the opposite way
    • So the photoreceptor detecting light results in decreased activity
    • (I.e. there are many ways in which the visual system is "backwards")
  • Centre of visual field - bipolar cells collecting information from a small group of photoreceptors
    • More laterally - horizontal cells receive information from very many photoreceptors (less visual acuity)
    • Amacrine cells help set up contrast and colour (see Phase 2)
    • Information ultimately gets to ganglion cells, which carry information up to the optic nerve
  • Optic radiation
    • Links the thalamus and cortex
    • Splits into paths that follow the retrolenticular part and sublenticular parts of the internal capsule
  • Lesions at different areas along the optic pathway can determine the effect on the visual field
    • Before the optic chiasm destroys vision in one eye
    • At the optic chiasm cuts vision of the lateral half of each eye
    • After the optic chiasm removes the left or right visual field
    • The sublenticular part causes loss of the upper quadrant of the left or right visual field
    • The retrolenticular part causes loss of the lower quadrant of the left or right visual field
      • May get macular sparing where the centre of the eye remains clear

Photoreceptors

  • Have different distributions in different parts of the visual field
    • At the fovea (centre of visual field)– highest concentration of cones
    • Peripherally – low concentration of cones, high concentration of rods
      • Thus if want to look at faint stars, better to look indirectly so that rods can pick up light
  • Rods and cones have different light sensitivities (because cones have a smaller end with opsins etc, so less likely to pick up photons in the dark; these cells are more likely to determine fine detail when there is light). Rods = more likely to pick up photons in dark, low acuity (as peripheral in retina), and are black and white.
    • Scoptopic light range – rods (eg: moonless night sky
    • mesopic – both (eg: comfortable reading)
    • photopic – cones (eg: white paper in full sun)
    • Rods and cones have different proportions in different parts of the eye (see chart).
    • 3 types of cones (R, G, B - preferential), one type of rod. This is because we have different types of opsins in them. But a particular type of cone will respond to a range of colours, but at different levels of activity
      • hence colour blind at night, because a rod has only one type of opsin and can't distinguish both intensity and frequency on its own (it will have some response to all colours) (and green colours tend to look brighter than others)
    • need both cones and rods to tell between colours
  • Colour
    • Protanopia – loss of red cones
    • Deuteranopia – loss of green vision deficiency cones
    • Tritanopia – loss of blue cones
  • Note that dogs have only 2 types of cones
  • Anomalous trichromancy
    • Most common form of red-green colour blindness
    • Red and green cones are present, but peak absorbances of one of the photopigments (opsins) has been shifted closer
      • Red and green opsins are thought to be evolutionary recent
  • Primates, but not carnivores so that we could see fruit ripeness
  • Found on the X-chromosome
    • Thus males are more common to have colour blindness
    • Reds and greens appear very similar (as well as purples and greys)
      • Most colours in context are still distinguishable