Refraction of the eye

  • Myopia (90% of people with refractive errors have myopia)
  • Light from distance focuses in front of retina
  • Light from an object at the “far point” is focused on the retina
  • Far Point = the point in distance at which when light rays hit the eye, image focuses ON the retina (calculated by using 1/Rx (m))
  • A negative lens that makes the distance light appear to come from the far point will put distant objects into focus
  • Hypermetropia/hyperopia
  • Light from near (& possibly distance) does not focus on retina (lens cannot accommodate enough to bring near into focus)
  • Only point from which light can be focused on the retina is behind the eye – far point is virtual not real (so there is no distance in real space where you can place an object where they can see it clearly/so that it focuses on the retina)
  • A positive lens focuses distant light towards the far point & the eye focuses it on the retina
  • Accommodation: The eye can increase its power to focus on near objects (this happens via crystalline lens in eye – ciliary muscles contract (suspensory ligaments relax) & it bulges to increase in refractive power to focus on near, vice versa)
  • Accommodation reduces with age from ~16D to 0.50D – we have very strong accommodation when we’re young but it reduces from the day we’re born (reduces our ability to focus at near – presbyopia)
  • Lens does not have a blood supply & it never stops growing
  • As we age more & more fibres are added to the lens, so lens becomes more dense which causes it to lose its elasticity
  • Presbyopia happens to everyone & is not related to the Px’s refractive error
  • Reading glasses (positive spheres) make near objects clear (& distant objects blurred)
  • Regardless of refractive error, when wearing the distance correction near objects are blurred
  • Presbyopes therefore need different correction for distance & near
  • Myopes have poor distance vision
  • If the myopia is not too high then near vision is clear without any spectacles
  • Objects at the far point can be seen clearly & those closer than the far point can be made clear by accommodating
  • A young myope wearing spectacle lenses can see clearly at all distances
  • Hypermetropes ‘ eyes have too little dioptric power
  • When young hypermetropes accommodate the dioptric power increases & distant objects become clear hence giving them good unaided vision at distance
  • If hypermetropia is much less than the amplitude of accommodation, then they will have accommodation left over to allow clear vision at near as well as distance (if hypermetropia is too high then they cannot accommodate enough to see at near)
  • In later life, when accommodation is reduced, near & then distance unaided vision are reduced

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Optometrist Examination A Revision Notes

Positive lenses converge light – thicker in middle, thinner at edge

Negative lenses diverge light – thinner in middle, thicker at edge

Distance (f) at which light rays come to focus is the focal length

A dioptre (the unit of focal power) is the reciprocal of the focal length in m

Emmetropia = no refractive error – all light (from every direction) converges onto one focal point that lies on retina

Ametropia = abnormality of the eye (near-sightedness/far-sightedness/astigmatism)

Myopia = short-sightedness – light waves focus in front of retina therefore focal point is much shorter than the eye

Hyperopia/hypermetropia = long sightedness – light waves focus behind retina therefore focal point is longer than the eyeball

Astigmatism = two focal lines are formed

Visual Acuity

  • A term to describe the eye’s ability to discriminate detail in an object
  • Depends on two factors:
  • Magnitude of refractive error (dioptres of defocus)
  • Quality of the retina the image is focused upon
  • Vision = how well you see without any correcting lenses
  • If vision is bad could be refractive blur (&/or neural disorder)
  • VA = how well you see with an optimum correction
  • If VA is bad it is a neural (retina or brain) disorder
  • Measuring vision & VA:
  • Measure of the eye’s ability to discriminate detail: the smallest angle (α) subtended at the nodal point of the eye by two points which can just be resolved as separate = Min Angle of Res. (MAR) – where you can just about discriminate between two separate points
  • (w is the angle made at the eye between the two points)

If w > MAR then the two points are easily resolved

If w < MAR then two points not resolved

If w = MAR two points are just resolved

  • Bar grating method:
  • View at a distance where it appears uniformly grey (so lines are not made out) – cannot be resolved
  • Subject approaches grating stopping when it can be just resolved
  • Repeat six times & calculate average distance
  • Calculate w using the width of the bar & the average distance
  • Snellen letters: Notary Public London Solicitors
  • Measure resolution acuity in exactly same way as bar grating
  • An angular subtense of 1 arc minute forms the basis for the construction of the chart
  • This 1 min of arc angular separation is taken as the ‘norm’ for a measure of VA using letters
  • (each degree out of 360° is divided into 60 mins (‘) of arc, & each min of arc is divided into 60 secs (“) of arc)
  • Limbs & spaces subtend 1 arc minute at specified distances & letter height subtends 5 arc minutes (limb widths are 1/5th of the letter height)
  • These distances are (in m): 6, 9, 12, 18, 24, 36 & 60
  • The purpose of measuring VA is to obtain a figure which indicates state of the imaging system of eye
  • Snellen defined VA as: (Testing distance in m)/(Distance in m at which letters on the best line subtended 5 arc mins)
  • Usual testing distance = 6m
  • Most Px’s under 60 achieve better than 6/6
  • Recording Snellen Acuity: very smallest row of letters that subject can read should always be recorded for only then we can tell if there has been a deterioration between consultations

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Mob boss review

The Time Traveller realises that due to devolution, the human species is dying out as time goes on. It becomes clear how little impact human kind had on Earth. This is evident when the Time Traveller says, “I cannot convey the sense of abominable desolation that hung over the world… slow stirring monsters, the uniform poisonous – looking green of lichenous plants, the thin air that hurts one’s lungs”. Wells is telling us how the start and end of Earth will be the same. The Earth started off barren and simple and it will end the same as all the change and developments humans made will become nothing in the future and there’ll be no trace of technology at all. Wells is also suggesting that the world and mankind’s place on Earth is coming to an end. We are devolving into a world of weakness and due to this, we won’t be able to survive against nature. The Time Traveller was expecting an advanced world in the future however, he was met with an ancient world with no evidence of intelligence at all. Therefor we should stop placing such value in material things and advancements as they are futile.

Near the end of the novella, the Time Traveller says, “Then my eye travelled along to the figure of the white sphinx upon the pedestal of bronze… the rising moon grew brighter.” The sphinx is a monument that seems to constantly be watching over the Time Traveller whilst he in exploring the future. The ancient riddle of the sphinx associates this monument with the ageing and desolation of mankind. This suggests how human impact will now show in the future and how little effect we have on the universe. As the sphinx is a symbol of the ancient world, this continues to show how the world will look the same as when it was first created due to devolution removing all signs of intelligence and technology. Wells is trying to make us realise that our efforts to become the best now wont benefit us in the future – it will be the reason for our downfall and possible extinction as we won’t be able to survive against nature since technology will make us lazy and stupid.

 

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