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Nervous System > How We See 1 - Refraction and Accomodation > Flashcards

How We See 1 - Refraction and Accomodation Flashcards

1
Q

What is refraction?

A

Bending of light when it passes from one optical medium to another

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2
Q

What parts of the eye are the refractive media?

A

Cornea

Aqueous humour

Lens

Vitrous humor

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3
Q

What parts of the eye are transparent to allow light to fall on the retina?

A
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4
Q

What does a sharp image form on the retina due to?

A

Light waves from an object bend at the cornea

Then bend some more at the lens to form a clear image on the retina

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5
Q

Is more ‘bending power’ needed when an object is close or further away from th eye?

A

Closer, so the lens becomes thicker and clear image is formed on the retina again

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6
Q

Does the lens become thicker or thinner when an object moves closer towards the eye?

A

Thicker, becomes thinner when an object moves further away form the eye

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7
Q

Is the cornea or the lens the most powerful ‘bender’ of light?

A

Cornea (45D), lens is only 15D

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8
Q

Although the cornea is more powerful at bending light than the lens, what is an advantage the lens has that the cornea does not?

A

It can change its shape to adjust its ‘bending’ power

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9
Q

From what ranges can our eyes focus on objects?

A

Distant objects (infinite distance) to close objects (20cm)

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10
Q

What is accomodation?

A

The ability of the eye to change its focus from distant objects to near objects and vice versa

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11
Q

What 3 things happen at the same time to comprise accomodation when going form a distant object to a near object?

A

Lens changes shape (becomes thicker when going from distant to close object)

Pupils constrict

Eyes converge

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12
Q

Explain the process of the lens changing its shape for accomodation?

A

1) Ciliary muscle contracts making ciliary body bulge
2) Space in the middle decreases
3) Suspensory ligament becomes lax
4) Lens is no longer under stretch
5) Lens becomes thicker

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13
Q

Why does the lens become thicker to focus on close objects?

A

A thicker lens is more powerful so can focus on close objects

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14
Q

Why do the pupils constrict when we are focusing on close objects?

A

Pupils constrict to allow only a few rays (from those objects) to pass through

Done by pupillary constrictor controlled by parasympathetic innervation (IIIn)

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15
Q

Explain the process of eyes converging to focus on near objects?

A

1) Eyes need to turn to look at object (convergence)
2) Medial rectus muscles of both eyes do this
3) Which is why medial rectus is thicker than lateral rectus (we spend a lot of time doing ‘close’ work)

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16
Q

Is the medial or lateral rectus a thicker muscle?

A

Medial rectus

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17
Q

What are examples of refractive errors?

A

Myopia (short sightedness)

Hyperopia (long sightedness)

Astigmatism (non-spherical cuvature of cornea or lens)

Presbyopia (long sightnedness of old age)

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18
Q

What is the medical term for short sightedness?

A

Myopia

19
Q

What is the medical term for long sightedness?

A

Hyperopia

20
Q

What is the medical term for non-spherical curvature of cornea or lens?

A

Astigmatism

21
Q

What is the medical term for long sightedness of old age?

A

Prebyopia

22
Q

What is the medical term for perfect vision?

A

Emmetropia

23
Q

In myopia, what objects appear clear and what objects appear hazy?

A

Close objects look clear and distant objects appear hazy

24
Q

What is the most common cause of myopia?

A

Eye is too long so when cornea and lens bend light they make an image form in front of the retina so far off objects are not seen clearly

25
Q

What are symptoms of myopia?

A

Headaches

Infants and preverbal children have divergent squint

Toddlers show loss of interest in sports/people an more interest in books

Teacher may ontice child losing interest in class

26
Q

What is the correction of myopia?

A

Bending power needs to be decreased:

spectacles

contact lenses

laser eye surgery

27
Q

In hyperopia, what objects appear clear and what objects appear hazy?

A

Close objects look hazy and distant objects appear clear

28
Q

Is hyperopia, does the light come to a focus behind or in front of the retina?

A

Behind

29
Q

In myopia does the light come to a focus behind or in front of the retina?

A

In front

30
Q

What is the most common cause of hyperopia?

A

Eyeball is too short and the lens is too flat so the image of distant object is formed behind the retina

31
Q

In hyperopia, is accomodation used when it normally wouldnt be?

A

Yes, image of distant object is formed behind the retina so the person starts to use accomodative power and make the lens thicker to bring the formation of the image on the retina

So they are using lens power to see far off things that are normally seen wihout using any power

When seeing close up objects they use more and more power until the power is all used up, so they cannot see near objects

32
Q

In myopia, is accomodation used when it usually wouldnt be?

A

No, because light focuses in front of the retina from far away objects, so when they are brought closer divergent rays from th eobject reach the cornea and an image is formed without the need to increase the curvature of the lens (using accomodative power)

33
Q

What are symptoms of hyperopia?

A

Eyestrain after reading/working on the computer in young individual

Convergent squint in children/toddlers

34
Q

When children/toddlers have a squint due to hyperopia or myopia what needs to be done?

A

Immediate correction with glasses/lenses to preserve vision in both eyes and prevent a lazy eye

35
Q

What is the correction for hyperopia?

A

Biconvex glasses alleviates use of glasses for focussing distant objects and rests accomodative power

Contact lenses

Laser eye surgery

36
Q

What kind of lens is used in glasses for myopia and hyperopia?

A

Myopia - biconcave

Hyperopia - biconvex

37
Q

What objects appear clear and hazy in astigmatism?

A

Both close and distant objects appear hazy

38
Q

What is the most common cause of astigmatism?

A

Surface having different curvatures in different meridians (compare a rugby ball to a football)

39
Q

What is the consequence of astigmatism in terms of the bending of light?

A

Bending of light rays along one axis will never be the same as the other axis

Image formed is always hazy whatever the distance of the object

40
Q

What is the correction of astigmatism?

A

Special glasses (cylindrical glasses) which are curved only on one axis

Layer eye surgery can be used to correct thing]

Special contact lenses called toric lenses

41
Q

What happens to the lens with age?

A

Becomes less mobile/elastic

So when the ciliary muscle contracts it is not as capable of changing its shape as before

So near objects starts to become difficult and glasses are needed

42
Q

With age, does seeing close objects or far away objects become more difficult generally?

A

Close objects

43
Q

When does presbyopia usually begind?

A

5th decade of life

44
Q

What is the correction of presbyopia?

A

Biconvex “reading glasses”

Nervous System (48 decks)

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