How We See 1 - Refraction and Accomodation Flashcards
What is refraction?
Bending of light when it passes from one optical medium to another
What parts of the eye are the refractive media?
Cornea
Aqueous humour
Lens
Vitrous humor
What parts of the eye are transparent to allow light to fall on the retina?
What does a sharp image form on the retina due to?
Light waves from an object bend at the cornea
Then bend some more at the lens to form a clear image on the retina
Is more ‘bending power’ needed when an object is close or further away from th eye?
Closer, so the lens becomes thicker and clear image is formed on the retina again
Does the lens become thicker or thinner when an object moves closer towards the eye?
Thicker, becomes thinner when an object moves further away form the eye
Is the cornea or the lens the most powerful ‘bender’ of light?
Cornea (45D), lens is only 15D
Although the cornea is more powerful at bending light than the lens, what is an advantage the lens has that the cornea does not?
It can change its shape to adjust its ‘bending’ power
From what ranges can our eyes focus on objects?
Distant objects (infinite distance) to close objects (20cm)
What is accomodation?
The ability of the eye to change its focus from distant objects to near objects and vice versa
What 3 things happen at the same time to comprise accomodation when going form a distant object to a near object?
Lens changes shape (becomes thicker when going from distant to close object)
Pupils constrict
Eyes converge
Explain the process of the lens changing its shape for accomodation?
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
Why does the lens become thicker to focus on close objects?
A thicker lens is more powerful so can focus on close objects
Why do the pupils constrict when we are focusing on close objects?
Pupils constrict to allow only a few rays (from those objects) to pass through
Done by pupillary constrictor controlled by parasympathetic innervation (IIIn)
Explain the process of eyes converging to focus on near objects?
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)
Is the medial or lateral rectus a thicker muscle?
Medial rectus
What are examples of refractive errors?
Myopia (short sightedness)
Hyperopia (long sightedness)
Astigmatism (non-spherical cuvature of cornea or lens)
Presbyopia (long sightnedness of old age)
What is the medical term for short sightedness?
Myopia
What is the medical term for long sightedness?
Hyperopia
What is the medical term for non-spherical curvature of cornea or lens?
Astigmatism
What is the medical term for long sightedness of old age?
Prebyopia
What is the medical term for perfect vision?
Emmetropia
In myopia, what objects appear clear and what objects appear hazy?
Close objects look clear and distant objects appear hazy
What is the most common cause of myopia?
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
What are symptoms of myopia?
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
What is the correction of myopia?
Bending power needs to be decreased:
spectacles
contact lenses
laser eye surgery
In hyperopia, what objects appear clear and what objects appear hazy?
Close objects look hazy and distant objects appear clear
Is hyperopia, does the light come to a focus behind or in front of the retina?
Behind
In myopia does the light come to a focus behind or in front of the retina?
In front
What is the most common cause of hyperopia?
Eyeball is too short and the lens is too flat so the image of distant object is formed behind the retina
In hyperopia, is accomodation used when it normally wouldnt be?
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
In myopia, is accomodation used when it usually wouldnt be?
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)
What are symptoms of hyperopia?
Eyestrain after reading/working on the computer in young individual
Convergent squint in children/toddlers
When children/toddlers have a squint due to hyperopia or myopia what needs to be done?
Immediate correction with glasses/lenses to preserve vision in both eyes and prevent a lazy eye
What is the correction for hyperopia?
Biconvex glasses alleviates use of glasses for focussing distant objects and rests accomodative power
Contact lenses
Laser eye surgery
What kind of lens is used in glasses for myopia and hyperopia?
Myopia - biconcave
Hyperopia - biconvex
What objects appear clear and hazy in astigmatism?
Both close and distant objects appear hazy
What is the most common cause of astigmatism?
Surface having different curvatures in different meridians (compare a rugby ball to a football)
What is the consequence of astigmatism in terms of the bending of light?
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
What is the correction of astigmatism?
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
What happens to the lens with age?
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
With age, does seeing close objects or far away objects become more difficult generally?
Close objects
When does presbyopia usually begind?
5th decade of life
What is the correction of presbyopia?
Biconvex “reading glasses”
