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)