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Flashcards in Vision 1 Deck (37)
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What is refraction?

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

Light rays bending to form a sharp image on the retina


What are refractive errors?

Mismatch between how much we bend light rays


What parts of the eye does light pass through and therefore contribute to refraction

Aqueous humor
Vitreous humor

All these transparent to allow light to fall on retina


How does a share image form on the retina?

Light waves from an object bend at the cornea, bend some more at the lens to form a clear image on the retina.
This bending of light waves is called Refraction.

When an object comes closer, the eye needs more bending power to focus on an object (the lens becomes thicker and hence more powerful, and a clear image is formed on the retina again)


What is better at refraction cornea or lens?

Cornea is the most powerful "bender" of light (45D) but lens (15D) has the capacity to change its "bending power"


Explain accommodation

Our eyes have the capacity to change the focus from distant objects (infinity) to close objects (20cm)

The changes occurring in both eyes as it changes focus from a distant to a close object is called accommodation.


What 3 things happen simultaneously and comprise accommodation?

-LENS CHANGES SHAPE (becomes thicker & more spherical)
-Pupil constricts
-Eyes converge


Describe the process of lens thickening

-Ciliary muscle contracts making the ciliary body bulge (parasympathetic).
-Space in the middle decreases
-Suspensory ligaments become lax
-Lens is no longer under stretch


Describe the process of pupil constriction

-When we are looking at an object up close, we need a sharp focus.
-To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through.
-Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation IIIn.


Describe the process of eye convergence

When focusing on an object up close, our eyes have to turn in to look at the object (convergence)

We use our medial rectus muscles of both eyes to converge (IIIn)

Humans spend a lot of time doing "close" work, so our Medial recti muscles are thicker than our lateral recti muscles.


What are the 4 refractive errors?

-Myopia: short sightedness
-Hyperopia: long sightedness
-Astigmatism: non-spherical curvature of cornea (or lens)
-Presbyopia: long-sightedness of old age

If you are lucky enough to have perfect vision you are an emmetrope (i.e. you have emmetropia)


What is myopia?

Shortsightedness: Close objects look clear, distant objects appear hazy.

Picture focuses in front of retina

Basically the bending power of cornea + lens are too much for that eyeball

When the object is brought closer the rays coming from it are divergent; and then this "bending power" comes to use. The image is formed on the retina without needing to increase curvature of lens.


What is the most common cause of Myopia?

Eyeball too long

So the cornea and lens, when the bend rays of light, make the image to form IN FRONT of the retina. So far off objects not seen clearly.

When the object is brought closer the rays coming from it are divergent; and then this "bending power" comes to use. The image is formed on the retina without needing to increase curvature of lens.


What are the symptoms of myopia?

Headaches, Complain of not being able to see blackboard/ distant objectsInfants & preverbal children divergent squint

Infants and preverbal children -> divergent squint

Toddlers -> loss of interest in sports/people. More interest in books, pictures.

Teachers may notice child losing interest in class


How to correct myopia

Bending power needs to be decreased.

Biconcave lenses
-Contact lenses
-Laser eye surgery


What is hyperopia?


Image focuses behind the retina

Close objects look hazy, distant objects appear clear.


What causes hyperopia?

Eyeball too short or cornea + lens too flat

So when an image of a distant object is formed on the retina it lies BEHIND THE RETINA.

The person then automatically starts to use his accommodative power and makes the lens thicker.
This causes the image to form on the retina.

So he is using the lens power to see far off thing (that he should normally be seeing without using any power).

When seeing closer objects, he uses more and more power until ultimately, his power is all used up.


What are the symptoms of hyperopia?

Eyestrain after reading/ working on the computer in a young individual.

Convergent squint in children/ toddlers - needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent lazy eye


What is the correction of hyperopia?

Biconvex glasses alleviates use of glasses for focusing distant objects and "restS" the accommodative power

Contact lenses
Laser eye surgery


What is astigmatism?

Both close and distant objects appear hazy

Multiple images are formed


What causes astigmatism?

Surface of the lens has different curvatures in different meridians

So the bending of light rays along one axis will never be the same as that of the other axis.

So image formed is always hazy, whatever the distance of the object


How do you correct astigmatism?

Special glasses - called cylindrical glasses (which are curved in only one axis)

Laser eye surgery can also be used to correct the defect.

Need special contact lenses called topic lenses


Explain presbyopia

Longsightedness of old age.

With age the lens gets less mobile/elastic.
So when the ciliary muscle contracts, it is not as capable as before to change shape.
So seeing near objects/ reading the newspaper starts to become difficult -> needs glasses to read

Usually starts in 50s


How do you correct presbyopia?

Biconvex "reading glasses"


What is the process of us actually seeing called?



What is phototransduction?

Visible light is an electromagnetic wave.

The energy in light waves need to stimulate photoreceptor cells on the retina (phototransduction)

Phototransduction is defined as the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)

The phototransduced rods and cones need to activate optic nerve neurones (generate an action potential)


Explain the structure of rods and cones

Outer segment contains stacks of discs/laminae which have photoreceptors
-Cones this is like an ice-cream cone shape
-Rods this is rod shaped

Inner segment just behind with cell organelles

Outer limiting membrane divides cell. Just inside this is nucleus connected to axon and synaptic terminal.
-This area is the muller cell


Explain the visual pigment in photoreceptor outer segments

Each lamellae is made up of cell membrane.
Integrated into this cell membrane is the visual pigment rhodopsin in rods and small, medium and large wave cone-opsins .
These molecules differ in their spectral sensitivity


What are the different visual pigments and what do they correspond to on the spectrum?

Small wave cone-opsin = blue cone


Medium wave cone-opsin = green cone

Large wave cone-opsin = red cone


Explain the visual pigment Rhodopsin

Opsin + 11-cis Retinal

Integral transmembrane helical protein in the lamellae.
Chromophore nests in the opsin of this protein (11-cis retinal).

When light falls on 11-cis retinal, it isomerism to all- trans retinal

All-trans retinal cannot fit into the opsin. So rhodopsin splits.
this results in BLEACHING of the visual pigment