How we see

Question 1

What is refraction?

Answer 1

light rays bend to form a sharp image on the retina

Question 2

What is accommidation?

Answer 2

We can focus on far or near objects by changing how much we bend the light rays

This process is achieved by the lens changing its shape

Question 3

What are refractive errors?

Answer 3

simetimes there is a mismatch between how much we bend light rays leading to refractive errors

Question 4

What is refraction?

Answer 4

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

(An optical medium is material through which electromagnetic waves propagate)

Question 5

How does a sharp image form on the retina?

Answer 5

light waves form an object bend at the cornea, bend some more at the lens to form a clear image on the retina and this bending of light is called refraction

Question 6

What happens when an object comes closed to the eye?

Answer 6

when an object comes clsoer to the eye, the eye needs more bending power to focus on an object so the lens becomes thickers and thereofre more powerful, and a clear image is formed on the retina again

Question 7

What things in your eye bend light rays?

Answer 7

The cornea is the most powerful “bender” of light (45D) but lens (15D) has the capacity to change its “bending power”

Question 8

Our eyes have the ability to change focus form distant objects to close objects

the changes are occuring in both eyes as it changes focus form a distant to a close object

What is this called?

Answer 8

accomidation

Question 9

what are the 3 things that happen all togther and make up accomidation?

Answer 9

lens change shape (become thicker and more spherical)

pupil constricts

eyes converge

Question 10

What is the process of the lens thickening in accomidation?

Answer 10

the ciliary body contracts (parasympathic - CN III) causing the space inbetween the ciliary body and the lens to decrease and the suspensory ligamens to become lax meaning the lens is no longer streched leading to the lens thickening and becoming more spherical

a thicker lens is more powerful and can focus on close objects

Question 11

When we look at an object close up we need a sharp focus so how is the focus sharpened?

Answer 11

pupil constricts to only allow a few rays to pass through

Question 12

What is responsible for constricting the pupil?

Answer 12

pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation

Question 13

What does it mean when the eyes converge as part of accomidation?

Answer 13

when focusing on a object close up, our eyes have to turn to look at the object

Question 14

What muscles do we use to converge our eyes?

Answer 14

medial rectus muscles of both eyes

(IIIn on both sides)

Question 15

How do humans medial recti msucles compare to their lateral recti muscles?

Answer 15

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

Question 16

What are 4 common refractive errors?

Answer 16

myopia

hyperopia

astigmatism

presbyopia

Question 17

What is prefect vision called?

Answer 17

emmetrope, i.e. you have emmetropia

Question 18

What is myopia?

Answer 18

short sightedness

struggle to see things that are far away

Question 19

What is the most common cause of myopia

Answer 19

eyeballs are too long

Question 20

Explain the cause myopia?

Answer 20

It occurs when the shape of your eye causes light rays to bend (refract) incorrectly, focusing images in front of your retina instead of on your retina

So when the cornea + lens bend rays of light, they make the image form IN FRONT OF the retina so far away objects not seen clearly

^{(Basically the “bending power” of cornea + lens is too much for that eyeball)}

Question 21

In a myopic person, can they see closer up objects clearly?

Answer 21

Yes

When the object is brought closer, divergent rays from the object reach the cornea and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens (i.e. without using the accommodative power)

Can see near things clearly i.e. near-sighted

Question 22

What symptoms may be seen in myopia?

Answer 22

headaches, complain of not being able to see the blackboard/distant objects

infants/preverbal children - divergent squint

toddlers - loss of intrest in sports/people and mor eintrest in books/pictures

teacher may notice child loosing interest in class

Question 23

Why may someone with myopia develop a divergent squint?

Answer 23

Normally eyes converging when using accommodating power but in myopics they don’t use accommodating power so eyes may divert

Question 24

How can myopia be corrected?

Answer 24

bending power of the eyes need to be decreased and this is done by using biconcave lenses

e.g. spectacles, contact lenses and laser eye surgery

Question 25

What is hyperopia?

Answer 25

long-sightedness

struggle to see things up close but far objects are seen normally

opposite of muopia

Question 26

What causes hyperopia?

Answer 26

Often it occurs when the eyeball is too short, or the lens or cornea is too flat so the image of a distant object is formed 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 his lens power to see far off things (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!

So he can’t see nearby objects ie. “longsighted”

Question 27

What are the symptoms of hyperopia?

Answer 27

Symptoms of 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 a “lazy eye”

Question 28

How can hyperoipa be corrected?

Answer 28

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

Contact lenses

Laser Eye surgery

Question 29

What is astigmatism?

Answer 29

non-spherical curvature of cornea (or lens)

an imperfection in the curvature of your eye’s cornea. This hinders your eye’s ability to focus light onto your retina, resulting in blurred and distorted vision

Question 30

What is the cause of astigmatism?

Answer 30

a persons lens or cornea has an irregular curve so the bending of light rays along one axis will never be the same as that of the other axis

this leads to the image formed always being hazy no matter what distance the object is

Question 31

How is astigmatism corrected?

Answer 31

So to correct it we need 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 toric lenses

Question 32

What is presbyopia?

Answer 32

long-sightedness of old age

Question 33

What causes presbyopia?

Answer 33

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

Question 34

When would someone most commonly experince presbyopia?

Answer 34

Usually starts in 5th decade of life

Question 35

How is presbyopia treated?

Answer 35

Correction is by using biconvex “reading glasses”

Question 36

Study this image

Answer 36