Biology: cells and control - the eye

whats the first structure the light meets

the cornea

how does light pass through the cornea

its completely transparent, with no blood vessels

how does the cornea get oxygen

oxygen diffuses into it from the outside air

what does the cornea do

it cases all the light passing through it to refract to a certain amount

what is the second structure in the eye

the iris

what does the iris do

controls how big or small the pupil is

is the pupil a structure of the eye

no, its a gap in the middle of the iris to allow the light to pass through to the lens

what is the third structure in the eye

the lens

what does the lens do

to refract light, and the lens can change shape, it fine tunes the refraction so the light rays always converge directly on to the fovea

what does it mean if the lens can change shape

it means it can control how strongly it refracts the light

what does changing shape help the lens do

perfectly focus light on to the retina

what is the fourth structure in the eye

the retina

what are the two receptor cells in the retina

cone and rod cells

what do cone cells allow us to see

colour

whats the problem with cone cells

they dont work well in low light conditions

what do rod cells allow us to see

everything in black and white, and are more sensitive to light intensity

why cant you see colours in the dark

only the rod cells are working

what is the fifth structure in the eye

the fovea

what is the sixth structure in the eye

the optic nerve

what is the fovea

a special spot on the retina which is full of only cone cells

what do we try to focus light on

the fovea

what is the optic nerve

it takes all of the impulses generated by the receptor cells and transmits them to the brain

what is the iris reflex

controlling the light entering the eye

what happens when the light intensity it really high

the pupil constricts to stop damage on the retina

what happens when the light intensity it really low

the pupil dilates to allow lots of light into the eye

how does the iris control the iris reflex

its made up of two different types of muscles that constrict and relax

what muscles are on the inside of the iris

circular muscles that stretch around the pupil

what muscles are around the outside of the iris

the radial muscles

what do the radial muscles do

stretch from the inside to the outside

how does the pupil constrict in bright light

the circular muscles contract, squeezing the pupil smaller and the radial muscles are relaxed so they can be stretched longer in the iris

how does the pupil dilate when their is low light

the circular muscles relax so the pupil and the radial muscles contract, making them shorter, pulling the pupil open

what is accomodation

a reflex that changes the refractive power of the eye

what does accommodation allow

you to see distant and near objects

what do the ciliary muscles and suspensory ligaments control

the shape of the lens

how is light mainly directed onto the fovea

the cornea does most of the refracting as it always refracts by the same amount

what will the light rays be like if the object is far away

parallel

how will the light rays his the eye if the object is close by

at a wide angle

if an object is close by what does the lens have to be

short and fat so it is more curved, this means it can refract more strongly

how is the lens short and fat

the ciliary muscles contracts inwards towards the lens. because the ciliary muscles are closer to the lens the suspensory ligaments slacken

what happens if the suspensory ligaments slacken

they are no longer pulling on the lens so it can return to its fatter more natural shape

wht do the suspensory ligaments do

attach the lens to the ciliary muscles

what are the ciliary muscles

a muscle that stretches all the way around like a ring

if the lens doesn’t need to do as much wat does this mean

the object is far away and the cornea has already refracted the light most of the way

how do you reduce the refractive power of the lens

you need to stretch it out

what happens for the lens to stretch out

the ciliary muscles relax moving away from the lens, and pulling the suspensory ligaments taut

what happens when the suspensory ligaments are taut

the lens stretches out

what happens when a ciliary muscle contracts

it moves inwards to wards the lens, never outwards

are the suspensory ligaments muscles

no so they can nly e pulled taut, and slacken, not relax and contract

what does t mean if you are long-sighted

you can see long distances but not close up and the lens doesnt refract enough, light focuses behind the retina

what does it mean if you are short sighted

you can see near by objects but not objects far away and the lens doesnt refract enough, light focuses infront of the retina

what lens is in glasses to help people who are long sighted

a convex lens, which provides extra refracting power

what lens is in glasses to help people who are short sighted

a concave lens, which refracts light outwards so it counteracts the over refraction of the lens

what is the medical name for longsightedness

hyperopia

what is the medical name for short sightedness

myopia