the eye

check notes+ for diagrams and more info

where are the light sensitive receptor cells

retina, back of the eye - causes changes in receptors that send impulses to the brain along the optic nerve.

which parts of the eye refract light

cornea and lens

which part of the eye are the rays of light are focused to get a sharp image

fovea

function of iris

coloured ring of tissue behind the cornea

function of pupil

hole in the centre of the iris through which light enters the eye 

function of conjunctiva

a thin, semi-transparent membrane that covers the exposed white part of the eyeball (sclera) and lines the inner surface of the eyelids

function of lens

helps focus the image

function of cornea

transparent part of sclera at the front of the eye

function of ciliary muscle

muscles attached to suspensory ligaments

function of optic nerve

carries electrical impulses to the brain

function of sclera

tough out coat of eye (which is the visible white part)

function of choroid

dark, pigmented layer beneath sclera, contains blood vessels

function of retina

innermost layer of of the back of the eye - light sensitive layer where light energy is traduced into electrical energy of nerve impulses - contains rods and cones

function of fovea

high concentration of cones to provide a sharp image

function of blind spot

optic nerve attaches to retina, contains no light sensitive cells

function of aqueous humour

water fluid between cornea and lens

function of vitreous humour

clear gel that fills space between retina and lens, maintains shape of eyeball (front to back) so focusing distance doesn’t change

accommodation

changes that occur in the eye that allow us to focus on objects at different distances 

types of light sensitive cells

rods and cones

function of rods

works in dim light, uniformly distributed along the retina, responds to light intensity

cones

only work in hight light intensities, responds to wavelength of light and detects colour, mainly found in the fovea

what happens when light hit the cells

impulses are sent along the optic nerve to the brain

how does light bend (accommodation)

most done by cornea, lens changes shape slightly

describe how the eye helps you thread a needle (near object) 

• light rays are diverging from a nearby object, lens has to be more convex in order to refract the light rays enough to focus them on the retina   

• ciliary muscles contract and suspensory ligaments become slack 

• reducing tension on the lens which becomes fatter/rounder shape 

• refracting the light more  

describe how the eye helps you to see a far object

• rays of light are almost parallel when they reach the cornea 

• cornea refracts the light rays, lens does not need to refract the light rays much more      

• ciliary muscles relax; suspensory ligaments are pulled taut   

• lens is pulled thin and flat. refracting light less

what happens to the image of the retina

is inverted, brain turns it the right way up

what happens if there’s too much light

damage the light sensitive cells of the retina (rods, cones)

where does light enter

pupil (hole in the centre of the iris)

types of muscles in the iris

circular muscles and radial muscles (dilate pupil - one makes them smaller and one makes them larger)

role of iris

control the amount of light entering the eye by changing the size of the pupil

what happens in bright light

• circular muscles contract 

• radial muscles relax 

• pupil contracts 

• less light enters the eye 

what happens in dim light

• circular muscles relax 

• radial muscles contract 

• pupil dilates 

• more light enters the eye

iris reflex

automatically without conscious thought 

what happens to rods in bright light

they lose the ability to respond (colour pigment in cells that responds to light is changed to another form)

what happens to rods when light become dimmer

pigment changes into a form that can detect light (takes up to half an hour)

what is a cataract

when lens becomes cloudy due to proteins deposited

treatment for cataracts

removing the lens and replacing it with an artificial one which helps you see objects at a distance, but one needs glasses to see near objects as the artificial lens cannot change shape like the real one

when do the lens need to be more convex

the closer you are trying to see