4.5.2.3 The Eye (biology only) Flashcards
Eye:
a sense organ containing receptors sensitive to light intensity and colour
Diagram of the eye:
Function of the retina:
- layer of light sensitive receptor cells (rods and cones) found at back of eye - rods detect black and white contrast/light and cones detect colour + cones less effective in dim light but rods more effective in dim light
- when light hits this, the cells are stimulated and stimulus converted to electrical signal by receptor cells
- electrical impulses are sent to brain which interprets the information to create an image
Function of pupil:
hole which lets light into the eye
Function of cornea:
- the see-through layer at the front of the eye
- allows light through and the curved surface bends and focuses light onto the retina (where 70% of light is refracted onto retina)
Function of lens:
focuses light onto retina
Function of sclera:
tough, white outer coating which supports the structures inside the eye - strong to protect and prevent some damage to the eye
Function of optic nerve:
- nerve that leaves the eye and leads to the brain
- where the info from receptor cells is collected and nerve impulses are carried from retina to the brain to create an image
Function of iris:
- muscles that surround the pupil
- control the amount of light entering the eye by contracting or relaxing to alter the size of the pupil
Function of fovea:
- fovea/yellow spot - a concentration of rods and cones where the light ideally focuses
Function of ciliary muscles and suspensory ligaments:
hold the lens in place and control its shape
Function of vitreous humour:
back of the eye is filled with vitreous humour - keeps the eyes shape
Function of choroid:
The choroid absorbs the light and stops it being reflected back, and also have a copious blood supply which provides the retina with oxygen and food
Accommodation:
the process of changing the shape of the lens to focus on near or distant objects
How do eyes focus on near objects?
Through accommodation
- Diverging rays of light from a near object
- Cornea refracts light rays
- The ciliary muscles contract
- So ciliary muscles have a smaller diameter
- The suspensory ligaments loosen/slacken
- The lens is then thicker and more curved so refracts the light rays strongly/lens is more convergent
- Rays focused onto the retina
Ray diagram of eyes focusing on near objects:
How do eyes focus on distant objects?
Through accommodation
- Parallel rays of light from a distant object
- Cornea refracts light rays
- The ciliary muscles relax
- So ciliary muscles have a larger diameter
- The suspensory ligaments are pulled taught
- The lens is then pulled flat and thin and only slightly refracts light rays
- Rays focused onto the retina
Ray diagram of eyes focusing on distant objects:
How does the eye adapt on bright light?
- radial muscle fibres and circular muscle fibres work antagonistically
- radial muscle fibres relax to and circular muscle fibres contract close iris up and make pupil smaller letting less light into eye avoiding damage to the retina
How does the eye adapt to dim light?
- radial muscle fibres and circular muscle fibres work antagonistically
- radial muscle fibres contract and circular muscle fibres relax causing pupil to dilate allowing more light to enter the eye to create a better image
How does the eye protect itself?
- tear gland gives out a fluid which acts like a windscreen cleaning fluid - contains the enzyme lysozyme which kills germs
- the eyelids and eyelashes stop sweat and dust running into the eye
What are two common defects of the eyes?
myopia (short-sightedness) and hyperopia (long-sightedness) in which rays of light do not focus on the retina
Myopia:
medical condition called short-sightedness in which people cannot clearly see objects far away
What is myopia caused by?
- eye muscle cannot relax enough to make the lens thin enough
- the eye-ball is too long for the lens system + lens is too curved so rays from a distant object come to a focus just in front of the retina - so distant objects appear blurry
Hyperopia:
medical condition called long-sightedness in which people cannot clearly see objects close to them
What is hyperopia caused by?
- caused by weak eye muscles and often develops as the muscles weaken with age - muscles unable to contract enough around lens to make it thick enough to focus near objects
- the eye-ball is too short for the lens system + lens is too flat so cannot refract light enough and so the light rays from a close object try to converge and ‘focus’ behind the retina
Treatment for eye defects:
- spectacle lenses
- contact lenses
- laser eye surgery
- replacement lens
What spectacle lens is used to treat myopia?
- concave spectacles as they cannot focus on things far away
- concave lens - refracts light out slightly before refracting back into the eye to ensure light rays are refracted onto and focus on retina
What spectacle lens is used to treat hyperopia?
convex lens to refract rays of light inwards so that lens refracts light rays onto and focus on retina
How do contact lenses treat eye defects?
work in same way as spectacle lenses but allow activities such as sport to be carried out - hard or soft contact lenses last for dif lengths of time
How does laser eye surgery treat eye defects?
lasers can either reduces the thickness of the lens (so it refracts light less) to treat myopia of change its curvature (so it refracts light more strongly) to treat hyperopia
How can replacement lenses treat eye defects?
- hyperopia can be treated by replacing the lens with an artificial one made of clear plastic (or adding the plastic on top of the natural lens)
- the risks include damage to retina or cataracts developing
