The Eye Flashcards
Definition of Iris
A coloured ring of circular and radial muscle that controls the size of the pupil and so controls the amount of light entering the eye
Definition of Cornea
A transparent, curved layer at the front of the eye that refracts the light entering and helps to focus it
Definition of Lens
A transparent, convex, flexible, jelly-like structure that refracts light to focus it onto the retina and to produce an image. This final adjustment is called accommodation.
Definition of Retina
A light-sensitive layer at the back of the eye made up of rods, which detect light of low-intensity, and cones, which detect different colours.
Definition of Optic Nerve
Transmits electrical impulses from the retina to the brain
Definition of Conjunctiva
Thin layer covering the front of the eye which protects the cornea
Definition of Eye Muscles
Allow the eyeball to move from side to side and up down
Definition of Sclera
Tough non-elastic coat, that is white in colour and protects the eyeball
Definition of Ciliary Muscles and Suspensory Ligaments
Cause the lense to change shape and so allow fine focusing onto the retina
Definition of Choroid
Layer of blood vessels inside the sclera which supplies food and oxygen to the eye
How does they eye respond to bright light?
Circular muscles of iris contract to reduce diameter of pupil.
Less light can enter and the retina is protected from bleaching.
How does they eye respond to dim light?
Radial muscles of iris contract to make the diameter of the pupil larger, allowing a larger amount of light to reach and stimulate the light-sensitive cells in the retina.
What is the job of the aqueos / vitreous humour?
They create outward pressure on the sclera, which keeps the spherical shape of the eyeball.
What does the ciliary body produce?
Aqueos humour
What is the blind spot?
Part of the retina on top of the optic nerve which doesn’t contain any light-sensitive cells.
How do our eyes detect light
STAGE 1
Light from a source travels towards the eye
STAGE 2
Light passes through the cornea and enters the eye. The cornea refracts the light rays.
STAGE 3
Light passes through the lens, which changes shape to refract the light even more. This is how the eye focuses light.
STAGE 4
Light rays hit the back of the retina. The light receptors detect the light and a nerve impulse is sent along the optic nerve to the brain.
What type of organ is the eye?
A sense organ
What type of receptors does it contain?
Light receptors
What are rod cells sensitive to?
They’re sensitive to light at LOW INTENSITIES.
Rod cells are more sensitive to light than cone cells.
They do NOT detect colour.
What are cone cells sensitive to?
They’re sensitive to COLOUR and light at HIGH INTENSITIES.
Humans have 3 different types of cone cells, each sensitive to a different range of colours (ones which respond to red, green or blue)
Definition of suspensory ligaments
Attachment between the lens and the ciliary muscles which keeps the lens in place and allows its shape to be changed by the ciliary muscles.
Definition of cillary muscles
Small muscles attached to suspensory ligaments which alter the shape of the lens
How does the eye focus on distant objects?
- Ciliary muscles relax, eyeball becomes more spherical
- Suspensory ligaments tighten, pulling lens thin
To focus distant objects, the lens must be thin so light rays are only slightly refracted
How does the eye focus on close objects?
- Ciliary muscles contract, pull eyeball inwards (bulges forward)
- Suspensory ligaments loosen, causing the lens to thicken
To focus on close objects, the lens needs to be thick to refract the light rays greatly
In the Iris Reflex, in what way do the circular and radial muscles in the iris work together?
They work antagonistically
- they oppose each other in their actions
- when circular muscles contract, pupil constricts
- when radial muscles contract, pupil dilates
In what type of vision do the rod cells help?
Night vision
Where are the cone cells concentrated?
In the fovea
What is the fovea’s purpose?
- allows objects to be examined in fine detail
- the centre only contains cones, this is where colour discrimination occurs
When does the fovea work in full efficiency?
In bright light
What happens if the image falls on the blind spot?
It cannot be seen
Definition of long-sightedness
When a person can see distant objects clearly, but close objects appear blurred
Definition of short-sightedness
When a person can see near objects clearly, but distant objects appear blurred
Definition of colour blindness
When a person cannot tell the difference between certain colours, often red or green
Why can long-sighted people not focus clearly on objects close-by?
The image is focused BEHIND their retina, not ON their retina
Due to weak eye muscles which are unable to contract enough to make lens thick enough to focus near objects.
What is the scientific name for long-sightedness?
Hyperopia
Why can near-sighted people not focus clearly on objects in the distance?
the image is formed IN FRONT of their retina
due to overstrong eye muscles which cannot relax enough to make the lens thing enough leading to light rays being bent too much so image is blurred
What is the scientific name for short-sightedness?
Myopia
In what 2 ways can eye-sight be corrected?
Glasses / contact lenses
How do lenses help?
Help focus light onto the retina
In what 2 more modern ways can sight problems be corrected?
Laser eye surgery
- involves using a highly precise laser to change shape of cornea
- alters refraction of light as it enters eye, enabling it to be focused onto the retina correctly
Artificial lens
- faulty eye lens can be removed and replaced by an artificial lens that focuses light onto the retina properly
What type of lens is used to correct long-sightedness?
A CONVEX lens
What type of lens is used to correct near-sightedness?
A CONCAVE lens
What causes colour-blindness?
Caused by a lack of a certain type of light receptor (cone) in the retina
What colours are hard to distinguish for a colour blind person?
red and green
Why do cataracts occur?
Occur as a result of changes in the lens
What happens to the lens if there’s a cataract?
The lens becomes less transparent, which leads to a decrease in vision. Eyes will appear cloudy.
What age group is most likely to develop cataracts?
Old people
How is cataracts treated?
Affected lens is removed and replaced with a new, artifical lens
When are objects most clearly seen by the rod cells?
At night by not looking directly a them
When are objects most clearly seen by the cone cells?
During daylight by looking directly at them
Why is there a layer of pigment behind the rods and cones?
Prevents internal reflection which might lead to multiple or blurred images.
How does the image differ when formed on the retina?
it’s inverted and diminished (smaller)
- brain corrects the inversion and change in size
INTERGRATION
How can we see colours which aren’t just, red, green or blue?
Seen by light waves hitting more than one type of cone
What is presbyopia?
A condition of old age (over 40) where people lose the ability to focus on near objects as the lens becomes less elastic
