3.10.1 Physics of the Eye Flashcards
What is the eye
An optical refracting system
What wavelengths is the eye sensitive to
380nm - 760nm
What does the choroid do
Contains blood vessels that supply food and oxygen
Contains a black pigment to reduce reflection
The end contains muscle fibres
What do the ciliary muscles do generally
Adjusts the shape of the lens to enable focussing on objects at different distances
What happens when ciliary muscles are relaxed
The suspensory ligaments pull the lens into a thin, flatter shape
The eye is ‘un accommodated’
Used to focus on objects at a distance >5m
What is the far point
Maximum focal length
(Infinity)
What happens when ciliary muscles contract
Tension is released and the lens takes on a fat, more spherical shape
The eye is ‘accommodated’
Used to focus on objects at a distance <5m
What is the near point
The minimum focal length
(10cm)
What is the vitreous humour
A transparent, jelly-like substance that fills the inside of the eye
What does the vitreous humour do
The pressure keeps the eye spherical
Reduces refraction at the lens due to a similar refractive index
What does the lens do
Adjusts focus only
What does the cornea do
It is the outer clear convex structure where most refraction takes place due to its (high) refractive index that directs light rays into the eye and helps them focus on the retina
What does the aperture/pupil do
Changed the intensity (size varies with length of iris)
Allows light to enter the eye and be focussed on retina, diffraction occurs
What factors change the size of the pupil and how/why
Darker = bigger pupil
Closer object = smaller pupil - to reduce superfluous (external) rays enter in, and sharpen the image
What does the retina do
Produced electrical signals when light shines on it
The photoelectric cells / photoreceptors (rods and cells) convert light energy to electrical
What are rods
There are several
They are responsible for vision at low light levels (scotopic vision)
They are greyscale - they do not mediate colour vision and have a low spacial acuity
What are cones
They are singular
They are responsible for vision at higher light levels (photopic vision)
They are capable of colour vision
They are responsible for high spacial acuity
What are nerve fibres
They travel along the retina surface
They make up the optic nerve
What is the yellow spot (and fovea)
The yellow spot is a sensitive region of the retina
In the centre of the spot is the fovea, which is the part of the retina with the highest concentration of cones
(As we move out from this spot, the density of cones decreases and the density of rods increases)
Has good spacial resolution
What is the blind spot
Where all the nerves converge
There are no rods or cones
What happens when we look directly at an object
A clear detailed image is formed on the fovea
Everything else is not clear
What happens when we look at objects in poor light
The outside area is used (due to high rod density) so when we look at something directly (fovea area), the object seems to disappear
What is the persistence of vision
Where an aftermath image remains for a short amount of time if the retina after a bright image is removed
(Light stimulates the cells, and this sensation takes times to decay)
What is the decay time in low intensity light and what does this mean in imaging
About 5s
Images flashing faster than 5Hz appear continuous
What is the decay frequency in high intensity light and how is this used
About 50Hz
Useful in fluorescent lighting and TV/film imaging where it is seen continuously
What is spacial resolution
A measure of the ability to form separate images form object that are close together
(The minimum separation angle to resolve individual points)
What is needed to distinguish two objects from each other
At least one rod or cone between the light from each of them,
Otherwise the brain ‘sees’ them as one
What happens to diffraction and resolution if the pupil is small
The smaller the pupil:
The greater the diffraction through the pupil
The less the resolution
Where in the eye is the best resolution and why
At the fovea - highest density of cones
What are the three types of light/colours cones are sensitive to?
Red, green and blue light
What do the red, green and blue cones do
Absorb a range of wavelengths
What do lens do
Change the direction of light rays by refraction
What are the two types of lens
Converging (convex)
Diverging (concave)
What is the principle axis
A straight line that passes through the centre of the lens, perpendicular to its surface on both sides
What is the principle focus
The point at which light rays parallel to the principle axis meet/ converge
Normally, where is the principle focus for a convex lens
In front of the lens on the principle axis
What is the focal plane
Where rays that are parallel to each other but not the principle axis converge (somewhere on the focal plane)
The plane is perpendicular to the principle axis
What is the focal length
The distance between the lens axis and the focal plane
Converging = positive because the focal point is in front of the lens
Diverging = negative because the focal point is behind the lens
What is a real image
Rays pass through from the object to another point in space
The image can be captured
Produced by converging (convex) lenses
What is a virtual image
Rays appear to have come from another point in space (not the object)
The image cannot be captured
Produced by diverging (concave) lenses
What is myopia
Short sightedness
Unable to focus on distant objects
Where is the far point for those with myopia
Closer than infinity
When does myopia occur?
When the cornea and lens are too powerful
(Images of distant objects are brought into focus in front of the retina)
What kind of lens and power is needed to correct myopia?
Negative power
Diverging (concave) lens
How does a diverging lens correct mypoia?
Spreads the rays in front of the cornea to move the focal point back.
Brings the infinity far point (u) to the uncorrected far point (v) of the eye.
Brings u forward and makes a virtual image at v.
What is hypermetropia?
Long sightedness
Unable to focus on near objects
Where is the near point for those with hypermetropia?
Further away than normal
(Further than 25cm)
Why does hypermetropia occur?
The cornea and lens are too weak
(Images of near objects are brought into focus behind the retina)
What kind of lens and power is needed to correct hypermetropia?
Positive power
Converging (convex) lens
How does a converging lens correct hypermetropia?
Brings the ray in before the cornea to bring the focal point nearer to the front of the eye.
Puts the near point (u) at the uncorrected near point (v) of the eye.
What is the cause of astigmatism?
An irregular shaped cornea or lens (rugby ball)
This means different t focal lengths for different planes
What does astigmatism cause in vision?
Overlapping images
Overall blurred vision
How can astigmatism be fixed?
By a cylindrical lens which refract light at different angles into the eye.
