3.10.1 Physics of the Eye

Question 1

What is the eye

Answer 1

An optical refracting system

Question 2

What wavelengths is the eye sensitive to

Answer 2

380nm - 760nm

Question 3

What does the choroid do

Answer 3

Contains blood vessels that supply food and oxygen
Contains a black pigment to reduce reflection
The end contains muscle fibres

Question 4

What do the ciliary muscles do generally

Answer 4

Adjusts the shape of the lens to enable focussing on objects at different distances

Question 5

What happens when ciliary muscles are relaxed

Answer 5

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

Question 6

What is the far point

Answer 6

Maximum focal length
(Infinity)

Question 7

What happens when ciliary muscles contract

Answer 7

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

Question 8

What is the near point

Answer 8

The minimum focal length
(10cm)

Question 9

What is the vitreous humour

Answer 9

A transparent, jelly-like substance that fills the inside of the eye

Question 10

What does the vitreous humour do

Answer 10

The pressure keeps the eye spherical
Reduces refraction at the lens due to a similar refractive index

Question 11

What does the lens do

Answer 11

Adjusts focus only

Question 12

What does the cornea do

Answer 12

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

Question 13

What does the aperture/pupil do

Answer 13

Changed the intensity (size varies with length of iris)
Allows light to enter the eye and be focussed on retina, diffraction occurs

Question 14

What factors change the size of the pupil and how/why

Answer 14

Darker = bigger pupil
Closer object = smaller pupil - to reduce superfluous (external) rays enter in, and sharpen the image

Question 15

What does the retina do

Answer 15

Produced electrical signals when light shines on it
The photoelectric cells / photoreceptors (rods and cells) convert light energy to electrical

Question 16

What are rods

Answer 16

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

Question 17

What are cones

Answer 17

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

Question 18

What are nerve fibres

Answer 18

They travel along the retina surface
They make up the optic nerve

Question 19

What is the yellow spot (and fovea)

Answer 19

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

Question 20

What is the blind spot

Answer 20

Where all the nerves converge
There are no rods or cones

Question 21

What happens when we look directly at an object

Answer 21

A clear detailed image is formed on the fovea
Everything else is not clear

Question 22

What happens when we look at objects in poor light

Answer 22

The outside area is used (due to high rod density) so when we look at something directly (fovea area), the object seems to disappear

Question 23

What is the persistence of vision

Answer 23

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)

Question 24

What is the decay time in low intensity light and what does this mean in imaging

Answer 24

About 5s
Images flashing faster than 5Hz appear continuous

Question 25

What is the decay frequency in high intensity light and how is this used

Answer 25

About 50Hz Useful in fluorescent lighting and TV/film imaging where it is seen continuously

Question 26

What is spacial resolution

Answer 26

A measure of the ability to form separate images form object that are close together (The minimum separation angle to resolve individual points)

Question 27

What is needed to distinguish two objects from each other

Answer 27

At least one rod or cone between the light from each of them, Otherwise the brain ‘sees’ them as one

Question 28

What happens to diffraction and resolution if the pupil is small

Answer 28

The smaller the pupil: The greater the diffraction through the pupil The less the resolution

Question 29

Where in the eye is the best resolution and why

Answer 29

At the fovea - highest density of cones

Question 30

What are the three types of light/colours cones are sensitive to?

Answer 30

Red, green and blue light

Question 31

What do the red, green and blue cones do

Answer 31

Absorb a range of wavelengths

Question 32

What do lens do

Answer 32

Change the direction of light rays by refraction

Question 33

What are the two types of lens

Answer 33

Converging (convex) Diverging (concave)

Question 34

What is the principle axis

Answer 34

A straight line that passes through the centre of the lens, perpendicular to its surface on both sides

Question 35

What is the principle focus

Answer 35

The point at which light rays parallel to the principle axis meet/ converge

Question 36

Normally, where is the principle focus for a convex lens

Answer 36

In front of the lens on the principle axis

Question 37

What is the focal plane

Answer 37

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

Question 38

What is the focal length

Answer 38

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

Question 39

What is a real image

Answer 39

Rays pass through from the object to another point in space The image can be captured Produced by converging (convex) lenses

Question 40

What is a virtual image

Answer 40

Rays appear to have come from another point in space (not the object) The image cannot be captured Produced by diverging (concave) lenses

Question 41

What is myopia

Answer 41

Short sightedness Unable to focus on distant objects

Question 42

Where is the far point for those with myopia

Answer 42

Closer than infinity

Question 43

When does myopia occur?

Answer 43

When the cornea and lens are too powerful (Images of distant objects are brought into focus in front of the retina)

Question 44

What kind of lens and power is needed to correct myopia?

Answer 44

Negative power Diverging (concave) lens

Question 45

How does a diverging lens correct mypoia?

Answer 45

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.

Question 46

What is hypermetropia?

Answer 46

Long sightedness Unable to focus on near objects

Question 47

Where is the near point for those with hypermetropia?

Answer 47

Further away than normal (Further than 25cm)

Question 48

Why does hypermetropia occur?

Answer 48

The cornea and lens are too weak (Images of near objects are brought into focus behind the retina)

Question 49

What kind of lens and power is needed to correct hypermetropia?

Answer 49

Positive power Converging (convex) lens

Question 50

How does a converging lens correct hypermetropia?

Answer 50

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.

Question 51

What is the cause of astigmatism?

Answer 51

An irregular shaped cornea or lens (rugby ball) This means different t focal lengths for different planes

Question 52

What does astigmatism cause in vision?

Answer 52

Overlapping images Overall blurred vision

Question 53

How can astigmatism be fixed?

Answer 53

By a cylindrical lens which refract light at different angles into the eye.

Question 54

What are the lines drawn in a ray diagram (3)?

Answer 54

1. Horizontal, then through the focal point. 2. Through the centre of the lens. 3. Through the focal point, then horizontal.

Question 55

What does a ray diagram for a converging (convex) lens look like for a far object?

Answer 55

For a far object that is outside the focal point (e.g looking out a window), the image is: - Real (other side of the lens) - Upside down - Shorter (than object)

Question 56

What does a ray diagram for a converging (convex) lens look like for a near object?

Answer 56

For a near object that is inside the focal point (e.g looking at a book), the image is: - Virtual (wrong side of the lens) - Upright - Larger (than object)

Question 57

What does a ray diagram for a diverging (concave) lens look like?

Answer 57

For both sides of the focal point, the image is: - Virtual (wrong side of lens) - Upright - Shorter (than object)

Question 58

What is the equation for lens power for myopia?

Answer 58

Power = 1/f = 1/u + 1/v (u = infinity) (v = negative) Power = 1/f = 0 + 1/-v

Question 59

What is the equation for lens power for hypermetropia?

Answer 59

Power = 1/f = 1/u + 1/v (u = 0.25m) (v = negative)