Physics of the Eye

Question 1

Describe converging lens.

Answer 1

Convex in shape - bulge outwards. Brings parallel light rays together.

Question 2

Describe diverging lens.

Answer 2

Concave in shape - cave inwards. Causes light rays to spread out (diverge).

Question 3

Definition of principal axis.

Answer 3

Straight line which passes through the lens centre, perpendicular to the lens.

Question 4

Definition of principal focus.

Answer 4

The point which the light rays meet at. For converging, point is found after lens. For diverging. point is found before lens.

Question 5

Definition of focal plane.

Answer 5

Plane perpendicular to principal axis, found where principal focus is.

Question 6

Definition of focal length.

Answer 6

Distance between lens axis and focal plane.

Question 7

Definition of real image.

Answer 7

Image formed when light rays from object are made to pass through another point in space.
- image forms on opposite side of object.

Question 8

Definition of virtual image.

Answer 8

Image formed when light rays from object seem to come from another point in space.
- image forms on the same side as object.
- will have negative v and focal length.
- diverging lenses always produces virtual image.

Question 9

State Lens equation.

Answer 9

1/u + 1/v = 1/f
u = distance between object and lens axis
v = distance between image and lens axis
f = focal length

Question 10

State linear magnification equation.

Answer 10

m = v/u
m = linear magnification
v = distance between object and lens axis
u = distance between object and lens axis

Question 11

State power of lens equation.

Answer 11

P = 1/f
P = lens power in dioptres, D
f = focal length, m

Question 12

What happens if the lens power is higher?

Answer 12

The lens will be thicker and converge light rays more. Results in shorter focal length.

Question 13

What kind of lens power does a diverging lens have?

Answer 13

A negative power due to a negative focal length.

Question 14

What is the cornea?

Answer 14

Transparent ‘window’ - has a convex shape and high refractive index.

Question 15

Role of lens.

Answer 15

Fine focuses, controlled by muscles. Muscles contract, tension is released, lens become fat, more spherical.

Question 16

Role of the retina.

Answer 16

Where the image forms. Has rods and cones which are light-sensitive cells.

Question 17

Describe the yellow spot.

Answer 17

Sensitive region of retina. Its centre is called the fovea - has the highest concentration of cones.

Question 18

Role of optic nerve.

Answer 18

Carries signals from rods and cones to the brain.

Question 19

Definition of far point.

Answer 19

The furthest distance which the eye can see.

Question 20

Definition of near point.

Answer 20

The closest distance the eye can focus on - around 9cm for younger people.

Question 21

Role of cones and rods.

Answer 21

Photoreceptors which detect light and convert it to electrical signal.
- contains a chemicals which bleach when light falls upon it.

Question 22

State difference between rods and cones.
MUST KNOW CONE GRAPH!

Answer 22

Rods:
- more sensitive to light,
- don’t detect colour
- relied on in dark environments
- only one type
Cones:
- three types; green, red, blue
- absorbs a range of wavelengths
- detects colour

Question 23

How is Myopia treated?

Answer 23

Diverging lens is used.

Question 23

Definition of spatial resolution.

Answer 23

a measure of ability to form separate images of objects very close to each other.

Question 23

What is Myopia?

Answer 23

• short-sightedness
• can’t focus on distant objects.
• occurs when the eyeball is too long or when the lens to too strong
• results in diverging lens in front of retina

Question 24

What is Hypermetropia?

Answer 24

• long-sightedness
• cannot focus on near objects (normal far point is ~25cm)
• occurs when lens is too weak or when eyeball is too short

Question 25

How is Hypermetropia treated?

Answer 25

Converging lens is used, to increase power of the system.

Question 26

What is Astigmatism?

Answer 26

• caused by irregularly shaped cornea or lens
• different focal lengths for different planes

Question 27

How is Astigmatism treated?

Answer 27

Using a cylindrical lens, which adds power in only one plane.