Physics of the Eye Flashcards
Learning key terms and concepts
Describe converging lens.
Convex in shape - bulge outwards. Brings parallel light rays together.
Describe diverging lens.
Concave in shape - cave inwards. Causes light rays to spread out (diverge).
Definition of principal axis.
Straight line which passes through the lens centre, perpendicular to the lens.
Definition of principal focus.
The point which the light rays meet at. For converging, point is found after lens. For diverging. point is found before lens.
Definition of focal plane.
Plane perpendicular to principal axis, found where principal focus is.
Definition of focal length.
Distance between lens axis and focal plane.
Definition of real image.
Image formed when light rays from object are made to pass through another point in space.
- image forms on opposite side of object.
Definition of virtual image.
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.
State Lens equation.
1/u + 1/v = 1/f
u = distance between object and lens axis
v = distance between image and lens axis
f = focal length
State linear magnification equation.
m = v/u
m = linear magnification
v = distance between object and lens axis
u = distance between object and lens axis
State power of lens equation.
P = 1/f
P = lens power in dioptres, D
f = focal length, m
What happens if the lens power is higher?
The lens will be thicker and converge light rays more. Results in shorter focal length.
What kind of lens power does a diverging lens have?
A negative power due to a negative focal length.
What is the cornea?
Transparent ‘window’ - has a convex shape and high refractive index.
Role of lens.
Fine focuses, controlled by muscles. Muscles contract, tension is released, lens become fat, more spherical.
Role of the retina.
Where the image forms. Has rods and cones which are light-sensitive cells.
Describe the yellow spot.
Sensitive region of retina. Its centre is called the fovea - has the highest concentration of cones.
Role of optic nerve.
Carries signals from rods and cones to the brain.
Definition of far point.
The furthest distance which the eye can see.
Definition of near point.
The closest distance the eye can focus on - around 9cm for younger people.
Role of cones and rods.
Photoreceptors which detect light and convert it to electrical signal.
- contains a chemicals which bleach when light falls upon it.
State difference between rods and cones.
MUST KNOW CONE GRAPH!
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
How is Myopia treated?
Diverging lens is used.
Definition of spatial resolution.
a measure of ability to form separate images of objects very close to each other.
What is Myopia?
- 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
What is Hypermetropia?
- long-sightedness
- cannot focus on near objects (normal far point is ~25cm)
- occurs when lens is too weak or when eyeball is too short
How is Hypermetropia treated?
Converging lens is used, to increase power of the system.
What is Astigmatism?
- caused by irregularly shaped cornea or lens
- different focal lengths for different planes
How is Astigmatism treated?
Using a cylindrical lens, which adds power in only one plane.
