AAD vergence & refraction

Question 1

how does light travel?

Answer 1

• light rays travel in a straight line through a homogenous medium

Question 2

wavefronts

Answer 2

• direction of wavefronts are depicted by light rays
• sampled wavefronts get flatter as they move away from the source
• at infinity, wavefronts are flat
• optical infinity = 6m + beyond

Question 3

vergence

Answer 3

• describes a directional relationship; are the things being described coming together (convergence) or moving away from each other (divergence)
• with light, it describes the path/curvature of the pencil of light rays
• collection of pencils is called a beam of light
• diverging, parallel, converging

Question 4

vergence

Answer 4

• the more curved a wavefront, the greater the vergence
• diverging pencil of rays

Question 5

vergence
equation

Answer 5

L = n/l

where:

L = vergence, Dioptres (D)
n = refractive inde of the medium
l = distance of the object from the surface, in metres (m)

Question 6

vergence in air

Answer 6

• refractive index of air is 1.00
• therefore vergence in air can be defined by:

L = 1/l

Question 7

assumptions

Answer 7

• light is always travelling from left to right; this means that objects will always be on the left of a refractive/reflecting boundary
• always measure from the refractive/reflective boundary; e.g. if measuring the distance between a light source and a lens, measure from the lens to the light source

Question 8

sign convention

Answer 8

• if distance from boundary to light source is measured in same direction as light, then numerical value of distance (l) is positive; convergence
• if distance from boundary to light source is measured in opposite direction as light, then numerical value of distance (l) is negative; divergence

Question 9

parallel vergence

Answer 9

0 vergence
- objects will never have convergence; will always be parallel or divergence

Question 11

relationship between vergence + power

Answer 11

• optical surfaces (e.g. lenses) can refract light; this means that the vergence of light rays can be altered

L’ = L + F

where:

L’ = image vergence
L = object vergence
F = surface power

Question 12

summary

Answer 12

• Negative vergence (like 𝐿) describes diverging rays
• Positive vergence (like 𝐿′ ) describes converging rays
• Negative distances (like 𝑙 ) are away from surface in opposite direction to light ray
• Positive distances (like 𝑙′) are away from surface in same direction as light ray

Question 13

refraction

Answer 13

• describes the change in direction of a light wave due to a change in its velocity

Question 14

refractive index

Answer 14

• describes how much a material changes the velocity of light
• denoted by n
• dependent on the wavelength of the light
• indirect measure of density; higher RI materials are typically more dense

all transparent media slow down light + so always have a refractive index larger than 1

clinical consideration: in the UK, n is determined by Helium ‘d’ line (587.562 nm) because it’s close to the wavelength the human eye is most sensitive to in daylight (photopic)

Question 15

refractive index
equation

Answer 15

n = (velocity vac) / (velocity mat)

Answer 16

incident light ray
emergent light ray
normal = angle perpendicular to surface; refraction doesn’t occur when light rays fall on normal
i = angle of incidence
i’ = angle of refraction
primary medium
secondary medium

Answer 17

Velocity can change due to change in refractive index (𝑛) between optical mediums (e.g. air – glass, or even hot air – cold air!)

Question 18

laws of refraction

Answer 18

1) The incident and refracted light rays lie in one plane which is normal to the refracting surface at the point of refraction

2) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This is called Snell’s Law

sin(i) / sin(i’) = constant

Answer 19

Bend happens due to change in refractive index – related to density

If the secondary medium (𝑛′) has a higher refractive index (more dense) than the primary medium (𝑛), the light ray will bend towards the normal

If the secondary medium (𝑛′) has a lower refractive index (less dense) than the primary medium (𝑛) , the light ray will bend away from the normal

if n > n’, then i < i’

if n < n’, then i > i’

Question 20

snell’s law for transparent material

Answer 20

n (sin i) = n’ (sin i’)

where:

𝑛 - refractive index of primary medium𝑛 ′ - refractive index of secondary medium𝑖 – angle of ray prior to entering medium (incidence)𝑖′ – angle of ray after entering medium (refraction)