3. Refraction of Light Flashcards
define refraction
chane in driection of light when is passess from one transparent medium into another of different optical density
what impacts the velocity of light
density of the medium it travels through
denser medium
slower light
what happens when a beam of light strikes a more dense medium obliquelys
edge of a beam which arrives first is slowed down when entering the medium
the other side continues at the same speed
when the other side hits the medium it is deviated towars the normal as it enters
Optical density
comparison of velocity of light in a vacuum vs in the medium
what is the measurement of optical density
measurement - absolute refractive index (n) of the medium
absolute refractive index =
velocity of light in a vacuum / velocity of light in medium
air and vacuum with the refractive index
as optical density in air as a medium is neglible under normal conditions
air = vacuum
refractive index of air
1
refractive index of water (incl aqeuous)
1.33
refractive index of cornea
1.37
refractive index of crystallinelens
1.386 - 1.406
refractive index of crown glass
1.52
refractive index of flint glass
1.6
refractive index of diamond
2.5
how can you measure refractive index
using a refractometer
on entering an optically dense medium from a less dense medium
light is deviated towards the normal
what governs / influences the angle of incidence and reflection
refractive indices of the media according to Snell’s law
Snell’s law
incident ray, refracted ray and normal all lie in the same plan
angles of incidence and refraction are related to the refractive index of the media
Snell’s law equation
medium1x n x medium 2 = sin i / sin R
where n is the refractive index
Snell’s law equation in air to glass
air x N x glass = sin i / sin r
if the interface is between two denser mediums (glass and air) then the value of n for that interface can be calculated b
water N glass = n glass / n water
if passing from medium 1 to medium 2 the index of refraction can be given by
1n2 = n2 / n1
What happens when light passes air -> glass -> air
Air -> glass = deviated towards the normal
glass -> = deviated away from the normal
BUT overall it is laterally displaced
light passing across a curved interface between two media of different refractive indices
obeys snell’s law
a convex spherical cruved surface
causes parallel light to conver or diverege
convex curved surface cause CONVERGENCE if
n1 < n2
if the media passing to is greater than from
e.g. air : cornea
convex curved surface cause DIVERGENCE if
n1 > n2
if the media passing to is LESS than from
e.g. glass : air
refracting power of a surface formula
surface power = (n2-n1) / r
where r is the radius of the curvature of the surface in METRES
what is refracting power measured in
dioptres
surface power is positive for
converging surfaces
surgace power is negative for
diverging surfaces
example of a refracting surface
anterior surface of the cornea
objects situated in an optically dense mediua appear
displaced when viewed from a less dense medium
this is due to refraction
What happens to rays emerging from a denser medium
- strike at 90 degree and leave undeviated
- strike at less than 90 degree and are refracted
- meets at the citical angle and run parallel with the inferface
- less than critical angle and are internally reflectid
how is the critical angle determined
by the refractive indcies of thhe media and can be calculated using Snell’s law
critical angle for tear film / air interface
48.5 degrees
critical angle for crown glass / air interface
41
example of how total internal refleciton is used pratically
fibre opitc cables
what prevents visulation of the angle of the anterior chamber and peripheral retina
total internal reflection
how to overcome total internal reflection when examining the ey
use a contact lens that has a HIGHER refractive indice than the eye and filling the space between the eye and the lens
changes the refracting surface to allow visulaision of the angle of the anterior chamber
dispersion of light
light of shorter waves are deviated more than longer
blue more than red
dispersive power
indicate by the anlge formed between red and blue light leaving (with yellow in betwee)
dispersive power and refractive index
are NOT related
