Light
Refraction of Light
Learning Objectives
You will learn to
•recall and use the terms used in refraction, including normal, angle
of incidence and angle of refraction.
•recall and apply the relationship sin i/sin r = constant to new
situations or to solve related problems.
•understand relative refractive index and absolute refractive index.
•explain refraction by means of a change in speed of light in different
optical media.
•explain the terms critical angle and total internal reflection.
•identify the main ideas in total internal reflection and apply them to
the use of optical fibres in telecommunication and state the
advantages of their use.
Refraction
Definition:
The change in direction, or
bending of light when it
passes from one medium to
another medium of different
optical density.
Do you notice something
interesting about this picture?
refraction of light
i
air (less dense)
glass (denser)
r
Incident ray
is the light ray in 1st medium (air)
Refracted ray
is the light ray in the 2nd medium (glass)
Optical density of a
material is the ability
of a material to allow
light to pass through it
Normal
is a line drawn perpendicular to the surface
Angle of incidence, i
is the angle between the incident ray & the normal
Angle of refraction, r
is the angle between the refracted ray & the normal
refraction of light
Example:
(b) From denser TO less dense medium:
(a) From less dense TO denser medium:
incident ray
incident ray
i
normal
air (less dense)
i
glass (denser)
r
glass (denser)
air (less dense)
r
normal
refracted ray
refracted ray
 i will be greater than  r
 r will be greater than  i
 When a ray of light enters a denser medium, it bends TOWARDS the normal.
 When a ray of light enters a less dense medium, it bends AWAY from the
normal.
refraction of light
r is lesser than i
Ray of light bends towards
the normal when it enters a
denser medium at an angle
r is greater than i
Ray of light bends away from
the normal when it enters a
less dense medium at an
angle
refraction of light
During refraction, light bends first on passing from
air to glass and again on passing from the glass to
the air.
1. When light moves from
air to glass (a denser
material), it slows down
and is refracted towards
the normal.
2. When light moves from
glass to air (a less dense
material), it speeds up
and is refracted away
from the normal.
refraction of light
Try this….
Complete the following diagrams to show the path of light rays through
the glass blocks
(a)
(b)
(c)
air
glass
(d)
air
glass
air
glass
glass
air
air
Laws of Refraction
First Law: The incident ray, the normal and the refracted
ray all lie on the same plane.
Second Law: For 2 given media, the ratio sin i / sin r is a
constant, where i is the angle of incidence and r is the
angle of refraction. (This is also known as Snell’s Law).
n1sinθ1 = n2sinθ2
Reflection vs Refraction
Reflection
Refraction
Needs only 1 medium
Needs 2 media
i =
sin i/sin r = constant
r
2 Laws of Reflection
2 Laws of Refraction
Refractive Index of Materials
Medium
Vacuum
Air
Water
Ethanol
Glycerine
Crown Glass
Quartz
Flint Glass
Diamond
Refractive Index (n)
1.00
1.003
1.33
1.36
1.47
1.52
1.54
1.61
2.42
Relative Refractive Index
The relative refractive index is the ratio of the absolute
refractive index of one material compared to that of
another, for example from water to glass.
Absolute Refractive Index
The absolute refractive index is the ratio compared with the
refractive index of a vacuum. (n for a vacuum = 1.00)
n1sinθ1 = n2sinθ2
Example
A light ray strikes an air/water surface at an angle of 46° with respect to
the normal. The refractive index for water is 1.33. Find the angle of
refraction when the direction of the ray is (a) from air to water and
(b) from water to air.
(a) The incident ray is in the air, so θ1 = 46° and n1 = 1.00. The refracted
ray is in water, so n2 = 1.33. Snell’s law can be used to find the angle
of refraction θ2:
sinθ2 = (n1sinθ1)/n2 = (1.00 x sin46°)/1.33 = 0.54
θ2 = sin-10.54 = 33°
(b) With the incident ray in the water, we find that
sinθ2 = (n1sinθ1)/n2 = (1.33 x sin46°)/1.00 = 0.96
θ2 = sin-10.96 = 74°
Therefore, for the case where the light ray is passing
from vacuum into a given medium, we could
simplify our equation to: n = sin i / sin r.
For the case where the light ray is passing from a
given medium to vacuum, we could simplify our
equation to: n = sin r / sin i.
Example
A ray of light is travelling from water (n = 1.33) to
glass (n = 1.52) with an incident angle of 45.0°.
Calculate the angle of refraction when the ray of
light enters the glass slab.
n1sinθ1 = n2sinθ2
1.33 x sin 45.0° = 1.52 x sin θ2
sin θ2 = (1.33 x sin 45.0°) / 1.52
θ2 = 38.2°
Refractive Index (n) of a Medium:
n = sin i / sin r = c / v
where i: angle of incidence (in less dense medium)
r: angle of refraction (in denser medium)
c: speed of light in vacuum (=3x108m/s)
v: speed of light in medium
Example
If the speed of light in air is 3.0 x 108 ms-1, find the
speed of light in diamond (refractive index of
diamond = 2.42)
n = c/v
2.42 = 3.0 x 108 / v
v = 3.0 x 108 / 2.42
= 1.24 x 108 ms-1
To an observer standing at the side of a swimming pool,
objects under the water appear to be nearer the surface
than they really are. A similar effect can be seen when
"looking through" glass or any other transparent substance.
n = real depth / apparent depth.
Refraction - Effects of Refraction
Apparent
depth
An object seen in the water will usually appear to be at a different
depth than it actually is, this is due to the refraction of light rays as
they travel from the water into the air.
The first diagram shows that the observer ‘perceives’ that the chest
appear to be closer to the surface than it really is.
Effects of Refraction
In the diagram, refraction causes point A to appear nearer
to the surface at B. So to the eyes, the straw appears to
bend towards the surface of the water.
eye sees the
virtual image of
the stick
shallower than
it actually is
Total Internal Reflection
What happens when light passes from an optically denser
medium to an optically less dense medium?
The critical angle is defined as the angle of incidence in
the optically denser medium for which the angle of
refraction in the optically less dense medium is 90°.
Total internal reflection is an optical phenomenon that
occurs when a ray of light strikes a medium boundary at an
angle larger than a particular critical angle with respect to
the normal to the surface. If the refractive index is lower on
the other side of the boundary, no light can pass through
and all of the light is reflected.
For total internal reflection to occur, the following conditions
must be satisfied:
1) The light ray must travel from an optically denser medium
towards an optically less dense medium.
2) The angle of incidence must be greater than the critical
angle.
Critical Angle & Refractive Index
Consider a light ray going from a denser medium to a less dense
medium (i.e. from glass to air). When angle of refraction is 90°, the
angle of incidence is known as the critical angle.
n = sin r / sin i = sin 90° / sin c
= 1 / sin c
sin c = 1 / n
Example
A right-angled prism (one of the angles of the prism is
90°) is made of glass of refractive index 1.5. A ray of light
enters the prism. Calculate the critical angle of the prism.
Using sin c = 1/n = 1/1.5 = 2/3
c = sin-1 (2/3)
= 41.8°
The Optical Fibres
An optical or fibre is a glass or plastic
fiber that carries light along its length.
Optical fibers are widely used in fiberoptic communications, which permits
transmission over longer distances and at
higher bandwidths (data rates) than other
forms of communications. Fibers are
used instead of metal wires because
signals travel along them with less loss,
they are also immune to electromagnetic
interference and data security issues.
How optical fibre works
Light is kept in the core of the optical fiber by total internal
reflection. This causes the fiber to act as a waveguide.
The core is optically more dense than the cladding. The
light ray will undergo total internal reflection as it strikes
the interface between the core and cladding as the incident
ray has exceeded the critical angle.
References:
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http://www.photo.school.nz/lenses/bent_spoon.jpg
http://media.tiscali.co.uk/images/feeds/hutchinson/ency/dept0001.jpg
http://media-2.web.britannica.com/eb-media/73/1573-004-4FEB1C43.gif
http://www.optics4kids.com/terms/images/rightangleprism.gif
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p028.shtml
http://commons.wikimedia.org/wiki/File:Total_internal_reflection_of_Chelonia_m
ydas_.jpg
http://www.biocrawler.com/w/images/index.php?dir=e%2Fec%2F
http://www.fas.harvard.edu/~scidemos/LightOptics/FishTankTIR/FishTankTIR.ht
ml
http://www.bd-associates.net/product/fiber.htm
http://laser.physics.sunysb.edu/~wise/wise187/2001/reports/andrea/report.html