Transcript Refraction of Light
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
Optical density
of a material is the ability of a material to allow light to pass through it
Incident ray
is the light ray in 1st medium (air)
Refracted ray
is the light ray in the 2nd medium (glass)
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: (a) From less dense TO denser medium:
incident ray
i
air (less dense) glass (denser)
r
normal
(b) From denser TO less dense medium:
incident ray glass (denser) air (less dense) normal
i r
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) air glass (b) (c) glass air glass air (d) air glass 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).
n
1
sinθ
1
= n
2
sinθ
2
Reflection vs Refraction
Reflection
Needs only 1 medium i = r 2 Laws of Reflection
Refraction
Needs 2 media sin i/sin r = constant 2 Laws of Refraction
Refractive Index of Materials
Medium Refractive Index (n)
Vacuum 1.00
Air Water Ethanol Glycerine Crown Glass Quartz Flint Glass Diamond 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)
n
1
sinθ
1
= n
2
sinθ
2
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 θ 1 ray is in water, so n 2 of refraction θ 2 : = 46° and n 1 = 1.00. The refracted = 1.33. Snell’s law can be used to find the angle sinθ 2 θ 2 = (n = sin -1 1 sinθ 1 )/n 2 0.54 = 33° = (1.00 x sin46°)/1.33 = 0.54
(b) With the incident ray in the water, we find that sinθ 2 θ 2 = (n = sin -1 1 sinθ 1 )/n 2 0.96 = 74° = (1.33 x sin46°)/1.00 = 0.96
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.
n 1 sinθ 1 = n 2 sinθ 2 1.33 x sin 45.0
° = 1.52 x sin θ 2 sin θ 2 = (1.33 x sin 45.0
° ) / 1.52
θ 2 = 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 (=3x10 8 m/s) v: speed of light in medium
Example
If the speed of light in air is 3.0 x 10 8 ms -1 , find the speed of light in diamond (refractive index of diamond = 2.42) n = c/v 2.42 = 3.0 x 10 8 / v v = 3.0 x 10 8 / 2.42
= 1.24 x 10 8 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 fiber optic 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:
• • • • • • • • • • 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