light continued
DESCRIPTION
Refraction of light is the bending of light as it goes from one optical medium to another Less dense to more dense: bends towards normal More dense to less dense: bends away from normalTRANSCRIPT
Light continued
Refraction of light is the bending of light as it goes from one optical medium to another
Less dense to more dense: bends towards normal
More dense to less dense: bends away from normal
A medium is
Index of RefractionMaterial Index of RefractionVacuum 1.0000
Air 1.0003Ice 1.3100
Water 1.3330Ethyl Alcohol 1.3600
Plexiglas 1.5100Crown Glass 1.5200
Light Flint Glass 1.5800Dense Flint Glass 1.6600
Zircon 1.9230Diamond 2.4170
Rutile 2.9070Gallium phosphide 3.5000
Incident ray
Refracted ray
Glass block
i
r
The Laws of Refraction of Light
1. The incident ray, the normal and the refracted ray all lie in the same plane
2. where n is a constant
This is called Snell’s Law
nrsin isin
When light travels from a rarer to a denser medium it is refracted towards the normal e.g. air to water
when light travels from a denser to a rarer medium it is refracted away from the normal e.g. glass to air
Experiment to Verify Snell’s Law and determine the refractive index of glass
12
34
Block of glass
i
r
Experiment to Verify Snell’s Law and determine the refractive index of glass
Method1. Outline the glass block on paper2. Stick pins 1 and 2 in the paper in front
of the block3. Stick pins 3 and 4 in line with the
images of 1 and 2, as seen through the block
4. Remove the pins, join the pinholes and draw the normal
5. Measure angles i and r6. Repeat for different angles of incidence7. Draw a graph of sin i vs. sin r8. The slope of the graph is the refractive
index of glass
Result
i r Sin i Sin r Sin i/Sin r
Sin i
Sin r
Slope of graph =
Thus the refractive index of glass is ____
12
22
xxyy
ConclusionThe graph is a straight line graph
through the origin, thus verifying Snell’s Law
The slope of the graph gave the refractive index of glass
Real and Apparent Depth
A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water
We can calculate the refractive index of a liquid by using
n = depthApparent depth Real
Refractive index
n =
rsin isin
depthApparent depth Real
CSin 1
mediumin light of speedairin light of speed
Question 1
When light passes from air into a liquid, the angle of incidence and refraction are 57˚and 30˚ respectively. Calculate the refractive index of the liquid.
Answer Formula:
nSinrSini
68.13057
SinSin
Question 2
A ray of light is incident at 35˚ on (a) a glass surface and (b) a water surface. Calculate the angle of refraction given that,
ng=1.52 nw=1.33 Answer
Formula: nSiniSinrn
SinrSini
Textbook Examples
Questions 1-3 on page 31
Total Internal Reflection
This may occur when light goes from a denser to a less dense medium
As i is increased so is rEventually r = 90˚At this point i has reached the ‘critical
angle’
r = 90˚
i = critical angle
Total Internal Reflection!!!
If i is increased beyond the critical angle, the ray does not enter the second medium
It is reflected back into the first mediumWe can also find the refractive index of a
material using
n = CSin 1
C = critical angle
Example
The critical angle of glass is 41.81˚Find the refractive index of glassn =
n = 1/0.666
n = 1.5
CSin 1
Refractive index
n =
rsin isin
depthApparent depth Real
CSin 1
mediumin light of speedairin light of speed
ExampleThe refractive index of glass is 1.5This value is for a ray of light
travelling from air into glass
So ang = 1.5 =
Or gna = =
Asin Bsin
5.11
Bsin Asin
Applications of Total Internal Reflection
Periscopes (using a prism)Diamonds and bicycle reflectorsOptical fibres – in telecommunications
and by doctors
Prisms
Prisms are considered to be better than mirrors for reflecting light. This is because they use total internal reflection.
This is the reason prisms are used in periscopes and binoculars.
Optical Fibres
Total Internal Reflection has given rise to the use of optical fibres.
The signal is fired at an angle greater than the critical angle so that it is reflected all the way along the fibre.
The light is therefore trapped within the fibre. This technology is used to transmit information over long distances.
Endoscopes
These are used in medicine. They contain two bundles of optical fibres, one to carry light to the organ being viewed and the other to carry the image of the organ back to the operator.
44
Optic Fibre
Glass core of high refractive index
Glass cladding of low refractive index
45
Normal
NN N
Glass of high refractive index
Glass of low refractive index
Remember that rays are path-reversible
AIR
GLASS
A
B
Mirages
Mirages are caused by the refraction of light in air due to temperature variations
SKY
LENSES
Made of transparent material, usually glass
Convex lens (converging)
Thick in centre and thin at edges
Concave lens (diverging)
Thin in the centre and thick at the edges
Ray diagrams for lenses
1. Ray incident parallel to principal axis is reflected out through focus
2. Ray incident through focus is reflected out parallel to axis
3. Ray incident through optic centre continues in straight line
f2f f
RAY DIAGRAMS FOR CONVEX LENSES
Optic centre
f2f f
RAY DIAGRAMS FOR CONCAVE LENSES
Optic centre
Geometric Optics 2.03
The image in a concave lens is always virtual, erect, and diminished
Concave lens always has negative f and v
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Lens formulae
m =
m =
uv
object ofheight image ofheight
vuf111
Example
An object is placed 10cm away from a convex lens of focal length 12 cm. Calculate the nature, position and magnification of the image.
f = 12u = 10v = ?
v = -60cm
vuf111
v1
101
121
v1
101
121
v1
601
m =
m = = 6
So the image is formed 60cm from the lensIt is virtualMagnification is 6
uv
1060
Example 2
An object is placed 40cm from a concave lens of focal length 50cm. Find the position, nature and magnification of the image.
u = 40cmf = -50cmv = ??
v = -22.2 cm
vuf111
v1
401
501
v1
401
501
v1
2009
m =
m = = 0.56
So the image is formed 22.2cm from the lensIt is virtualMagnification is 0.56
uv
402.22
Experiment to Measure the Focal Length of a Convex Lens
F
Plane mirror
Pin
Convex lens
Experiment to Measure the Focal Length of a Convex Lens
Method A rough estimate of the focal length may first
be obtained by focusing the image of a distant object on a sheet of paper
1. Set up the apparatus as in the diagram2. Move the pin in and out until there is no
parallax between the pin and its image3. Measure the distance from the pin to the
centre of the lens. This is the focal length
ResultThe distance from the pin to the centre of
the lens was ______
ConclusionThe focal length of the lens is ______
Experiment to Measure the Focal Length of a Concave Lens
F
Plane mirror
Pin
Convex lens
Concave lens
Method1. Set up the apparatus as in the diagram2. The focal length of the convex lens is first
measured3. The concave lens is combined with the convex
lens (of shorter focal length), and so overall it behaves as a convex lens
4. Move the pin in and out until there is no parallax between the pin and its image
5. Measure the distance from the pin to the centre of the lens. This is the focal length of the combination
Results
The focal length of the convex lens is 22cm The focal length of the combination is 51cm. The focal length of the concave lens can be
calculated from the formula
Where F = focal length of combinationf1 = focal length of convex lensf2 = focal length of concave lens
21
111ffF
F = 51cmf1 = 22
f2 = ??
21
111ffF
f1
221
511
f1
221
511
f1
11225122
f1
112229
cmf 69.38
Conclusion
The focal length of the concave lens is 38.69cm