Chapter 10 Light
10.1 Speed of Light (c)
The speed of light for the universe is calculated to be 299,792,458 m/s
Although there is a slight difference between
taken to be 3.00 X 108 m/s. Why quibble?
c in air and a vacuum, both are
Exercises 10.1 p.202
Measuring c
Assignment:
1. Read p 203, 204.
Draw simple diagrams of the Roemer / Huygens calculations and the
Michelson experiment. Explain the rationale behind both.
2. Do an Internet search and report on the contributions of
¾ Gallileo – Roberts
¾ Fizeau – Shpely
¾ Foucault – Therrien
…to our understanding of c. Keep this to a few sentences.
3. Is the speed of light constant? What are the ‘experts’ saying?
4. Does the speed of light always travel in straight lines?
10.2 Linear Propagation
Light essentially travels in straight lines. This phenomenon explains the shape
and appearance of shadows based on the distance and angle of an object in
relation to a source of light.
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Investigation 10.1
10.3 Reflection
Images produced by the reflection of an object off a mirrored surface appear
to be behind the mirror at the same distance as the distance of an object in
front of the reflective surface.
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Investigation 10.2, 10.3
10.3 Refraction
Light bends as it travels from one transparent medium to another. The degree
to which a ray of light bends depends on:
¾ The temperature of the media
¾ The degree of purity of the media
¾ The type of media
¾ The wavelength of the incident light
Investigation 10.4 – Use Physics Exploration Software – I. Snell’s law.
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Snell’s Law
The Index of Refraction
The ratio of the angle of incidence to the angle of refraction is a constant for
light traveling from one transparent medium to another.
The values of Indices of Refraction listed in tables are for light traveling from a
vacuum into the medium.
Indices of refraction depend upon colour of light, purity, temperature and
composition
Snell's
Law
Index of refraction (n) =
sin i
sin r
angle of incidence ( r )
angle of refraction ( I )
Common Indices of Refraction
Water (20oC)
Diamond
Glass
Air
Quartz crystal
1.333
2.42
1.50-01.9
1.00029
1.54
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Snell’s law and Critical Angle.
Critical angle – any angle equal to or greater than the angle at which all light
coming from the source is reflected back into the medium from which it
originated. I.e total internal reflection.
Investigation 10 – 5:
Use Physics Exploration Software – I. Snell’s Law.
NOTE: this software is set up for different wavelengths of light NOT
WHITE LIGHT; so results are not accurate. Also, you have to go about the
procedure sort of backwards. You should be able to come to the same
conclusions.
Set up “n” for glass, water and diamond. Determine the following:
1) What do you notice about the index of refraction for the different
colours of light?
2) How (or Why) does this relate to the refraction of light into its
component parts?
3) What is the Critical Angle for each of the following wavelengths:
425, 465, 490, 525, 575, 630, 680nm. These wavelengths are my
poor estimates of the average wavelength for each colour. What do
you suppose the Critical Angle for white light would be in each
medium?
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Thinking question: How do you suppose jewelers can tell the difference
between diamond and fakes?
Calculating the Critical Angle
Air
Water
From Snell’s Law we know that the Index of Refraction is
___sin i___ = n
sin r
Okay, this equation can be used to calculate critical angle for light going
from air (LD) into water(HD), but what about the other way around? In that
case the following is true:
___sin i___ = _1_
sin r
n
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Not only that, but we know that, by definition the critical angle MUST be
where refracted ray is at the interface and therefore the angle of
refraction is 90o.
Substituting the values:
___sin ic___ = _1_
sin 90o
n
Solving for sin ic:
sin ic = ___sin 90o x 1___
n
***Where n for water is 1.33
sin ic = ___1.000 x 1___ = 0.750
1.33
Arcsine 0.750 = 48.6o
Exercises 10.5 p.221
10.5 Light Polarization
The filtering of different directions of wave propagation from a beam of light
Investigation 10.6 Use Exploration of Physics II. Polarization of Light.
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10.6 Dispersion
The breaking up of white light into it’s component wavelengths (colours)
Investigation 10.7
10.7 The Rainbow
primary rainbow
secondary rainbow
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10.8 Colour and Wavelength
The different colours of visible light are associated with differing wavelengths
Colour
Range of λ
red
orange
yellow
green
blue
violet
630 - 760 nm
590 - 630 nm
560 - 590 nm
490 - 560 nm
450 - 490 nm
380 - 450 nm
The reason why light disperses into various wavelengths is due to the fact that
the speed of light varies for the different wavelengths when it is being
transmitted in media with refractive indices greater than 1. The more a wave
of visible light slows down, the more it refracts. Red light slows the least so it
refracts (bends) the least; violet light slows the most, so it bends the most.
Wave Speed and Index of Refraction
Simply put:
The Index of Refraction
n = ___sin i___ = ___v1___
sin r
v2
Where:
¾ v1 is the speed of light in medium1 (low density)
¾ v2 is the speed of light in medium2 (high density)
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