Chapter 35 The nature of light and the laws of geometrical optics

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Chapter 35 The nature of light and the laws of
geometrical optics November 12 Nature of
light 35.1 The nature of light

• Newtons particle theory the light particles
  leave the object and stimulate the sense of sight
  upon entering the eyes.
• Huygens argued that light might be some sort of a
  wave motion.
• Young (1801) demonstrated the wave nature of
  light. He showed that light rays interfere with
  each other.
• Maxwell asserted that light is a form of
  electromagnetic wave.
• The photoelectric effect was a major phenomenon
  which cannot be explained by waves. The kinetic
  energy of the ejected electron is independent of
  light intensity. Einstein (1905) proposed an
  explanation using the idea of quantization.

Quantization model The energy of a light wave is
present in particles called photons. E h.
Plancks constant h 6.63 10-34 Js
Dual nature Light exhibits the characteristics
of a wave in some situations and the
characteristics of a particle in other
situations. Nature prevents testing both
qualities at the same time.
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35.2 Measurements of the speed of light

• 1) Roemers method
• Period of the revolution of Io (a moon of
  Jupiter) are longer when the earth was receding
  from Jupiter and shorter when the earth was
  approaching Jupiter.
• Huygens estimated the lower limit of the speed of
  light to be 2.3 108 m/s.
• 2) Fizeaus method (1849)
• Rotating toothed wheel. c 2d / ?t
• Fizeau found c 3.1 108 m/s.
• 3) Measuring the speed of light at WIU
• Using nanosecond light pulses. The setup is in
  Currens 402. It takes less than 10 minutes.
  Everyone is welcome to have a try. Please contact
  me for an appointment.

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35.3 The ray approximation in geometric optics
Geometric optics The study of the propagation of
light using rays. Ray approximation A wave
moving through a medium travels in the direction
of its rays which are lines perpendicular to the
wave fronts.

• A plane wave incident on a barrier with an
  opening
• When ? ltlt d, the rays continue in a straight-line
  path, and the ray approximation remains valid.
• When ? ? d, the rays spread out after passing
  through the opening (diffraction).
• When ? gtgt d, the opening behaves as a point
  source.

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Read Ch351-3 Homework Ch35 (1-3) 2,3 Due
November 21
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November 17 Reflection and refraction 35.4
Reflection
Reflection When a ray encounters a boundary
between two media, part of the incident ray is
reflected back. Specular reflection Reflection
from a smooth surface. The reflected rays are
parallel to each other. Smooth ? surface
variations ltlt l. Diffuse reflection Reflection
from a rough surface. The reflected rays travel
in a variety of directions.

• Law of reflection
• The angle of reflection is equal to the angle of
  incidence ?1' ?1
• 2) The incident ray, the reflected ray and the
  normal are all in the same plane.

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35.5 Refraction
Refraction When a ray encounters a boundary
between two media, part of the ray enters the
second medium and is bent at the boundary. The
bending of the ray is called refraction. Index
of refraction The speed of light in any material
is less than its speed in vacuum
As light travels from one medium to another, its
frequency does not change, while its wavelength
does.
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• Law of refraction
• The incident ray, the reflected ray, the
  refracted ray, and the normal all lie on the same
  plane.
• 2) Snells law

• The path of light is reversible in reflection and
  refraction.
• If n2 gt n1, q2 lt q1 , the ray bends toward the
  normal.
• If n2 lt n1, q2 gt q1 , the ray bends away from
  the normal.
• Table 35.1
• Quiz 35.2,3

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Example 35.3
Example 35.4 displacement of light through a slab
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Read Ch35 4-5 Homework Ch35 (4-29)5,7,9,13,18,
19,20 Due December 5
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November 19 Total internal reflection 35.7
Dispersion
Dispersion The refractive index n for a given
material varies with the wavelength ? of the
light passing through it. This is called
dispersion. In the visible region, the index of
refraction for a material generally decreases
with increasing wavelength.
Dispersion by a prism 1) The ray emerges
refracted from its original direction by an angle
d (angle of deviation). 2) Violet deviates the
most, red deviates the least. 3) White light
spreads out into a spectrum.
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Measuring refractive index n using the minimum
angle of deviation
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34.8 Total internal reflection
When light goes from a medium with a higher
refractive index to a medium with a lower
refractive index,
there is a critical angle of incidence that
results in an angle of refraction of 90
Total internal reflection If the angle of
incidence is larger than the critical angle, the
beam will be entirely reflected at the boundary.
This is called total internal reflection. It
occurs only when n1 gt n2. Example 35.6 the
critical angle from water to air. Question
Total internal reflection occurs a) from water (n
1.33) to glass (n 1.51), or b) from glass to
water? What is the critical angle?
Application Optical fibers.
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Read Ch35 6-9 Homework Ch35 (31-)
25,31,28,36,60,63 Due December 5