Light Waves

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Light Waves
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What is Light?

• Light is the range of frequencies of the
  electromagnetic spectrum that stimulate the
  retina of the eye.

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Light Matter

• Transparent (Glass) Materials that allow light
  to pass through without distorting images.
• Translucent (Cloudy Glass) Materials that allow
  light to pass through them, but do not allow them
  to be seen clearly.
• Opaque (Brick) Material in which all light is
  absorbed or reflected. Not is transmitted
  through.

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Reflection and Absorption of Light and Color

• The color observed by any object is the same as
  that not absorbed by the object. For example, a
  red block will absorb all colors of the EM
  visible light spectrum except for red.
• How does this apply to the clothes we wear?

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Continuous Waves

• When a wave impacts a boundary, some of the
  energy is reflected, while some passes through.
• The wave that passes through is called a
  transmitted wave.
• A wave that is transmitted through a boundary
  will lose some of its energy.
• Electromagnetic radiation will both slow down and
  have a shorter wavelength when going into a
  denser media.
• Sound will increase in speed when transitioning
  into a denser media.
• Speed of Light in different mediums

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Continuous Waves Higher Speed to Lower Speed

• Note the differences in wavelength and amplitude
  between of the wave in the two different mediums

Displacement
Note This phenomena is seen with light traveling
from air to water.
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Continuous Waves Lower Speed to Higher Speed

• Note the differences in wavelength and amplitude
  between of the wave in the two different mediums

Incident Reflected Wave
Transmitted Wave
v2
-v1
v1
Boundary
Higher speed Longer wavelength
Lower speed Shorter wavelength
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Law of Reflection

• The angle of incidence with respect to the normal
  is equal to the angle of reflection.

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Specular Diffuse Reflection

• Light incident upon an object with a smooth
  surface will create specular reflection.
• Light incident upon an object with a rough
  surface will create diffuse reflection.

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Refraction of Light

• When light travels through a surface between two
  different media, the light will be refracted if
  the angle of incidence is greater than zero.
• If light is passing into a more dense media, it
  will bend towards a normal with the boundary.

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Law of Refraction (Snells Law)

• The ratio of the sine of the angle of incidence
  to the angle of refraction is a constant.
• n1 sin?1 n2 sin?2
• Where
• n1, n2 index of refraction
• ?1 Angle of incidence
• ?2 Angle of refraction
• Note, the incident ray will always
• bend towards the normal when
• transitioning from a material with
• a lower index of refraction to one
• with a higher index of refraction.

www.sol.sci.uop.edu
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Speed of Light and the Index of Refraction

• The index of refraction, by definition, is the
  ratio of the speed of light in a vacuum to the
  speed of light in a substance.
• The index of refraction is always greater than 1.

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Total Internal Reflection

• When the angle of incidence is such that the
  angle of refraction is equal to 90o, the critical
  angle (?c) has been attained.
• All rays will be reflected internally at all
  angles greater than this angle.
• ?c sin-1 (n2/n1)
• Note Internal reflection can
• only occur if n2 lt n1.
• Internal Reflection (Use PHET)
• Application fiber optic
• cable

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Homework Light I

• Light I Chapter 16 16, 18, 31, 32, 33, 41
•  
•  
• 16. Wavelength decreases as freq inc.
•  
• 18. Screen B has ¼ the illumination of Screen A
•  
• 31. 7x 10E-7m
•  
• 32. d vt 3E8m/sec 1.28s 3.84E8m
•  
• 33. d vt ? d/v t 1.5E8km/3E8m/sec
  1.5E11m/3E8m/sec 500 sec
•  
• d vt 3E8 0.1s 3E7m 3E4km.
• Since we are talking about a round trip time, a
  distance of 1.5E4km is needed.

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Reflection Lab Angles
Angle 1 Angle 2 Angle 3 Angle 4 Angle 5 Angle 6
A 10 50 15 25 20 30
B 30 20 45 25 35 30
C 55 40 30 10 35 45
D 45 65 50 40 10 55
E 40 55 45 60 50 30
F 60 50 65 45 30 60
G 20 30 60 45 55 15
H 50 20 40 60 10 30
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Have you ever seen this?
17

• Mirages can happen at any time of the year,but
  they are more commonly seen during the summer due
  to the hotter temperatures. The temperature
  difference between the layers of air is more
  important to the formation of a mirage than the
  actual temperature. The greater the difference
  in temperature the greater the bending effect.

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Science (cont.)

• There are two types of mirages. Superior and
  inferior mirages. Mirages are caused by light
  rays passing through a layer of hot air near the
  surface. This causes a bending effect. When the
  light rays are refracted a mirage is created.

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Light Passing Through Glass
Air
Air
Glass
Reflected Ray
Refracted Ray
?4
?2
?3
?1
Incident Ray
Note ?1 ?4 ?2 ?3 The light exiting the
glass is parallel to the light entering it.
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Start Period 1 here
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Sunset is an illusion
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Light II Chapter 17 p. 410 5, 6, 8, 10, 11 5.
Angle of incidence is greater than angle of
refraction 6. Angle of incidence is less than
angle of refraction. 8. index of refraction
times sine of angle of incidence index of
refraction times sine of angle of refraction 10.
Angle of incidence that produces an angle of
refraction whose sine is 90 degrees. 11. Total
internal reflection.
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Chromatic Dispersion

• When white light enters a medium, the different
  wavelengths that comprise the light will travel
  at different speeds.
• If the angle of incidence is greater than zero,
  the wave will exhibit chromatic dispersion.
• Note The shorter
• the wavelength, the
• greater the bending.
• What happens to
• the frequency?
• NOTHING

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Diffraction of Light

• When a wave front is incident on a barrier with
  an opening, the wave will spread out after
  crossing the barrier. This process is called
  diffraction.
• Diffraction is an interference phenomena.
• As the slit becomes narrower, the amount of
  diffraction will increase.
• As the wavelength of light increases, the amount
  of diffraction will increase.
• Diffraction

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Is light a Wave?

• Young Double-Slit Experiment
• The wave properties of light were first
  demonstrated by Thomas Young in 1801.
• Showed that light undergoes interference in and
  diffraction in much the same way that water and
  sound waves do.
• Used a source of monochromatic light so that only
  one wavelength was chosen. Also used light with
  no phase difference.

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Young Double-Slit Experiment
www.src.wits.ac.za
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Young Double Slit Experiment
www.hyperphysics.phy-astr.gsu.edu
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Young Double Slit Experiment

• ? (delta) r2 r1
• If r2 r1 is equal to some multiple of ?, then
  the image on the screen will be a maximum
  (constructive interference).
• If we assume that D is very big and r1 and r2 are
  parallel, then the angle between them will be ?.
• ? d sin ?
• or
• m? d sin ?

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Young Double Slit Experiment

• To determine the distance y between the central
  and first maximum, we will again assume that D is
  very large compared to y or d.
• From the diagram, we see that
• tan ? y/D
• However, for small angles of ?, tan? sin?
• Therefore, we
• can substitute
• y/D for sin?
• and get

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Young Double Slit Experiment

• What are the implications of the formula?
• As wavelength (?) and distance (D) to the screen
  increases, the distance between maximums
  increases.
• As the distance between slits (d) increases, the
  distance between maximums decreases.

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Key Ideas

• Transverse waves such as electro-magnetic
  radiation do not require a medium.
• Light Waves travel at different speeds in
  different mediums. It slows down when going from
  air to a liquid or solid.
• Waves can interfere with one another resulting in
  constructive or destructive interference.
• The law of reflection states that angle of
  incident wave equals the angle of the reflected
  wave.

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Key Ideas

• Snells Law / Law of Refraction A wave will bend
  toward the normal when transitioning from a media
  with a low index of refraction (e.g. air) to a
  media with a higher index of refraction.
• Total internal reflection occurs when the angle
  of incidence is greater than the critical angle.
  Consequently, no light will escape.
• Diffraction is the spreading out of a wave when
  it encounters a barrier.
• Thomas Youngs double slit experiment showed that
  light has wave properties similar to water and
  sound.