LIGHT

1
CHAPTER 14

• LIGHT

2
EARLY CONCEPTS OF LIGHT

• Geometrical Optics the study of light rays and
  their interactions with mirrors and lenses.
• Early Greeks thought that light consisted of tiny
  particles, which entered the eye to create vision.

3
EARLY CONCEPTS OF LIGHT

• Socrates and Plato thought that vision resulted
  from streamers or filaments emitted by the eye
  making contact with the object.
• In other words, you did not see something until
  your eyes fell upon the object.

4
EARLY CONCEPTS OF LIGHT

• Light is emitted from objects whether or not your
  eyes are there to receive it.
• What you see depends on the light that enters
  your eyes.

5
CHARACTERISTICS OF LIGHT

• Not all light is visible to the human eye.
• The most common type of light is white light (Ex
  sun).
• White light is a combination of the six
  elementary colors red, orange, yellow, green,
  blue, and violet.

6
CHARACTERISTICS OF LIGHT

• The spectrum of light includes more than visible
  light.
• A variety of forms of radiation exist X-rays,
  microwaves, radio waves, etc.
• All of which have many of the same properties as
  visible light.

7
CHARACTERISTICS OF LIGHT

• This is because they are all examples of
  Electromagnetic Radiation.
• Electromagnetic Radiation a form of energy that
  exhibits wavelike behavior as it travels through
  space.

8
CHARACTERISTICS OF LIGHT

• In the electromagnetic wave theory, light is a
  wave composed of oscillating electric and
  magnetic fields.
• These fields are perpendicular to the direction
  in which the wave moves.

9
CHARACTERISTICS OF LIGHT

• Therefore, electromagnetic waves are transverse
  waves.

10
CHARACTERISTICS OF LIGHT

• Electromagnetic waves are distinguished by their
  different frequencies and wavelengths.
• In visible light, these differences account for
  the different colors.

11
CHARACTERISTICS OF LIGHT

• These differences also distinguish visible light
  from invisible electromagnetic radiation.

12
THE ELECTROMAGNETIC SPECTRUM

• Radio Waves
• Range
• ? 30-cm
• f lt 1.0 x 109-Hz
• Applications
• AM and FM radio Television

13
THE ELECTROMAGNETIC SPECTRUM

• Microwaves
• Range
• 30-cm gt ? gt 1-mm
• 1.0x109-Hz ltflt 3.0x1011-Hz
• Applications
• Radar atomic and molecular research aircraft
  navigation microwave ovens

14
THE ELECTROMAGNETIC SPECTRUM

• Infrared Waves
• Range
• 1 mm gt ? gt 700 nm
• 3x1011 Hz lt f lt 4.3x1014 Hz
• Applications
• Molecular vibrational spectra infrared
  photographyphysical therapy

15
THE ELECTROMAGNETIC SPECTRUM

• Visible Light
• Range
• 700 nm gt ? gt 400 nm
• 4.3x1014 Hz lt f lt 7.5x1014 Hz
• Applications
• Visible light photography optical microscopy
  optical astronomy

16
THE ELECTROMAGNETIC SPECTRUM

• Ultraviolet Waves
• Range
• 400 nm gt ? gt 60 nm
• 7.5 x 1014 Hz lt f lt 5 x 1015 Hz
• Applications
• Sterilization of medical instrumentsidentificatio
  n of fluorescent minerals

17
THE ELECTROMAGNETIC SPECTRUM

• X-Rays
• Range
• 60 nm gt ? gt 10-4 nm
• 5 x 1015 Hz lt f lt 3 x 1021 Hz
• Applications
• Medical examination of bones, teeth, and vital
  organs treatment for types of cancer

18
THE ELECTROMAGNETIC SPECTRUM

• Gamma Rays
• Range
• 0.1 nm gt ? gt 10-5 nm
• 3 x 1018 Hz lt f lt 3 x 1022 Hz
• Applications
• Examination of thick materials for structural
  flaws treatment of types of cancer food
  irradiation

19
ELECTROMAGNETIC WAVES

• All electromagnetic waves move at the same speed
  of light.
• Speed of light (c)
• 2.997 x 108 m/s
• Speed of light frequency x wavelength
• c f?

20
ELECTROMAGNETIC WAVES

• Example
• The AM radio band extends from 5.4 x 105-Hz to
  1.7 x 106-Hz. What are the longest and shortest
  wavelengths in this frequency range?
• 555.0-m
• 176.3-m

21
ELECTROMAGNETIC WAVES

• Example
• What is the wavelength range for the FM radio
  band (88 MHz - 108 MHz)?
• 3.406-m
• 2.775-m

22
ELECTROMAGNETIC WAVES

• Example
• What is the frequency of an electromagnetic wave
  if it has a wavelength of 1.0-km?
• 299,700-Hz

23
ELECTROMAGNETIC WAVES

• Example
• The portion of the visible spectrum that appears
  brightest to the human is around 560-nm in
  wavelength, which corresponds to yellow-green.
  What is the frequency of 560-nm light?

24
SHADOWS

• By observing shadows and the position of the
  light sources and the objects causing the
  shadows, it is easy to deduce that light travels
  in straight lines.

25
SHADOWS

• Light Rays
• A line that represents the path of light in a
  given direction.

26
SHADOWS

• The darkest part of a shadow is known as the
  umbra.
• Umbra
• the darkest part of a shadow where no light from
  the source reaches.

27
SHADOWS

• The area surrounding the umbra is known as the
  pneumbra, where only some of the individual
  shadows overlap.

28
SHADOWS

• Pneumbra
• the transition region between the darkest and
  full brightness. Only part of the light from the
  source reaches this region

29
SHADOWS

• Example
• Imagine standing in a shadow looking back at the
  source.
• If your eye is in the umbra, you will not be able
  to see any portion of the light source.

30
SHADOWS

• If your eye is in the pneumbra, you will be able
  to see part of the light source.
• The shadows that we see on earth are not totally
  black, or devoid of light. Why?

31
SHADOWS

• This is due to the light that scatters in to the
  shadow from the atmosphere or from other objects.
• On the moon, the shadows are much darker because
  there is no lunar atmosphere.

32
SHADOWS

• So, you do not want to step into a shadow on the
  moon, due to the fact that you do not know what
  is hidden in the shadow. (sharp rocks, uneven
  terrain, deep holes, etc.)

33
SHADOWS

• ? What happens to the size of a shadow as the
  object moves closer to the light source? If the
  object moves farther away from the light source?
• Larger Smaller

34
SHADOWS

• SS SO(dS/dLS)
• SS Size of shadow
• SO Size of object
• dS Distance from light source
• to shadow
• dLS Distance from object to
• light source

35
SHADOWS

• Example
• A 20-cm diameter ball is located 50-cm from a
  point source and 100-cm from a wall. What is the
  size of the shadow on the wall?
• SS 20-cm(150-cm/50-cm)
• SS 60.00-cm

36
SHADOWS

• Example
• If the object in the previous example is moved
  20-cm closer to the wall, what is the size of the
  shadow?
• SS 20-cm(150-cm/70-cm)
• SS 42.86-cm

37
CAMERAS

• ? If we sat directly in front of a piece of
  photographic film, what would happen?
• The film would be completely exposed leaving no
  record of the scene.

38
CAMERAS

• The reason you can get images onto film is you
  can control the amount of light rays that hit the
  film.
• By controlling the amount of light, a
  recognizable image of the object is formed on the
  film.

39
CAMERAS

• Pinhole cameras were used before the invention of
  film. (Solar Eclipses)
• The size of the image produced by a pinhole
  camera depends on the objects size and the
  relative distance of the object and the image
  from the pinhole.

40
CAMERAS

• For our purposes, we will equate the ratio of the
  heights to the ratio of the bases.
• hi/ho di/do

41
CAMERAS

• hi height of image
• ho height of object
• di distance from the pinhole
• to the image
• do distance from the pinhole
• to the object

42
CAMERAS

• Example
• Calculate the size of the image of a person who
  is 2-m tall if the person stands 4-m from a
  pinhole camera. Assume that the back wall of the
  camera is 0.4-m from the pinhole.

43
CAMERAS

• hi/ho di/do
• hi ho(di/do)
• hi 2-m(0.4-m/4-m)
• hi 0.2000-m

44
CAMERAS

• Example
• What is the size of the image if the person moves
  2-m closer to the pinhole camera?
• hi 2-m(0.4-m/2-m)
• hi 0.4000-m

45
REFLECTION OF LIGHT

• Light traveling through a uniform substance
  always travels in a straight line.
• When light encounters a different substance, its
  path will change.

46
REFLECTION OF LIGHT

• This change in direction of the light is called
  reflection.
• Reflection the turning back of an
  electromagnetic wave at the surface of a
  substance.

47
REFLECTION OF LIGHT

• Most substances absorb at least some of the
  incoming light and reflect the rest.
• Mirrors reflect almost all of the incoming light.

48
REFLECTION OF LIGHT

• The texture of a surface affects how it reflects
  light.
• Light that is reflected from a rough, textured
  surface, is reflected in many different
  directions.

49
REFLECTION OF LIGHT

• This type of reflection is called diffuse
  reflection.
• Light reflected from smooth, shiny surfaces, such
  as a mirror or water, is reflected in one
  direction.

50
REFLECTION OF LIGHT

• This type of reflection is called specular
  reflection.
• A surface is considered smooth if its surface
  variations are small compared with the wavelength
  of the incoming light.

51
REFLECTION OF LIGHT

• ? If you shine a light on a flat mirror at an
  angle of 45, what angle do you think you will
  see the reflection of that light?
• 45 Incoming and reflected angles are equal.

52
REFLECTION OF LIGHT

• It is more convenient to consider the angles
  between the rays and the normal.
• Normal a line perpendicular to a surface or
  curve. An invisible line that touches the
  reflecting surface at the spot where the rays hit
  the surface.

53
REFLECTION OF LIGHT

• If a straight line is drawn perpendicular to the
  reflecting surface at the point where the
  incoming ray strikes the surface, the angle of
  incidence and angle of reflection can be defined
  with respect to the line.

54
REFLECTION OF LIGHT

• Angle of Incidence (?) the angle between a ray
  that strikes a surface and the normal to that
  surface at the point of contact.

55
REFLECTION OF LIGHT

• Angle of Reflection (?) the angle formed by the
  line normal to a surface and the direction in
  which a reflected ray moves.
• ? ?

56
FLAT MIRRORS

• When we look at smooth reflecting surfaces, we do
  not see light rays, we see images.
• We can see how images are formed by looking at
  the paths taken by the light rays.

57
FLAT MIRRORS

• Flat or plane mirrors produce images that are
  exactly the same size as the object.
• The distance between the object and the mirror is
  the same as the image and the mirror

58
FLAT MIRRORS

• Since flat mirrors do not change the size of the
  object, we say that it has a lateral
  magnification of 1.
• Lateral Magnification the ratio of the height of
  the image to the height of the object.

59
FLAT MIRRORS

• m hi/ho
• m Lateral Magnification
• hi Image Height
• ho Object Height

60
FLAT MIRRORS

• Example
• A box that is 0.75-m tall is sitting 0.5-m from a
  plane mirror. How far is the image recreated in
  the mirror? How tall is the image? What is the
  magnification of the image?

61
FLAT MIRRORS

• There are two types of images in mirrors
• Real Images
• Virtual Images

62
FLAT MIRRORS

• Real Image an image formed when rays of light
  actually intersect at a single point.
• Virtual Image an image formed by light rays that
  only appear to intersect.

63
FLAT MIRRORS

• Characterizing the image formed by a plane
  mirror
• 1. Nature REAL VIRTUAL
• 2. Orientation ERECT INVERTED
• 3. Size ENLARGED TRUE REDUCED
• 4. Position di do

64
FLAT MIRRORS

• ? How tall does a mirror have to be in order for
  you to be able to see your whole body?
• The size of the mirror would be your height
  divided by 2.

65
FLAT MIRRORS

• The image formed by a flat mirror appears to have
  right-to-left reversal that is, the right side
  of an object is the images left side.

66
FLAT MIRRORS

• This effect is due to the changes in direction of
  the light rays as they are reflected from the
  mirror.

67
FLAT MIRRORS

• ? Why does a flat mirror appear to reverse images
  left to right, but not up and down?
• An object facing the mirror produces an image
  that faces the object.

68
CURVED MIRRORS

• If the mirror surface is curved, it is possible
  to form magnified and smaller images, and also
  real images.
• The distortions are a result of the curvature of
  the mirrors.

69
CURVED MIRRORS

• Some cosmetic and rearview mirrors are simple
  curved mirrors that do not produce bizarre
  distortions, but do change the image size.
• Two different types of curved mirrors, Concave
  and Convex.

70
CURVED MIRRORS

• Concave (Spherical) Mirror
• An inwardly curved, mirrored surface that is a
  portion of a sphere and that converges incoming
  light rays.

71
CURVED MIRRORS

• A concave mirror can form two different types
  (real and virtual) depending on how close the
  object is to the mirror.

72
CURVED MIRRORS

• One factor that determines where the image will
  appear and how large the image will be is the
  amount by which the mirror is curved.
• This depends on the radius (R) or Center of
  Curvature (C) of the mirror.

73
CURVED MIRRORS

• The point where the light rays converge or meet
  is called the focal point (F) of the mirror.
• For a spherical mirror, the focal point is
  located halfway between the center of curvature
  and the vertex (A).

74
CURVED MIRRORS

• Vertex the highest point of something.
• Focal Distance Radius/2
• The focal point is always () when dealing with
  concave mirrors.

75
CURVED MIRRORS

• At a large distance from the mirror, you will see
  an image of yourself that is inverted and reduced
  in size (real image).
• As you move toward the mirror, the image gets
  larger.

76
CURVED MIRRORS

• When you pass the center point of the mirror, the
  image moves behind you.
• When you are closer to the than the focal point,
  the image is similar to that in a flat mirror,
  except that it is magnified (virtual image).

77
CURVED MIRRORS

• The line passing through the center of curvature
  and the center of the mirror (M) is known as the
  optic or principal axis.

78
CURVED MIRRORS

• Two Rules concerning concave mirrors
• 1.Any light ray approaching the mirror parallel
  to the principal axis is reflected through the
  focal point.

79
CURVED MIRRORS

• 2. Any ray that approaches the mirror through the
  focal point is reflected parallel to the
  principal axis.

80
CURVED MIRRORS

• The location of the image can be predicted with
  ray diagrams.
• Ray Diagrams are drawings that use simple
  geometry to locate an image formed by a mirror.

81
CURVED MIRRORS

• To pinpoint the location of the image, 3 light
  rays are commonly used
• All 3 rays begin at the tip of the object.
• 1. Along the radius.
• Reflects back on itself

82
CURVED MIRRORS

• 2. Parallel to the optic axis.
• Reflects back through the focal point.
• 3.Through the focal point.
• Reflects back parallel to the optic axis.

83
CURVED MIRRORS

• Where the 3 reflection lines intersect, that is
  where the tip of the image is located.
• Since the base of the object will be located on
  the optic axis, the image will also be located on
  the optic axis.

84
CURVED MIRRORS

• When the object is located beyond the focal
  point, a real image is formed.
• When the object is located within the focal
  point, a virtual image is formed.

85
CURVED MIRRORS

• Image location can be predicted with the mirror
  equation
• 1/do 1/di 2/R

86
CURVED MIRRORS

• Object images and distances have a positive value
  when measured from the center of the mirror to
  any point on the mirrors front side.

87
CURVED MIRRORS

• Distances for images that form on the back side
  of the mirror always have a negative value.
• The object and image heights are always positive
  when both are above the optic axis, and negative
  when either is below.

88
CURVED MIRRORS

• To find the magnification of the image in a
  concave mirror, all you need to know is the
  location of the image and the location of the
  object.
• M - (di/do)

89
CURVED MIRRORS

• For an image in front of the mirror, its M is
  negative and the image is inverted with respect
  to the object.
• When the image is behind the mirror, M is
  positive and the image is upright with respect to
  the object.

90
CURVED MIRRORS

• Convex Spherical Mirror
• An outwardly curved, mirrored surface that is a
  portion of a sphere and that diverges incoming
  light rays.
• Ex Side-view mirrors

91
CURVED MIRRORS

• Convex mirrors take the objects in a large field
  of view and produce a small image, so they are
  well suited for providing a fixed observer with a
  complete view of a large area.

92
CURVED MIRRORS

• The resulting images in a convex mirror are
  always virtual, and the image distance is always
  negative.
• Because the mirrored surface is on the side
  opposite the radius of curvature, it also has a
  negative focal length.

93
CURVED MIRRORS

• Ray Diagrams for Convex Mirrors.
• Differs slightly from a concave mirror.
• Dotted lines are extended along the reflected
  reference rays to points behind the mirrors
  surface.

94
CURVED MIRRORS

• A virtual, upright image forms where the 3
  apparent rays intersect.
• Magnification for convex mirrors is always less
  than 1.

95
COLOR

• The color of an object depends on which
  wavelengths of light shine on the object and
  which wavelengths are reflected.

96
COLOR

• If all wavelengths of incoming light are
  completely reflected by an object, that object
  appears the same color as the light illuminating
  it.

97
COLOR

• An object of a particular color absorbs all
  colors except the light whose color is the same
  as the objects color.
• Ex A green leaf.

98
COLOR

• An object that reflects no light appears black.

99
COLOR

• Additive primary colors produce white light when
  combined.
• The additive effects of color can be demonstrated
  by placing colored filters in front of spotlights
  or slide projectors and allowing the color beams
  from each one to overlap.

100
COLOR

• The primary additive colors are red, green, and
  blue.
• Combinations of the three primary colors produce
  new colors
• Red Green YELLOW
• Red Blue MAGENTA
• Blue Green CYAN

101
COLOR

• Red Blue Green WHITE
• This process is the basis of color television.
  TVs are covered with arrays of red, green, and
  blue dots or lines.

102
COLOR

• The primary subtractive colors are cyan, magenta,
  and yellow.
• When any two primary subtractive colors are
  combined, they produce either red, blue, or green
  pigments.

103
COLOR

• Subtractive primary colors filter out all light
  when combined.
• When blue and yellow light are mixed, white light
  results.
• However, when you mix blue and yellow paint, the
  resulting color is green, not white. Why?

104
COLOR

• When pigments are mixed, each one subtracts
  certain colors from white light, and the
  resulting color depends on the frequencies that
  are not absorbed.

105
COLOR

• When all three primary subtractive colors are
  combined, they produce black.
• This brief coverage of color perception allows us
  to answer the following questions

106
COLOR

• 1. Why is the sky blue?
• 2. Why is the sun yellow?

107
COLOR

• The sun radiates light that is essentially white.
  Since it appears yellow, we can assume that some
  of the complementary color has somehow been
  removed.

108
COLOR

• The complementary color of yellow is blue - the
  color of the sky.
• The molecules in the atmosphere are more
  effective in scattering blue light than red light.

109
COLOR

• As the sunlight passes through the atmosphere,
  more and more of the blue end of the spectrum is
  removed, leaving the transmitted light with a
  yellowish color.

110
COLOR

• When we look away from the sun, the sky has a
  bluish cast because more of the blue light is
  scattered into our eyes.

111
COLOR

• These ideas also account for the color of water.
• Because water absorbs red light more than the
  other colors, the water takes on the color that
  is complementary to red, which is cyan.