Chapter 13: light

1
Chapter 13 light
2
Do Now 4/14/09

• 1.) Are sound waves longitudinal or transverse?
• 2.) What is an electromagnetic wave?
• 3.) How is wave speed calculated?

3
Light

• Light has a dual nature - it is part wave and
  part particle.
• Light travels in a wave that is partly electric
  and partly magnetic. These types of waves are
  called electromagnetic waves.

4

• Gamma rays energetic enough to damage cells and
  kill.
• X-rays enough energy to penetrate flesh and used
  for medical diagnostics.
• Ultraviolet light energetic enough to burn skin.
• Visible light detectable by the human eye.
• Infrared light feels warm on the skin as heat.
• Microwaves used for cooking and communications.
• Radio waves used for communications.

5
Using chocolate to find the speed of light
½ wavelength
Wavelength
6
Speed of Light

• Speed of (light) wave frequency x wavelength
• c f ?
• All electromagnetic waves travel at the speed of
  light (300, 000 km/s)
• The AM radio band extends from 5.4 x105 Hz to 1.7
  x 106 Hz. What are the longest and shortest
  wavelengths in this frequency range?

7
Do Now 4/15/09

• 1.) Explain how the speed of light can be
  determined using a chocolate bar and a microwave.
• 2.) If an electromagnetic wave has a frequency of
  7.57 x 1014 Hz, what is its wavelength? To what
  part of the spectrum does this wave belong?

8
Reflection of Light

• A mirror is an object that reflects about 90 of
  light and absorbs the rest.
• When light hits a mirror the light is reflected,
  the direction of the light waves changes.
• The way in which light is reflected from a
  surface depends on the surfaces smoothness.
• Light that is reflected from a rough surface,
  such as paper or cloth, is reflected in many
  different directions. This type of reflection is
  called diffuse reflection.
• Light reflected from a smooth, shiny surface,
  such as a mirror or pond is reflected in only one
  direction. This type of reflection is called
  specular reflection.

9

• Which of the following are examples of specular
  reflection, and which are examples of diffuse
  reflection?
• (a) Reflection of light from the surface of a
  lake on a calm day
• (b) Reflection of light from a plastic trash bag
• (c) Reflection of light from the lens of
  eyeglasses
• (d) Reflection of light from a carpet

10
Reflection of Light
Angle of incidence ?
Angle of Reflection ?
11
Flat Mirrors

• The simplest mirror is a flat mirror
• If you place an object in front of a flat mirror,
  the image that you see in the mirror appears to
  be behind the mirror.

12

• The image looks to be the same distance from the
  mirror as the object is.
• The distance between the object and the mirror is
  the object distance, and the distance between the
  reflected image and the mirror is the image
  distance. These distances are equal, and the
  image of the object is the same size as the
  object.
• The image that appears to come from a point
  behind the mirror, but actually doesnt, is
  called a virtual image. A virtual image appears
  as if it is behind the surface of the mirror.

13
On loose leaf

• If one wall of a room (assume the room has four
  walls) consists of a large flat mirror, how much
  larger will the room appear to be? Explain your
  answer.

14
Do Now 4/16

• 1.) Light travels in the form of
• (a) Waves
• (b) Particles
• (c) Both A and B
• (d) None of the above
• 2.) A person stands 5 m from a flat mirrors
  surface. What is the object distance?
• What is the image distance?
• 3.) P. 476 10

15
Color Spectrum
When sunlight passes through a prism, the colors
of the spectrum appear.
16
Black and White

• All the colors ROYGBIV combined produce white
  light. Therefore, white is not a color itself,
  but rather the combination of all the colors of
  the spectrum.
• Black is also not a color itself, but rather the
  absence of light.
• Objects appear black when they absorb every color
  in light. Black objects absorb red, orange,
  yellow, green, blue, indigo, and violet.
• Since black objects become warmer in sunlight
  because they absorb all colors that make up
  sunlight.

17
Objects reflect Light

• Every object reflects light in a different way.
  For a red object, all the other colors of the
  spectrum may be absorbed besides red. Red is the
  only color that is reflected, so that is the
  color that you see.
• If an object reflects all colors, then the object
  appears to be white when illuminated with white
  light. If the object is illuminated with green
  light, then the white object will look green.

18
Some objects reflect more than one color

• A red rose reflects primarily red light, but also
  a little bit of blue. Since it reflects
  primarily red, that is the color you will see.
• The rose reflects both red and blue, so when you
  shine a red light on the flower, it will still
  appear red since it reflects red light. If you
  shine a blue light on the flower, it will appear
  blue since it reflects blue light. If you shine
  a green light on the red flower, the flower will
  appear black, because the red rose does not
  reflect the color green.

19

• When a red light shines on a red rose, why do the
  leaves become warmer than the petals?
• When a green light shines on a red rose, why do
  the petals look black?
• What color does a ripe banana appear when
  illuminated with red light? Yellow light? Green
  light? Blue light?

20
Mixing Colored Light

• The combination of all the colors of the spectrum
  produces white light. But the combination of
  just red, green, and blue can produce white
  light.

21
Additive Primary Colors

• These three colors (Red, Green, and Blue) are
  called the additive primary colors. When you add
  them together, they can produce the largest
  amount of different colors.
• Red, Green, and Blue produce white
• Red Green Yellow
• Red Blue Magenta
• Blue Green Cyan
• Yellow Blue ?
• Magenta Green ?
• Cyan Red ?

22
Complimentary Colors

• When TWO colors are added together to produce
  white, they are called complimentary colors.
• Therefore, Yellow Blue, Magenta Green, and
  Cyan Red are complimentary colors.

23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
The mixing of paints and dyes is a different
process from the mixing of colored light.

• Paints and dyes produce their colors by absorbing
  light of certain frequencies reflecting light
  of other frequencies.
• Material that absorbs light of a specific color
  is called a pigment.
• Blue paint, for example, reflects mostly blue
  light, but also violet and green. It absorbs
  red, orange, and yellow light.
• Yellow paint reflects mostly yellow light, but
  also red, orange, and green. It absorbs blue and
  violet light.

30
Color Mixing

• When blue and yellow paints are mixed, between
  them they absorb all colors except for green.
  The only color that they both reflect is green,
  which is why the mixture of blue and yellow paint
  is green.
• This process is called color mixing by
  subtraction. The color that is produced by the
  mixing of paints or dyes is the color that both
  paints reflect.
• It is called color mixing by subtraction because
  pigments (the materials that absorb a specific
  color) absorb or subtract colors from light.

31
Color Mixing

• For light mixing, red, green, and blue produce
  the largest amount of different colors. These
  are the three primary colors for color mixing by
  addition, or the additive primary colors.
• But in pigments (the materials that absorb
  specific colors), the combination of cyan,
  yellow, and magenta produce the largest amount of
  different colors. These are the three primary
  colors for color mixing by subtraction, or the
  subtractive primary colors.
• These three colors, when mixed together in
  different ways, can produce many different
  colors.

32
Do Now 4/17

• What color would an opaque magenta shirt appear
  to be under the following colors of light?
• (a) White (b) Red (c) Blue
• (d) Green (e) Yellow

33
Do Now 4/20

• 1.) Red Blue
• 2.) Green Blue
• 3.) Red Green
• 4.) Yellow Blue
• 5.) Magenta Green
• 6.) Red Cyan
• 7.) Red Blue Green
• 8.) Red Blue Green

34
Polarization

• Light from the sun is not polarized the
  particles vibrate in all directions.
• When sunlight passes through a polarized filter,
  the light becomes polarized.
• When light becomes polarized, the particles
  vibrate in one direction (either vertical or
  horizontal).

35
Polarization by Transmission

• The transmission axis (or polarization axis) of a
  filter is the line along which the light is
  polarized
• The polarization axis is in the same direction as
  the polarized wave. If the filter has a vertical
  polarization axis, the light waves will vibrate
  in the vertical direction. If the filter has a
  horizontal polarization axis, the light waves
  will vibrate in the horizontal direction.
• When two polarization axes are perpendicular to
  each other, no light can pass through. But if
  the polarization axes are aligned, light (about
  50) can pass through.

36
Polarization by Reflection

• Light can also become polarized when it is
  reflected.
• When light is reflected at a certain angle from a
  surface, the reflected light is polarized
  parallel to the surface.
• If the surface is parallel to the ground, the
  light will be horizontally polarized. If the
  surface is perpendicular to the ground, the light
  will be vertically polarized.