Chapter 27: Light

1
Chapter 27 Light
2
Do Now 3/24/09

• A wave has a frequency of 20,000 Hz and a
  wavelength of 4 cm. What is the speed of the
  wave? (Hint You need to convert the wavelength
  to meters!)

3
Using chocolate to find the speed of light
½ wavelength
4
Speed of Light

• Olaus Roemer was the first to approximately
  calculate the speed of light in 1675 by using the
  orbit of Jupiters moon, Io, and the orbit of
  earth around the sun.
• Speed of light
• 300, 000 km/s (3 x 106 m/s)
• 300, 000, 000 m/s (3 x 108 m/s)
• 186, 000 miles per second.

5
Speed of Light

• Light is so fast that it would make 7 ½ trips
  around the earth in one second.
• Light takes 8 minutes to travel from the sun to
  earth and 4 years to travel from the next nearest
  star, Alpha Centauri.
• The distance that light travels in one year is
  called a light year. This would mean that Alpha
  Centauri is 4 light years away.
• Our galaxy has a diameter of 100, 000 light
  years. It would take 100, 000 years to travel
  across our galaxy.

6

• How far, in kilometers, would a beam of light
  travel in one year?
• v d/t
• v 300, 000 km/s
• t 1 year
• The distance of one light year is 9.5 x
  1012 km
• 9, 500, 000, 000, 000 km
• 9 trillion, 500 billion km

7
Do Now 3/25

• 1.) What is the speed of light and how was it
  discovered?
• 2.) How long does light take to travel from the
  sun to earth?
• 3.) How long does light take to travel a distance
  of one light year?
• 4.) What is the distance of one light year?

8
Electromagnetic Waves

• 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.

9
Electromagnetic Spectrum
10

• 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.

11

• An electromagnetic wave contains electrons. When
  light hits an object, the electrons in the object
  are forced into vibration by the vibrating
  electrons in the light.
• Different materials have different types of
  electrons some of the electrons in materials
  vibrate really fast on their own (high frequency)
  and some vibrate slowly on their own (low
  frequency).
• Depending on the type of electrons in an object,
  light will either be remitted (transferred from
  one object to another) or absorbed (in which case
  the object will become warmer).

12

• When ultraviolet light hits glass, it caused the
  electrons in the glass to vibrate at the same
  frequency that the electrons in the ultraviolet
  light are vibrating at. The glasss electrons
  have a natural frequency and the light is
  forcing the electrons to vibrate at this natural
  frequency (resonance occurs).
• Because of resonance, the amplitude of the
  vibrations of the electrons in the glass become
  larger. This results in an increase in energy.
• The more energy between the atoms, the more
  frequently they are going to collide with each
  other. Since the electrons now have a lot of
  energy, they are going to bump into other
  electrons in the glass. This transfer of energy
  between atoms causes the glass to become warmer.

13
Glass is not transparent to Ultraviolet Light

• Glass absorbs ultraviolet light. The ultraviolet
  light causes the glass to become warmer.
• Therefore, glass is not transparent to
  ultraviolet light. If it were transparent, the
  light would be remitted, or transmitted, instead
  of absorbed.
• Infrared waves act like ultraviolet waves in that
  they cause the glass to become warmer, so light
  is not remitted.

14
Glass is transparent to visible light

• When an electromagnetic wave has a lower
  frequency than ultraviolet light, such as visible
  light, the vibrations of the electrons in the
  glass have smaller amplitude. This means that
  they have less energy, so they wont collide as
  much. Therefore, less energy is going to be
  transferred as heat.
• The energy in the electrons is remitted, or
  transferred from an object on one side of the
  glass to one on the other side. Therefore, since
  visible light is remitted through glass, glass is
  transparent to visible light.
• When the light passes through glass, it emerges
  at the same speed that it went through 300, 000
  km/s. Although the speed of light may slow down
  while it is in the glass, it comes out of the
  glass at the same speed it went in.

15
Opaque materials absorb light

• Some materials dont remit light they dont
  transfer the light from one object to another.
• Some materials dont allow light to pass through
  at all, they just absorb the light.
• These materials that absorb light are called
  opaque.
• Wood, stone, and people are opaque to visible
  light when light hits these objects, the energy
  is turned into kinetic energy or internal energy,
  and the objects become warmer.

16
Do Now 3/26

• 1.) Why is glass opaque (not transparent) to
  ultraviolet light?
• 2.) What materials are opaque to light?
• 3.) Explain one way that the speed of light can
  be found. (Hint Think of the experiment we did
  in class!)

17
Umbra and Penumbra

• A beam of light is called a ray.
• When light shines on an object, some of the rays
  may be stopped, while others continue in a
  straight-line path.
• In this case, a shadow will occur.
• When an object is farther away from a light
  source, the light casts a sharp shadow of the
  object. A total shadow is called an umbra.
• When the object is close to the light source, the
  light casts a fuzzy shadow. A partial shadow is
  called a penumbra.

18
Solar and Lunar Eclipses

• A solar eclipse occurs when the moon is in
  between the sun and earth. The sun casts a
  shadow of the moon on earth. However, since the
  moon is smaller than the earth, this shadow can
  only be seen by few people on this side of earth.
  
• A lunar eclipse occurs when the earth is in
  between the sun and moon. The sun casts a shadow
  of the earth on the moon. The people who are on
  the side of earth that is away from the sun
  (those who are experiencing night time)
  experience the lunar eclipse.

19
Journal 3/26

• 1.) What is the difference between a lunar and a
  solar eclipse?
• 2.) Can lunar eclipses be seen by everyone on
  earth? Why or why not?
• 3.) Who on earth can see solar eclipses when they
  occur?

20
Do Now 3/27

• 1.) Does an umbra or penumbra occur when an
  object is far away from the light source?
• 2.) What is the difference between a solar and
  lunar eclipse?
• 3.) Who can see lunar eclipses? Solar eclipses?

21
Polarization

• Sound travels in longitudinal waves, but light
  travels in transverse waves.
• When a wave is confined to a vertical plane, it
  is said to be vertically polarized. When a wave
  is confined to a horizontal plane, it is said to
  be horizontally polarized.

22
Polarization

• Light that comes from the sun is not polarized
  the particles vibrate in all directions. But
  when that light hits a polarized filter, the
  light becomes polarized the particles vibrate
  in a vertical or horizontal direction.
• Light is full of electrons. When the electrons
  vibrate vertically up and down, a vertically
  polarized light wave occurs. When the electrons
  vibrate horizontally left to right, a
  horizontally polarized light wave occurs.
• The polarization axis is in the same direction as
  the polarized wave.

23
Polarization Axis

• 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 can pass
  through (about 50 of light).