1. Light: The Cosmic Messenger

2. What is light?
Newton showed that white light is composed of all the colors of the rainbow.

3. What is light?
Does the prism PRODUCE colors, or were they there already in the light?
How can you tell?
Newton showed that white light is composed of all the colors of the rainbow.

4. Observation: Color is property of light!
White light is composed of different colors when shone through glass….
FIGURE 3-1 Prisms and a Spectrum
(a) When a beam of
white light passes through a glass prism, the light is separated
or refracted into a rainbow-colored band called a spectrum.
The numbers on the right side of the spectrum indicate
wavelengths in nanometers (1 nm = 10-9 m). (b) Drawing of
Newton’s experiment showing that glass does not add to the
color of light, only changes its direction. Because color is not
added, this experiment shows that color is an intrinsic property
of light.
…but the glass is not creating those colors!

5. Observation: Light is FAST!
Galileo tried to measure with lanterns!
…but light was too quick!
1 mile there & back in seconds!

6. Observation: Light is FAST!
Roemer tried to measure it with Jupiter’s Moons!
…by timing when they passed in front or behind Jupiter

7. Observation: Light is FAST!
Hubble Space Telescope image of Ganymede being eclipsed by Jupiter

8. More Observations: Speed of Light?
FIGURE 3-4 Evidence that Light Travels at a Finite Speed
The times of the eclipses of Jupiter’s moons as seen from Earth
depend on the relative positions of Jupiter, Earth, and the Sun.
Rømer correctly attributed the variations in these times to the
variations in the time that it takes light from these events to
reach Earth.

9. More Observations: Speed of Light?
FIGURE 3-4 Evidence that Light Travels at a Finite Speed
The times of the eclipses of Jupiter’s moons as seen from Earth
depend on the relative positions of Jupiter, Earth, and the Sun.
Rømer correctly attributed the variations in these times to the
variations in the time that it takes light from these events to
reach Earth.

10. Speed of Light is not infinite!
FIGURE 3-4 Evidence that Light Travels at a Finite Speed
The times of the eclipses of Jupiter’s moons as seen from Earth
depend on the relative positions of Jupiter, Earth, and the Sun.
Rømer correctly attributed the variations in these times to the
variations in the time that it takes light from these events to
reach Earth.
Roemer’s observations of Jupiter’s Moon’s Eclipses demonstrated light moves at a finite speed

11. Even More Observations: Light has “energy”
Non-visible light (beyond the red end of the spectrum) has energy, too!
FIGURE 3-5 Experimental Evidence for Infrared Radiation
This photograph shows the visible colors separated by a
prism. The thermometer in the color region has a temperature
less than the thermometer to the right of red. Therefore, there
must be more radiation energizing (i.e., heating) the warmer
thermometer. This energy is what we call infrared radiation—
invisible to the human eye but detectable as heat. (NASA/JPLCaltech)

12. Observation: Water Waves naturally interfere & create noticeable patterns
FIGURE 3-2 Wave Travel
(b) The pattern created water
waves passing through two slits in a ripple tank. As with
light, the water waves interfere with each other, creating
constructive interference (crests) and destructive interference
(troughs) throughout the right side of the tank and on the far
right wall. (University of Colorado, Center for Integrated Plasma
Studies, Boulder, CO.)

13. Observation: Light has a wavelike property, too!
FIGURE 3-2 Wave Travel
(a) Electromagnetic radiation
travels as waves. Thomas Young’s interference experiment
shows that light of a single color passing through a barrier
with two slits behaves as waves that create alternating light
and dark patterns on a screen. (University of Colorado, Center for Integrated Plasma
Studies, Boulder, CO.)
Young’s Experiment (1801)

14. Wavelength and Frequency
wavelength  frequency = speed of a wave

15. Wavelength of light? Microwaves on buttered bread!??
See: The Naked Scientists Podcast

16. What we “see” is only a small part of what there is!
The entire EM Spectrum
What we “see” is only a small part of what there is!
FIGURE 3-6 The Electromagnetic Spectrum
The full array of all types of electromagnetic radiation
is called the electromagnetic spectrum. It extends
from the longest-wavelength radio waves to the
shortest-wavelength gamma rays. Visible light forms only a tiny
portion of the full electromagnetic spectrum. Note that 1 m
(micrometer) is 106 m, and 1 nm (nanometer) is 109 m. The
insets show how we are now using all parts of the electromagnetic
spectrum here on Earth. (Will and Deni McIntyre/Science
Photo Library; Edward Kinsman/Photo Researchers, Inc.; Chris Martin-
Bahr/Science Photo Library; Bill Lush/Taxi/Getty; Michael Porsche/
Corbis; Ian Britton, Royalty-Free/Corbis)

17. EM Spectrum Varies by… Size (wavelength, color) Energy
How the waves are detected
But not….
How fast they move through space!
FIGURE 3-6 The Electromagnetic Spectrum
The full array of all types of electromagnetic radiation
is called the electromagnetic spectrum. It extends
from the longest-wavelength radio waves to the
shortest-wavelength gamma rays. Visible light forms only a tiny
portion of the full electromagnetic spectrum. Note that 1 m
(micrometer) is 106 m, and 1 nm (nanometer) is 109 m. The
insets show how we are now using all parts of the electromagnetic
spectrum here on Earth. (Will and Deni McIntyre/Science
Photo Library; Edward Kinsman/Photo Researchers, Inc.; Chris Martin-
Bahr/Science Photo Library; Bill Lush/Taxi/Getty; Michael Porsche/
Corbis; Ian Britton, Royalty-Free/Corbis)

18. Atmospheric “Windows” to the stars & universe: Visible & Radio light
FIGURE 3-7 “Windows” Through the Atmosphere
Earth’s atmosphere allows different types of electromagnetic radiation
to penetrate into it in varying amounts. Visible light, radio
waves, short wavelength infrared, and long wavelength ultraviolet
reach all the way to Earth’s surface. The other types of
radiation are absorbed or scattered by the gases in the air at
different characteristic altitudes (indicated by heights of windows).
Although the atmosphere does not have actual “windows,”
astronomers use the term to characterize the passage
of radiation through it.
Atmospheric “Windows” to the stars & universe: Visible & Radio light

19. Observations of Nature
Electricity acts through space over a distance
Lightening, sparks on a doorknob

20. Observations of Nature
Magnetism acts through space over a distance
Two magnets attract or repel one another without touching

21. More Observations
If you spin a conductor in a magnetic field, you get electricity!
Electric Generators
Portable gas generators

22. More Observations
If you run electricity into a coil, you get a magnet!
“Electromagnetic” cranes
Auto solenoids
Electric Motors

23. Maxwell’s Observations
Change Electricity => create magnetism
Change Magnetism => create electricity
Continuously change both, continuously create radiation!
Radiation created moves at “c” – the speed of light!

24. The Wave Model of Light Electro-magnetic radiation!
FIGURE 3-3 Electromagnetic Radiation
All forms of electromagnetic
radiation (radio waves, infrared radiation, visible
light, ultraviolet radiation, X rays, and gamma rays) consist
of electric and magnetic fields oscillating perpendicular to
each other and to the direction they move. In empty space this
radiation travels at a speed of 3 x 105 km/s. These fields are
the mathematical description of the electric and magnetic
effects. The distance between two successive crests, denoted
by λ, is called the wavelength of the light.
Electro-magnetic radiation!

25. Light is an electromagnetic wave.
Use this slide to define wavelength, frequency, speed of light.

26. The Electromagnetic Spectrum
Use this tool from the Light and Spectroscopy tutorial to go over the inverse relationship between wavelength and frequency.

27. Our first key idea is that visible light is only a small part of the complete spectrum of light. You may wish to spend some time explaining the various things shown in this figure… You may also want to repeat this slide at various points to summarize other ideas.

28. Still More Observations of Nature
Einstein’s Photo-electric Effect showed light can eject individual electrons
Energy depended upon light’s color!
Single electrons emerge
Light shining onto metal
Metal plate

29. Still More Observations of Nature
Planck’s Energy Curve showed light can be modeled with specific “quanta” of energy
Peak depended upon light’s color!
Metal plate

30. The Particle Model of Light
Particles of light are called photons.
Each photon has wavelength & frequency.
Energy of a photon depends on frequency.

31. Wavelength, Frequency, & Energy
l  f = c
l = wavelength, f = frequency
c = 3.00  108 m/s = speed of light
E = h  f = photon energy
h =  10−34 joule  s Planck’s constant

32. Thought Question Which is correct? The higher the photon energy,
the longer its wavelength.
the shorter its wavelength.
Energy is independent of wavelength.

33. Thought Question The higher the photon energy,
B. the shorter its wavelength.
Which explains why…
X-rays can kill you!
Country-western music on the radio can’t