Light: The Cosmic Messenger

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

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.

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!

Observation: Light is FAST! Galileo tried to measure with lanterns! …but light was too quick! 1 mile there & back in 0.00001 seconds!

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

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

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.

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.

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

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)

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.)

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)

Wavelength and Frequency wavelength  frequency = speed of a wave

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

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)

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)

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

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

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

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

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

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!

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!

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

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

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.

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

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

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

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 = 6.626  10−34 joule  s Planck’s constant

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

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