Electromagnetic Radiation (Light)

1. Electromagnetic Radiation (Light) • A source of light produces packets of energy called “photons” • Each packet has a well defined wavelength (which we perceive as color at visible wavelengths), the separation between wavecrests of the electromagnetic wave.

2. Sources of Light vs. Reflected Light • The vast majority of the things we see are made visible by reflected light originating from one or more sources of light.

3. Sources of Light vs. Reflected Light • The Sun is the primary light source illuminating Solar System objects.

4. Sources of Light vs. Reflected Light • The Moon, planets, asteroids, etc. “shine” by reflected sunlight.

5. Electromagnetic Radiation (Light) • A source of light produces packets of energy called “photons” • Each packet has a well defined wavelength (which we perceive as color at visible wavelengths).

6. Wavelength • Wavelength alone distinguishes types of light • At visible wavelengths – short wavelengths are blue; long are red • Wavelength, color, and energy of a photon are all the same thing

7. Wavelength • Wavelength alone distinguishes types of light • At visible wavelengths – short wavelengths are blue; long are red • Wavelength, color, and energy of a photon are all the same thing • Short wavelength photons (the “bluer” ones) carry more energy than long wavelength photons (the “redder” ones). • Start thinking, now, about “blue” and “red” being directions in the spectrum rather than absolutes • “toward the blue...” = “toward shorter wavelengths”

8. The Electromagnetic Spectrum • Wavelength alone distinguishes types of light • Visible light covers a tiny range of possible wavelengths • We have used technology to make other wavelengths “visible” defining, in the process new regions of the spectrum. • Radio, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray are all forms of light of different wavelength (here from long wavelengths to short).

9. Spectra • Light can be sorted and binned by wavelength. The resulting spectrum can be projected on a screen or plotted on a graph.

10. Two Fundamental Types of Spectra • Spectra can be from one of two classes • Continuous – a smoothly varying distribution of all colors • Discrete – emission (or absorption) at precise wavelengths • Often a spectrum is a combination of both

11. The Solar Spectrum

12. Continuous Spectra: Thermal Radiation • Any hot object glows • The hotter the object the brighter and bluer the glow

13. The Nature of Temperature • Temperature is a measure of the energy of motion of particles in a gas or in a solid. • In a gas the particles (atoms or molecules) are independently flying about colliding with one another or with the walls of the chamber. • At high temperature the particles move quickly. At low temperatures they are sluggish. • In a solid the particles are vibrating in place. • The lowest possible temperature is the point at which all thermal energy has been removed – absolute zero.

14. The Nature of Temperature • Temperature is a measure of the energy of motion of particles in a gas or in a solid. • In a gas the particles (atoms or molecules) are independently flying about colliding with one another or with the walls of the chamber. • At high temperature the particles move quickly. At low temperatures they are sluggish. • In a solid the particles are vibrating in place. • The lowest possible temperature is the point at which all thermal energy has been removed – absolute zero.

15. Continuous Spectra: Thermal Radiation • Any hot object glows • The hotter the object the brighter and bluer the glow

16. Continuous Spectra: Thermal Radiation • Dense spheres of gas (stars) are good approximations to blackbodies as well. • The hot stars below are blue. Cooler ones are yellow and red.

17. Continuous Spectra: Thermal Radiation • The equations below quantitatively summarize the light-emitting properties of solid objects. • The hotter the object the “bluer” the glow. • The Sun (6000K) peaks in the middle of the visible spectrum (0.5 micrometers / 500 nanometers) • Room temperature objects (300K) peak deep in the infrared (10 um). • The hotter the object the “brighter” the glow. • The energy emitted from each square centimeter of the surface of a hot object increases as the fourth power of the temperature. • Double the temperature and the emission goes up 16 times!

18. Sunspots and Thermal Radiation • Sunspots are relatively cooler regions of the Sun's 6000K surface. • Being only about 1000K cooler than their surroundings, they do glow brightly, but due to the strong, T4, dependence of a hot solid object's brightness on its temperature they appear dark.

19. Spectral Line Emission/Absorption • Individual atoms produce/absorb light only at precise discrete wavelengths/colors (or specifically at certain exact energies). http://jersey.uoregon.edu/vlab/elements/Elements.html

20. Spectral Line Emission/Absorption • This property arises from the discrete nature of electronic “orbits” in atoms. • Electrons can only be in configurations that have a specific energy. • Jumping between these configurations (higher to lower energy) emits light. • A photon of exactly the right energy can kick an electron from a lower to higher energy. http://jersey.uoregon.edu/vlab/elements/Elements.html

21. Spectral Line Emission/Absorption • This property arises from the discrete nature of electronic “orbits” in atoms. • Electrons can only be in configurations that have a specific energy. • Jumping between these configurations (higher to lower energy) emits light. • Conversely, a photon of exactly the right energy can kick an electron from a lower to higher energy. http://jersey.uoregon.edu/vlab/elements/Elements.html

22. Spectral Line Emission/Absorption • This property arises from the discrete nature of electronic “orbits” in atoms. • Electrons can only be in configurations that have a specific energy. • Jumping between these configurations (higher to lower energy) emits light. • A photon of exactly the right energy can kick an electron from a lower to higher energy.

23. Spectral Line Emission/Absorption • This property arises from the discrete nature of electronic “orbits” in atoms. • Electrons can only be in configurations that have a specific energy. • Jumping between these configurations (higher to lower energy) emits light. • A photon of exactly the right energy can kick an electron from a lower to higher energy. http://jersey.uoregon.edu/vlab/elements/Elements.html

24. Spectral Line Emission/Absorption • Spectral lines can reveal the elemental content of a planet or star's atmosphere. • Line intensity reveals both the quantity of the element as well as the temperature. http://jersey.uoregon.edu/vlab/elements/Elements.html

25. Spectral Line Emission/Absorption • Spectral line absorption arises when light from a continuous source passes through a cold gas. • The gas atoms selectively remove (actually scatter) specific colors/energies.

26. The Doppler Shift • The observed wavelength of a spectral line depends on the velocity of the source toward or away from the observer. • The amount of theshift is proportional to the object's velocity relative to the speed of light (so typically the shift is tiny but measurable).

27. The Doppler Shift • Objects approaching an observer have wavelengths artificially shifted toward shorter wavelengths – a blueshift. • Objects moving away toward longer wavelengths – a redshift • Note that these are directions in the electromagnetic spectrum, not absolute colors.

28. The Doppler Shift • Using the Doppler Shift we can measure the subtle motions (towards or away from us) of stars, galaxies and interstellar gas without ever seeing actual movement!