Light and Matter

1. Light and Matter © Sierra College Astronomy Department

2. Light and MatterIntroduction Astronomy – An Observation Based Science • In situ measurements by spacecraft are extremely limited for studying the objects of the cosmos • Vast majority of our understanding of the Universe comes from: • The acquisition of light with a variety of instruments • The application of physical principles discovered and refined locally and assumed universally valid to interpret the acquired light © Sierra College Astronomy Department

3. Light and MatterA First Glance How Do We Experience Light? • Energy and Power • Example: The warmth of the daytime Sun – a form of radiative energy • Units • Unit of energy: joule • Unit of energy rate (power): watt = 1 joule/sec • A human’s 10,000,000 joules/day is about 100 watts • Light and Color • Light split by a prism into colors gives a spectrum • All the colors (red, orange, yellow, green, blue, and violet) is roughly equal proportions gives white light • Black is the lack of color © Sierra College Astronomy Department

4. Light and MatterA First Glance How Do Light and Matter Interact • Interaction occurs in four basic ways: • Emission – The process by which an object creates and then emits light (e.g., the filament of a light bulb) • Absorption – The process by which an object destroys light by absorbing it (e.g., some of the light from the bulb hits your hand, is destroyed, and results in the warmth you feel) • Transmission – Light is neither created or destroyed by an object but simply passes through (e.g., that portion of the bulb light that passes through a window) • Reflection/scattering – Light that bounces off matter leading to what is called reflection (when the bouncing is all in the same general direction) and scattering (when the bouncing is in random directions) © Sierra College Astronomy Department

5. Light and MatterA First Glance How Do Light and Matter Interact (continued) • More terminology • Materials that transmit light are said to be transparent • Materials that absorb light are said to be opaque • Be careful • Some materials can be partially transparent (partially opaque) and to various degrees (e.g., sunglasses) • Different colors may behave differently with the same material (e.g., green grass absorbs most colors, but reflects/scatters green light) © Sierra College Astronomy Department

6. Light and MatterProperties of Light Particle vs Wave • A particle is a physical object that can be considered localized in space while characterized by certain measurable properties (e.g., a charged electron or a baseball) • A wave is a spatial and/or temporal variation of a certain property or substance that is generally associated with an extended volume (e.g., waves on the surface of a pond) • Light exhibits both a particle and wave nature – this seemingly contradictory observation is known as the wave-particle duality © Sierra College Astronomy Department

7. Light and MatterThe Wave Nature of Light Wave Motion in General • Wavelength(l)is the distance from a point on a wave to the next corresponding point. • Frequency(f or n) is the number of repetitions per unit time and often is given in cycles/second or hertz(Hz). • There is a relationship between f and l! © Sierra College Astronomy Department

8. Light and MatterThe Wave Nature of Light Light as a Wave • White light is made up of light of many wavelengths, but all wavelengths travel at 300 million meters/second (3.00 X108 m/s) in a vacuum. f l = c = speed of light wave • Nanometer (nm): unit of length = 10-9 m. • Angstrom (Å): unit of length = 10-10 m; it is a non-SI unit. © Sierra College Astronomy Department

9. Light and MatterThe Wave Nature of Light • 700 nm red light has f = 4.3 X 1014 Hz. 400 nm violet light has f = 7.5 X 1014 Hz • Frequencies range from 102 Hz (low) to 1024 Hz (high) • Wavelengths range from 106 m (long) to 10-16 m (short) • Based on frequency and/or wavelength, the Electromagnetic (EM) spectrum is usually broken into these regions: radio (AM/FM/microwave), infrared, visible, ultraviolet, X-ray, gamma ray • These waves are called “electromagnetic” because they consist of combined electric and magnetic waves that result when a charged particle accelerates Maxwell © Sierra College Astronomy Department

10. Light and MatterThe Photon Light as a Particle • Light can also be thought of as a energy particle or packet. • This was the conclusion to explain the photoelectric effect • Equation for energy of a photon: E = hf = hc/l where E is the energy of the photon, h is Planck’s constant, f is the frequency of the light, lthe wavelength and c is the speed of light. © Sierra College Astronomy Department

11. Light and MatterThe Structure of Matter Back to the Greeks • Democritus (c. 470-380 B.C.) introduced the idea of the atom – the smallest possible unit of any object • He then hypothesized four different types of atoms called elements, each of which had different shapes and properties © Sierra College Astronomy Department

12. Light and MatterThe Structure of Matter Basic Atomic Structure & Terminology • Parts of an Atom • Protons and neutrons are in the nucleus • Electrons in a“cloud” surround the nucleus • Electric charge (positive and negative) and force • Atomic Variations • Different elements are atoms with different atomic number (number of protons in nucleus) • Different isotopes of an element have different number of neutrons in the nucleus (atomic mass number = # of protons + # of neutrons) © Sierra College Astronomy Department

13. Light and MatterThe Bohr Atom Three postulates of the Bohr atom: • Electrons in orbit around a nucleus can have only certain specific energies (these energy levels are said to be quantized) • An electron can move from one energy level to another changing the energy of the atom • This change in energy of the atom is associated with a photon of equivalent energy, which in turn determines the frequency (or wavelength) of the photon [Note: This change in energy may also be associated with other processes – for example, atomic collisions] © Sierra College Astronomy Department

14. Light and MatterThe Bohr Atom • “States” of an electron: • An electron at it minimum energy level is said to be in its ground state • If a photon with just the right amount of energy interacts with the atom, the electron may be raised to a new level; the electron is said to be in an excited state. • While in an excited state, the electron can “relax” and fall down to a lower energy state releasing a photon. • Recall: E = hf = hc/l © Sierra College Astronomy Department

15. Light and MatterSpectra Examined Close Up Continuous spectrum • Contains an entire range of wavelengths rather than separate, discrete wavelengths. • Example: The heated filament of a lamp or a glowing piece of iron in the blacksmith’s forge. © Sierra College Astronomy Department

16. Light and MatterSpectra Examined Close Up Kirchhoff’s Laws - Background • In 1814 Fraunhofer analyzed the solar spectrum and found a number of dark lines across the continuous spectrum. The dark lines are caused by absorption. • Later it was discovered that if gases are heated until they emit light, a spectrum made up of bright lines appears. © Sierra College Astronomy Department

17. Light and MatterSpectra Examined Close Up • Kirchhoff’s laws summarize how the three types of spectra are produced: • A hot, dense glowing object (a solid or dense gas) emits a continuous spectrum. • A hot, low-density gas emits light of only certain wavelengths - a bright line spectrum. • When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum. © Sierra College Astronomy Department

18. Light and MatterSpectra Examined Close Up Emission and absorption lines explained • The Bohr model of the atom suggests that only certain wavelength photons will push electrons to higher levels and that only certain photons will be emitted when the electron fall back towards the ground state • This means that the atom will only emit and absorb certain wavelengths of light and will be the at same wavelengths © Sierra College Astronomy Department

19. Color from Reflection - Colors of Planets Planets have their colors because the material on their surfaces or in their clouds absorbs some of the wavelengths of sunlight and reflects a combination of wavelengths that appear, for example, as the rusty red of Mars or the blue of Neptune. Light and MatterSpectra Examined Close Up © Sierra College Astronomy Department

20. Light and MatterSpectra Examined Close Up Thermal Radiation • In a low pressure gas atoms will emit at discrete wavelength (Kirchhoff’s 2nd law) • If one increases the pressure these emission will become broader and slight randomized until the emission blur into each other and form a continuum (Kirchhoff’s 1st law) • Any dense object with T > zero K will emit a continuum of thermal radiation (sometimes called blackbody radiation) © Sierra College Astronomy Department

21. Light and MatterThe Colors of Planets and Stars Color as a Measure of Temperature • An intensity/wavelength graph, a thermal spectrum, of an object emitting electromagnetic radiation can be used to determine its temperature. • Therefore, the color of a star tells us about its surface temperature. In Cosmic Calculations: • A quantitative derivation is given by Wien’s Law: lmax= 2,900,000/T or T = 2,900,000/lmax where T is the temperature in Kelvin and lm is the wavelength where the thermal spectrum peaks in intensity in nanometers (nm) © Sierra College Astronomy Department

22. Light and MatterThe Colors of Planets and Stars Intensity per square meter • How much thermal energy is being emitted (per square meter) from an object with at temperature T? (see Cosmic Calculations 5.1) s = Stefan-Boltzmann constant © Sierra College Astronomy Department

23. Light and MatterThe Doppler Effect • Doppler effect is the observed change in wavelength from a source moving toward or away from the observer. • It is most well known as the change in pitch of sound waves when a speeding car or train blowing its horn passes by. • In front of the moving source one hears higher frequency (shorter wavelength) sound. • Behind the source one hears lower frequency (longer wavelength) sound. © Sierra College Astronomy Department

24. Light and MatterThe Doppler Effect • Redshift is the change in wavelength toward longer wavelengths. • Blueshift is the change in wavelength toward shorter wavelengths. • Except for very distant galaxies, most redshifts or blueshifts caused by the Doppler effect are very small. • It is spectral lines in stellar spectra that make the Doppler effect a powerful tool. © Sierra College Astronomy Department

25. Light and MatterThe Doppler Effect Doppler Effect: Measurement Technique • Measuring the amount of the shifting of stellar spectral lines can determine the radial velocity of the star relative to the Earth. • Radial velocity is velocity along the line of sight, toward or away from the observer. • Tangential velocity is velocity perpendicular to the line of sight. vt vr v To Earth © Sierra College Astronomy Department

26. Light and MatterThe Doppler Effect Dl = The difference or shift between the observed wavelength and the wavelength seen if there were no motion l0 is the wavelength emitted by the object l is the wavelength we observe vr is the radial speed of the emitting object c is the speed of light Note: A negative v means the distance between the source and observer is decreasing. A positive v means the distance is increasing. © Sierra College Astronomy Department

27. Light and MatterThe Doppler Effect Other Doppler Effect Measurements • The rotation rate of the Sun • Rotation rates of the planets and the rings of Saturn • The motion of other stars with planets around them • Speeding cars by police radar © Sierra College Astronomy Department

28. Light and MatterThe Doppler Effect • Speed measured by the Doppler effect is the speed of the object relative to the speed of the Earth. • All speeds are relative to something. All motion (or non-motion) is relative, too. • The understanding of the relativity of motion is called Galilean relativity or Newtonian relativity. © Sierra College Astronomy Department

29. Light and MatterLight Intensity • The energy flux of the wave is the rate at which the wave carries energy through a given area • The electromagnetic flux (F) decreases with square of the distance from the source of the waves (inverse square law of radiation): • Force of gravity also follows an inverse square relationship with distance. © Sierra College Astronomy Department