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The Nature of Light Electromagnetic Waves and the Electromagnetic Spectrum What is Light? Electric charges repel or attract each other. + + + This attraction / repulsion travels at the speed of light. If you move, say, an electron the change in force takes time to travel into the Universe. This change in the force field is an electromagnetic wave. — — — + — Light: Waves in What? A water wave is a disturbance in water’s surface. A sound wave is a variation in the air pressure. Light is fundamentally different. Light doesn’t need a substance to travel through. Light can travel thru the vacuum of space! Wave Properties The Range of Visible Light Visible light comes in many colors. Each color has a particular wavelength and frequency. Red: longest wavelength & lowest frequency Violet: shortest wavelength & highest frequency The Range of Visible Light The human eye is most sensitive to green light, with a wavelength of about 500 nanometers. (1 nanometer = 1 nm = 1 billionth of a meter.) The reddest light has a wavelength near 700 nm. The most violet light has a wavelength near 350 nm. The eye is sensitive to a very narrow range of wavelengths! The unit of frequency on this diagram is the Hertz. 1 Hertz = 1 Hz = 1 wave per second. The unit of wavelength is the meter, but the range spans from thousands of meters (km) to trillionths of a meter (pm). Electromagnetic Waves The speed of a wave is related to the wavelength and the frequency: speed = wavelength × frequency The speed of EM waves is a constant, so frequency and wavelength are inversely related: • High frequency waves have long wavelengths. • Low frequency waves have short wavelengths. The Speed of Light The speed of light (often abbreviated as “c”): c = 300,000 kilometers per second This is the same speed for all EM waves: Ø radio waves have the same speed as visible light Ø gamma rays travel at the same speed as x-rays Ø ultraviolet & infrared light have the same speed The Energy of Light Light carries energy. The amount of energy carried by a single “packet” of light depends only on the wave’s frequency. The energy (E) is related to frequency (f): E = h× f Higher frequency means more energy per wave. “h” is a tiny quantity known as Planck’s constant. Individual light waves carry a tiny bit of energy. Which of the following is not a form of light? A. radio waves B. x-rays C. ultraviolet light D. All of the above are forms of light. E. None of the above are forms of light. Which has the shortest wavelength? A. radio waves B. x-rays C. ultraviolet light D. blue light Which has the lowest frequency? A. radio waves B. x-rays C. ultraviolet light D. blue light Which carries the highest energy? A. radio waves B. x-rays C. ultraviolet light D. blue light Which travels at the highest speed? A. radio waves B. x-rays C. ultraviolet light D. All travel at the same speed. Temperature Scales In the U.S. and Trinidad & Tobago, the common temperature scale is the Fahrenheit scale: Water freezes at 32o F and boils at 212o F. The rest of the world uses the Celsius scale: Water freezes at 0o C and boils at 100o C. In astronomy, we discuss temperatures far beyond the range of everyday experience. Kelvin Temperature Scale • No negative Kelvin temperatures • No maximum Kelvin temperature • Room temperature is about 300 Kelvin Cores of stars (hotter than 10 million Kelvin) yikes! Surface temperatures of stars (3000 – 50,000 Kelvin) Dark interstellar gas cloud (10 - 100 Kelvin) Overall temperature of Universe 2.73 Kelvin Absolute zero = zero Kelvin (coldest possible temperature) brrrr! Thermal (or Blackbody) Spectrum A hot, dense object will emit a continuous range of colors known as a thermal spectrum (just like the hot filament of the light bulb). This type of spectrum is also called a blackbody. Intensity short Wavelength long Blackbody Temperature The height and wavelength of the peak shift as the temperature increases: brighter and bluer… Blackbody Temperature The frequency or wavelength of the peak of the blackbody curve depends on the temperature of the emitting object. The higher the temperature, the higher the energy and frequency of the peak. This means that the wavelength of the peak gets shorter. By observing an object’s thermal spectrum, we can determine the temperature of the object. A 15,000 K star is brightest in which part of the electromagnetic spectrum? A. radio B. infrared C. visible light D. ultraviolet What is the visual color of a star like the Sun? A. Greenish B. Bluish C. White-ish D. Reddish Which object is hotter? A. Object A B. Object B Which of these objects is largest in size? (Think back to luminosity vs. temperature vs. size) A. Object A B. Object B C. Object C D. Object D If an astronomer wanted to find the temperature of a distant object, which feature of the objectʼs spectrum should be examined?# A. which spectral lines are present# B. the total intensity of the objectʼs spectrum# C. the wavelength of the peak of the spectrum# Atomic Structure You will often see an atom drawn with electrons orbiting the nucleus like planets: However, electrons do not move on fixed orbits… Instead the electrons occupy an electron cloud. The electron orbits do have unique energy values. A Simple Spectrum Imagine a toy hydrogen atom with 1 proton and 1 electron. The electron is only allowed to have two energy values… let’s say 1 unit and 3 units. upper energy level = 3 units electron proton lower energy level = 1 unit A jump by an electron from one energy level to the other creates or destroys a photon, a single “packet” of light energy. Photons can have different amounts of energy, depending on the size of the electron’s jump: 2 energy units added to atom by photon (light) photon absorbed 2 energy units carried away by photon (light) photon emitted A Simple Spectrum In the case of our toy 1-electron atom, how much energy does the electron need to gain to jump up? E = 3 units E = 1 unit electron proton A Simple Spectrum How much energy does the electron lose when it jumps down? E = 3 units E = 1 unit proton electron If we observe a cloud of these “toy” 2-level atoms, we would see it emit light of just a single energy. (only one wavelength, frequency, and color, too.) The emission spectrum of this toy 2-level atom is a single bright line, called an emission line. The line’s energy/wavelength/frequency/color relates to the difference in energy between the two electron energy levels. Energy Levels of Hydrogen Real electrons in real hydrogen atoms have a very large number of energy levels. 13.6 eV Jumps to energy level 1 are in the ultraviolet. Jumps to energy level 2 are visible to the human eye. Jumps to energy level 3 = ? ? visible ultraviolet The electron-volt (eV) is the commonly used unit of energy for electrons and atoms. 12.8 eV 12.1 eV 10.2 eV 0 eV Energy Levels of Hydrogen What process occurs if the electron jumps up? è Absorption 13.6 eV What happens if the electron gains enough energy and jumps past the highest energy level? 12.8 eV 12.1 eV 10.2 eV è Ionization 0 eV An ionized atom (an ion) has lost an electron. Free electrons may later recombine with an ion. The electrons in an atom can be excited in a variety of ways: • absorption of light energy (radiative energy) • collision with fast-moving atoms (kinetic energy) • collision with free-floating electrons (kinetic) Excited electrons drop to lower energy levels and release the energy as light. A spectrum with bright lines on a dark background is known as an emission spectrum. Absorption of light energy is the exact opposite of emission of light energy. Emission lines and absorption lines match exactly. The light absorbed by the atoms is missing from the spectrum. An absorption spectrum appears as dark lines against a continuous background. hydrogen Clues in the Light The spectrum of a chemical element is unique and acts like a fingerprint. The spectral lines in the light from an object tell us the composition of the object! Here are the spectra of 3 different gases: Thermal spectrum: hot, dense matter Emission spectrum: hot, low-density gas Absorption spectrum: a thermal spectrum passing thru cooler cloud of gas Which type of spectrum would be emitted by a glowing gas cloud heated by a nearby O star?# O star# A. A continuous spectrum# B. An absorption spectrum# C. An emission spectrum# The spectrum of typical emission nebula# A normal star has a very hot core surrounded by a cooler atmosphere.# # Normal stars emit what kind of spectrum?# A. Emission# B. Absorption# C. Continuous# The spectrum of Procyon (A-type dwarf star)# The spectrum of the Sun# The spectrum of Arcturus (K-type giant star)# Doppler Shift: Observer and Source So far the light source and observer have been considered stationary relative to one another. If the source is approaching the observer (or vice versa), light waves will arrive more often. The light waves appear to be compressed. What happens to the apparent color of the light? If the source is receding from the observer (or vice versa), light waves will arrive less often. The light waves appear to be stretched out. What happens to the apparent color of the light? Doppler: a Subtle Change in Color The motion of an object changes the color of light emitted by the object. Approaching, light waves are compressed and blue-shifted. Receding, the light waves are stretched and red-shifted. This effect only depends on motion, not distance. It is visible for nearby planets or distant galaxies. You observe the spectrum of a distant unknown object. You recognize a spectral line of helium at a wavelength of 700 nm. In the lab (at rest), this line has a wavelength of 512 nm.# # What can you say about the objectʼs motion?# A. It must be moving toward us.# B. It must be moving away from us.# C. It must be moving sideways relative to our view.# D. It cannot be moving at all relative to us.#