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Homework 4

Homework 4. Unit 21 Problem 17, 18, 19 Unit 23 Problem 9, 10, 13, 15, 17, 18, 19, 20. Units 21 and 22 are covered. As a wave… A small disturbance in an electric field creates a small magnetic field, which in turn creates a small electric field, and so on…

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Homework 4

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  1. Homework 4 Unit 21 Problem 17, 18, 19 Unit 23 Problem 9, 10, 13, 15, 17, 18, 19, 20 Units 21 and 22 are covered

  2. As a wave… A small disturbance in an electric field creates a small magnetic field, which in turn creates a small electric field, and so on… Light propagates itself “by its bootstraps!” Light waves can interfere with other light waves, canceling or amplifying them! The color of light is determined by its wavelength. As a particle… Particles of light (photons) travel through space. These photons have very specific energies. that is, light is quantized. Photons strike your eye (or other sensors) like a very small bullet, and are detected. The Nature of Light • Light is radiant energy. • Travels very fast – 300,000 km/sec! • Can be described either as a wave or as a particle traveling through space.

  3. This is an inverse-square law – the brightness decreases as the square of the distance (d) from the source The Effect of Distance on Light • Light from distant objects seems very dim • Why? Is it because the photons are losing energy? • No – the light is simply spreading out as it travels from its source to its destination • The farther from the source you are, the dimmer the light seems • We say that the object’s brightness, or amount of light received from a source, is decreasing

  4. The atom has a nucleus at its center containing protons and neutrons Outside of the nucleus, electrons whiz around in clouds called orbitals Electrons can also be described using wave or particle models Electron orbitals are quantized – that is, they exist only at very particular energies The lowest energy orbital is called the ground state, one electron wave long To move an electron from one orbital to the next higher one, a specific amount of energy must be added. Likewise, a specific amount of energy must be released for an electron to move to a lower orbital These are called electronic transitions The Nature of Matter

  5. Measuring Temperature • It is useful to think of temperature in a slightly different way than we are accustomed to • Temperature is a measure of the motion of atoms in an object • Objects with low temperatures have atoms that are not moving much • Objects with high temperatures have atoms that are moving around very rapidly • The Kelvin temperature scale was designed to reflect this • 0  K is absolute zero –the atoms in an object are not moving at all!

  6. Results of More Collisions • Additional collisions mean that more photons are emitted, so the object gets brighter • Additional hard collisions means that more photons of higher energy are emitted, so the object appears to shift in color from red, to orange, to yellow, and so on. • Of course we have a Law to describe this…

  7. Wien’s Law: Hotter bodies emit more strongly at shorter wavelengths SB Law: The luminosity of a hot body rises rapidly with temperature Wien’s Law and the Stefan-Boltzmann Law

  8. Taking the Temperature of Astronomical Objects • Wien’s Law lets us estimate the temperatures of stars easily and fairly accurately • We just need to measure the wavelength (max) at which the star emits the most photons • Then,

  9. The Stefan-Boltzmann Law • If we know an object’s temperature (T), we can calculate how much energy the object is emitting using the SB law •  is the Stefan-Boltzmann constant, and is equal to 5.6710-8 Watts/m2/K4 • The Sun puts out 64 million watts per square meter – lots of energy!

  10. If a photon of exactly the right energy (corresponding to the energy difference between orbitals) strikes an electron, that electron will absorb the photon and move into the next higher orbital The atom is now in an excited state If the photon is of higher or lower energies, it will not be absorbed – it will pass through as if the atom were not there. This process is called absorption If the electron gains enough energy to leave the atom entirely, we say the atom is now ionized, or is an ion. Absorption

  11. Emission • If an atom drops from one orbital to the next lower one, it must first emit a photon with the same amount of energy as the orbital energy difference. • This is called emission.

  12. Sometimes it is more convenient to talk about light in terms of frequency, or how fast successive crests pass by a given point You can think of frequency as a measure of how fast you bob up and down as the waves pass. Frequency has units of Hz (Hertz), and is denoted by the symbol  Long wavelength light has a low frequency, and short wavelength light has a high frequency Frequency and wavelength are related by: Frequency ‘c’ is the speed of light.

  13. The Electromagnetic Spectrum I • There is more to light than just the visible part of the spectrum • Radio waves are very long wavelength photons (not sound!) with wavelengths longer than a meter or so • Microwaves (yes, the ones we cook with) are at the upper end of the radio part of the spectrum • Infrared wavelengths are just longer in wavelength than the visible spectrum

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