5.1 Basic Properties of Light and Matter
- Light is an electromagnetic wave. It is a form of energy composed of many different colors that combine to form white light.
- Wavelength: the difference between adjacent crests.
- Amplitude: half the distance in height between a crest and a trough
- Frequency: the number of crests that pass through a point each second
- wavelength x frequency= the speed of light= constant
- Particles of light are called photons
- Each photon has a wavelength and a frequency
- The energy of a photon depends on its frequency
- Lambda x f= c
- Lambda= wavelength
- F= frequency
- c= 3.00 x 10^8 m/s = speed of light
- E= h x f= photon energy
- Atomic number= # of protons
- Atomic mass number= # of protons + neutrons
- Isotope= same number of protons, different number of neutrons
- Molecule= compound of two or more atoms
- Emission
- Absorption
- Transmission: Transparent objects transmit light, opaque objects absorb light
- Reflection or scattering
- Interactions between light and matter determine the appearance of everything around us.
5.2 Learning from Light
- Spectra of astrophysical objects are usually combinations these three basic types: Emission line spectrum, Continuous Spectrum, and Absorption Line Spectrum
- Continuous: The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.
- Emission Line: A thin or low density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.
- Absorption Line: A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.
- Each type of atom has a unique set of energy levels
- Each transition corresponds to a unique photon energy, frequency, and wavelength
- Downward transitions produce a unique pattern of emission lines
- Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths
- Each type of atom has a unique spectral fingerprint
- Observing the fingerprints in a spectrum tells us which kinds or atoms are present.
- Nearly all large or dense objects emit thermal radiation, including stars, planets, and even you
- An objects thermal radiation spectrum depends on one factor, its temperature
- Hotter objects emit more light at all frequencies per unit area
- Hotter objects emit photons with a higher average energy
- Generally measure the Doppler Effect from shifts in the wavelengths of spectral lines
- Blue shift: objects moving towards
- Red shift: Objects moving away
- Effects tell us how fast an object is moving towards or away from us.
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