Properties of Light

Particle Theory

• Light can behave as particles called photons

• Photons are little packets of energy, they have no mass, but their energy can be calculated using

• E = hf • h is Planck’s Constant 6.626E-34 J.s• Higher energy means higher frequency.

Newton’s Light Experiments

• Newton showed visiblelight can be separated intodifferent colors. He did not know these were wave-lengths.

Wave Theory of Light

• Each color corresponds to a different wavelength.

The catch for astronomers: only certain parts of the entire EMS can penetrate the earth’s atmosphere. The windows are visible and radio.

Stars are basically gravitationally bound balls of gas

• Since astronomers are no longer “stuck” on earth they can use the entire spectrum to figure out what chemical composition stars are made of.

• Fact: A very dense gas or a solid will give off a continuous spectrum that changes smoothly in brightness from one color to the next.

Spectra of a cluster of stars

Blackbody Radiation

• Blackbodies are perfect absorbers. They will absorb all incoming radiation and none is reflected.

• They are also perfect emitters of their own internal radiation.

• This depends only on temperature, not its chemical composition.

Wien’s Law

• Spectrum of a hot blackbody • peaks at a shorter• wavelength than that • of a colder blackbody.

• The product of temperature and the wavelength at which the spectrum peaks is a constant: (always the same)

lpeakT = 2.9E7 A.K

Stefan-Boltzmann Law

• Blackbody radiation property 2: per unit of surface area a hot blackbody emits much more energy per second that a cold blackbody.

• This energy is proportional to the fourth power of the temperature:

• E = sT4

• Temp in kelvins, = s 5.67E-8 W/m2.K4

• Energy in J/area

Just a note:

• Brightness of a blackbody (L) luminosity can be found by multiplying its surface area (S) by energy emitted E

• L = SE • Surface area = 4pR2

• Energy = sT4

• Then L = 4pR2sT4 and so if we know the brightness of any star and its temperature, then its radius can be found.

The bad news:

• Humans reflect visible light from the Sun or from room lamps so we are NOT perfect

blackbodies. But we do emit thermal (blackbody) radiation which is most intense at infrared wavelengths. So we can be found:

Reflection

• Angle of incidence: this means the first ray that hits the barrier such as a mirror will form and angle with the barrier.

• Angle of reflection: • the angle with which the second ray bounces

back.• In reflection both angles of incidence and

reflection are equal and made with the NORMAL.

Refraction

• The direction of light rays can be changed at the boundary of two media of different densities. (Ex: air to water)

• General rule:• Away from the perpendicular if medium 2 is less

dense than medium 1 • Toward the perpendicular if medium 2 is more dense

than medium 1 • Such effects form the basis of the refracting telescope,

and of optical devices using lenses in general.

Diffraction

• The capability of light to bend around corners. • For astronomersdiffraction

Diffraction can give spectrums.

• A diffraction grating can be used to separate light into its constituent colors, and that diffractive effects set an absolute limit on the quality of an image observed through an optical instrument such as a telescope. This diffractive limit occurs because the lenses of such objects are of finite size and diffract light because they cut off part of the light wave.

Sun spectrum

Fraunhofer Lines

• The dark linesare from coolergas absorbing radiation behinda star such as the sun.By seeing what wavelengths are absorbed will tell what elements are in the gas.

Absorption and Emission Spectra

• https://www.khanacademy.org/partner-content/nasa/measuringuniverse/spectroscopy/a/absorptionemission-lines

• http://www.livescience.com/41548-spectroscopy-science-fair-project.html

Doppler Effect and OrbitsLight also can be described as a wave, and relative motion of the source of light waves leads to a corresponding Doppler effect for light. In this case it is not the pitch but the color (that is, the wavelength) that is shifted by the motion of the source. The wavelength is shifted to larger values if the motion of the source is away from the observer and to smaller values if the motion is toward the observer. The shift to larger wavelengths by motion away from the observer is called a red shift by astronomers.A shift to shorter wavelengths caused by motion toward the observer is called a blue shift. The terminology is borrowed from the visible part of the spectrum where blue is toward the short wavelength end and red is toward the long wavelength end, but the Doppler effect occurs for all wavelengths of light, not just the visible spectrum.