Light

Review Question What is light?

Review Question How can I create light with a cow magnet?

Review Question What is blackbody or thermal radiation?

Review Question What are the three properties of thermal/blackbody emission?

Blackbody emission

Review Question How does thermal emission differ from line emission?

Discussion The Sun and stars emit radiation like a blackbody. However being made of gas at high temperatures, they have atmospheres of cooler gas surrounding them. What type of spectrum would you expect stars to have?

Solar Spectrum

Iron Absorption lines in the Solar Spectrum

Molecular emission and absorption Photons can interact with molecules as well as electrons, causing them to vibrate and/or rotate. Most of these molecular emission/absorption bands are lower energy and are thus in the infrared or radio part of the spectrum.

Discussion How does a microwave heat you food?

Review Question What causes the colors of objects in this room?

Colors of objects All objects both absorb and reflect some of the light that hits them. If an object absorbs red and reflects blue it will appear blue, the color of the object being the color that is least absorbed and best reflected.

Spectrum from Mars

Difference between a Mirror and Scattered Reflection

Discussion A mirror consists of a glass substrate onto which a thin layer of metal is deposited. Why do you think metals are good reflectors of light?

Atmospheric scattering Nitrogen molecules in Earth’s atmosphere scatter blue light more effectively than red, so the sky appears blue.

Discussion Why does the sun appear reddish at sunrise or sunset?

Scattering and reddening

Pleiades reflection nebula

Trifid nebula (M20) in Sagittarius

Doppler Effect The frequency of electromagnet radiation depends on the relative motion of the observer and the source.

Water Waves

Discussion If the leaf is moving toward the source of the water waves, will the frequency be greater or less than when the leaf was still?

Doppler Shift with Light If you are moving toward the source, light is shifted to higher frequencies, or toward the blue end of the spectrum. Astronomers call is a blueshift. Likewise if you are moving away from a source, the light is shifted toward the red end of the spectrum and is called a redshift.

Doppler shifted spectra

Important points 1.The light need not appear red or blue. You can have a red spectral line which is blueshifted, or a blue line which is redshifted. 2.The Doppler effect depends on your relative velocity toward or away from an object, not on your distance from it.

Important points 3. The Doppler effect can only measure the radial velocity, the velocity toward or away from a source. It does not depend on the velocity of the object across the plane of the sky.

Telescopes

Discussion If you had a Coke bottle and you wished to fill it with rainwater falling from the sky, how would you do it?

Telescopes Serve 3 Functions 1.To collect light 2.To resolve fine detail 3.To magnify the image

Discussion Of the three telescope functions which do you think is the most important in astronomy and why?

Refractor

Refraction (Marching Band Analogy) What happens to the marching band if it hits a muddy part which causes the individual band members to slow?

Refraction (Marching Band Analogy)

Speed of light in glass Light moves slower in glass than in a vacuum Speed of light through glass is different for different frequencies and each frequency has a slightly different focus.

Chromatic aberration

Discussion A mirror consists of a glass substrate onto which a thin layer of metal is deposited. Why do you think metals are good reflectors of light?

Mirrors

Reflectors

Mt Palomar observing cage

Resolution The ability to separate two closely spaced objects, such as a double star. The bigger your telescope objective the better your resolution will be.

Resolution

Alcor and Mizar

Resolution and the atmosphere Atmospheric turbulence limits the resolution of even the largest telescopes on Earth to about that of a 6 inch telescope.

Points of light Stars are so far away that they appear as points of light no matter how big a telescope you observe them with. Bigger stars in photographs are simply brighter.

“Seeing” The apparent size of the stars as viewed through the telescope. Good seeing is typically 1 arcsec.

Atmospheric distortion

Seeing

Atmospheric distortion

Discussion What can astronomers do to try and improve resolution of their observations?

Hubble Space Telescope

Mauna Kea

Keck primary mirror

Adaptive optics Actuators deform the primary mirror hundreds of times a second to try and remove atmospheric distortion.

Discussion Adaptive optic telescopes work best in the infrared region of the spectrum and not in the visible. Why do you think that is?

Discussion What do you think happens when very high energy gamma rays from space hit the Earth’s atmosphere?

Discussion Why can’t the very long wavelength radio waves make it to Earth’s surface?

Atmospheric absorption