Light

Review Question What is light?

Review Question How can I create light with a magnet?

Review Question How can I create light with some electrons?

Synchrotron emission Jupiter

Review Question Why do all objects emit radiation?

Review Question What are the three laws of blackbody or thermal radiation?

Blackbody emission

Blackbody/Thermal Spectra

Sunspots are cooler regions of the Sun’s photosphere

Line emission

Discussion The Sun emits radiation like a blackbody. However it has an atmosphere of cooler gas surrounding it. What type of spectrum would you expect the Sun to have?

Solar Spectrum

Iron Absorption lines in the Solar Spectrum

Discussion Explain the difference between how thermal or blackbody radiation is created and how line emission spectrum is created.

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?

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

Radar measurement of Mercury’s rotation

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

Resolution The ability to separate two closely spaced objects, such as a double star.

Resolution

Resolution and the atmosphere In general, atmospheric turbulence limits the resolution of even the largest telescopes on Earth to that of a 6 inch telescope.

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

Bad seeing Good seeing

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

Hubble Space Telescope

Keck primary mirror

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

Adaptive optics turned off

Adaptive optics turned on

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

Atmospheric absorption