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 light2. To resolve fine detail3. 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)

Refraction (Marching Band Analogy)

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