Exam 1 Next Tuesday

Covers chapters 1, 2, S1, 4, 5 and 6

2 essay questions/25 multiple choice

Allowed one standard sized sheet of notes, writing on one side only.

• Explain how Newton came up with his law of gravity

• Explain tidal forces

• Explain what light is, how it is created

• Explain why wave speed, frequency, wavelength and energy are related

• Explain the properties of thermal/blackbody radiation

• Explain line absorption/emission from atoms and molecules

Discussion

What is light?

Discussion

How do we make electromagnetic waves?

Thermal Spectra

Continuous, Absorption and Emission Spectra

Solar Spectrum

Discussion

What kind of spectrum do we get from molecules? Why?

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?

Difference between a Mirror and Scattered Reflection

Atmospheric scattering

Molecules deflect or scatter light. Scattering is more effective for light that is closer in size to the molecule doing the scattering.

Discussion

Why is the sky blue, and why does the setting Sun appear red?

Red 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?

Red/blueshift

Redshift – if we are moving away from a light source, the frequency of spectral lines will be shifted slightly lower, toward the red end of the spectrum.

Blueshift – if we are moving toward a light source, the frequency of spectral lines will be shifted slightly higher, toward the blue end of the spectrum.

Important points

1. The light does not necessarily 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.

Doppler shifted spectra

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 across the sky.

Solar spectrum

Iron spectrum

Radial velocity

By measuring the wavelength of spectral lines in a star or galaxy and comparing it to the lines produced by lamps, we can determine the velocity of stars and galaxies either toward or away from us.

Proper motion

The “proper” motion of a star is the motion relative to other stars in the plane of the sky. In general one has to wait many years to determine the proper motion of even the closest stars. We can’t measure the proper motion of galaxies, because they are too far away.

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 to astronomers 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)

Discussion

If the marching band hits the slow part perpendicular, what happens to the distance between the rows?

Chromatic aberration

Reflectors

Mt Palomar observing cage

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.

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.

“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?

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?

Atmospheric absorption

• Explain how continuous spectrum and line spectra are created, physically and practically

• explain scattering and reflection

• explain the Doppler effect

• explain how lenses bend light

• how astronomers use telescopes and the effects of the atmosphere