Creating LightCreating Light

Light as a WaveLight (or electromagnetic radiation), can be thought of as either a particle or a wave. As a wave, light has

• a wavelength, (distance between waves)

• a frequency, (number of waves passing you each second)

• a speed, c = (this is always the same: 300,000 km/s)

• an energy, E = h (where h is just a constant)

Note that because the speed of light is a constant, , , and E are linked: if you know one, you know the other two.

Light as a Particle

Light can also behave as particle. Each packet of light is called a photon, and each photon carries a specific amount of energy (associated with the photon’s wavelength or frequency).

Photons emitted from a source will spread out in all directions at the speed of light. Since the amount of area surrounding a source increases as the distance squared, the density of photons will decrease as 1 / r2. This is the inverse square law of light.

Ways of Creating Light

There are 3 ways to produce light. • Through the blackbody process (a.k.a. thermal

emission)• Through line emission• Through synchrotron emission

(This last way is only for a few peculiar objects with strong magnetic fields. We will be ignoring this mechanism in this class.)

The Blackbody ProcessAnything that is hot (i.e., above absolute zero) produces light at all wavelengths – a continuous spectrum. But the amount of light given off at each wavelength is very sensitive to the object’s temperature. Specifically,

The hotter the object:

• the more high-energy photons are created

• the more light is created (MUCH more)

L T4

The Blackbody Temperatures

Temperatures

Sun: 6000° (optical)

People: 300° (IR)

Rigel: 44,000° (UV)

1,000,000° gas: x-ray

(all temperatures are in Kelvin)

The Bohr Model of the Atom

In the Bohr model of the atom, the nucleus contains protons and neutrons. Circling around the nucleus (in orbitals) are electrons. Since electrons are attracted to the protons, they normally orbit in their lowest energy state (i.e., closest to the nucleus).

Important: electrons can only orbit at very specific distances from the nucleus.

The exact positions of the orbitals are different for every element.

Creating an Emission Line

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level. Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off light.

Since each orbital has a very specific level, electron transitions between the orbitals emit very specific amounts of energy. The spectrum from this process would not be continuous.

Emission and Absorption

An electron does not have to be collided up to the higher orbital. If a photon comes by with an energy that is exactly equal to the difference between the two levels, the electron can absorb the photon, and use the energy to jump to the higher level.

The electron would, of course, later fall back down and re-emit the photon. (But not necessarily in the same direction.)

Ionization and Emission

Suppose a very high energy photon passes near an atom. If the photon has enough energy, it can kick the electron completely out of the atom, and create an ion. Eventually, the electron will recombine into (some level of) the ion, and cascade its way down to the lowest energy level. Each downward transition will produce a photon with the exact energy of the transition. This is not a continuous spectrum!

There is light only at very specific energies (i.e., colors), which correspond to each transition.

Emission Line SpectraSince every element has a different set of atomic orbital energies, each element creates a different emission line spectrum. They are as unique as fingerprints!

Absorption Line Spectra

An object (like a star) emits a hot blackbody spectrum. Somewhere between you and the star (like on the outside of the star) is some cooler gas. That gas can absorb the photons which correspond to the atom’s energy levels. The result is an absorption spectrum.

You observe the blackbody spectrum minus the energy that has been absorbed by the gas. (These photons have been re-emitted, but in other directions).

Emission versus AbsorptionAtomic emission and absorption are really two sides of the same coin. Photons that are absorbed can be re-emitted to produce an emission line spectrum.

Emission versus AbsorptionAtomic emission and absorption are really two sides of the same coin. Photons that are absorbed can be re-emitted to produce an emission line spectrum.

Telescopes Telescopes

Types of Telescopes

The purpose of a telescope is to collect as much light as possible and focus it into a small area. This can be done in 2 ways

• By using a lens to bend, or refract the light

• By using a mirror to reflect the light

Telescopes are characterized by their collecting area: a 36-in telescope has a lens (or mirror) that is 36-inches in diameter.

Atmospheric Seeing

The focal length of a telescope is the distance from its lens (or mirror) to the focus. In general, the larger the focal length, the larger the magnification. However, the atmosphere of the Earth blurs out all images. (This “barbeque on a hot day” effect is called “seeing”.)

Since the best seeing on Earth is slightly less than 1 arcsec, there is no point in trying to magnify any more than this.

Refracting telescopes use a lens to bring the light to a focus. Light passes through the primary to the eyepiece.

Refracting Telescopes

Refracting TelescopesRefracting telescopes are usually small, because

• It is difficult to physically support a big lens

• Light going through glass only gets bent a little, so large refractors have very long focal lengths. The result is very high magnifications, and physically large structures.

• When light passes through glass, the blue light gets bent more than red light. So the red-light focus of a large refractor is at a different place from the blue-light focus. This is chromatic aberration.

Reflecting TelescopesReflecting telescopes use mirrors to collect the light. They can be REALLY BIG, but the focus is high in the air. To solve this problem, one can place smaller mirrors (secondaries) in the beam.

To use a reflecting telescope, one could:

• Observe at prime focus

• Place a small pick-off mirror near the top, and observe off to the side (Newtonian)

• Reflect the light back down through a hole in the primary (Cassegrain)

• Make multiple reflections and observe somewhere comfortable (Coudé)

Reflecting TelescopesMirrors are not 100% reflective, so every reflection loses some light. Ideally, one would like to observe at prime focus, but for practical reasons, Cassegrain and Newtonian focii are often used.

Where to Put a Telescope

For an optical observatory, you need

• Dark Skies

• Stable air (for good seeing)

• Good weather

• High altitude (to get above as much of the atmosphere as possible). This is especially important if you also want to use the telescopes to observe in the infrared.

For a radio observatory, you need

• An area far from cell phones and TV/radio stations

Observatories

Mauna Kea ESO

Kitt Peak VLA

McDonald

For gamma-rays, x-rays and the ultraviolet, one has to observe from space. For the radio and optical, the ground is fine (except that the atmosphere blurs out optical light).

The infrared is tricky -- there are some “atmospheric windows”, where observations are possible. (Most things on Earth also glow in the IR.) But in general, the higher up you are, the better.

Atmospheric Windows

Space Observatories

Hubble Chandra

WMAPSpitzerGalex

Refracting Telescopes

Reflecting Telescopes

Prime Focus Observations

Newtonian Focus Telescopes

Cassegrain Focus Telescopes

Coudé Focus Telescopes

Next time -- REVIEW