Light

Light is almost our only tool in the study of the universe. In order to make progress in this study, we must understand as much as possible about the workings of light.

Galileo performed the first experiments on the nature of light. He tried to measure how long it took light to traverse a carefully measured path. After performing the experiment many times, Galileo concluded that he was measuring the reaction time of his observers, not the speed of light.

We would have to wait another century before Ole Roemer found a way to measure the speed of light. He found that the orbital period of Io going around Jupiter depended on where the Earth was in its orbit. The reason for the difference, he concluded, was the time it took light to travel the diameter of the Earth's orbit. Light takes 17 minutes to travel the diameter of the Earth's orbit, or c = 300,000 km/s or 7½ times around the Earth's equator every second.

In the 20^th century Albert Einstein showed that the speed of light was an absolute constant of the universe. It does not vary with our speed of the speed of the source of light. This is outside of our everyday experience, but has been very well verified.

Even the fundamental nature of light has been in dispute. Isaac Newton treated light as particle phenomenon, while Christian Huygens thought acted like a wave. Newton's view stood unchallenged until a famous set of experiments conducted by Thomas Young early in the 19^th century. His experiments relied on the fundamental nature of waves called interference. Light shows interference effects. Particles do not show interference. Young further showed that light is a transverse wave, where the medium moves perpendicular to the direction that the wave is traveling. Treating the wave nature of light is usually the preferred method, although light does show both wave and particle phenomena.

The next several properties we want to discuss are wave properties. One of these is the wavelength, the distance from one part of the wave to the next, identical part of the wave. From shortest to longest the wavelengths along the electromagnetic spectrum are:

Gamma Rays - very high energy form with wavelengths comparable to the size of the atomic nucleus. The atmosphere of the Earth blocks these rays.

X-rays - still very energetic with wavelengths about the size of the atom, X-rays can penetrate soft tissue, but not bone. The atmosphere also effectively blocks X-radiation.

Ultraviolet - wavelengths are now about the size of a virus. The small amount of UV that comes through the atmosphere from the Sun is responsible for your sunburn. Literally the light destroys skin cells.

Visible - wavelengths that the eye/brain respond to and about the size of a bacterium. From short to long wavelength within this band are Violet, Blue, Green, Yellow, Orange, and Red. WAVELENGTH MEANS COLOR

Infrared - wavelengths range to several millimeters and cause atoms and molecules to vibrate and move faster. Since motion and temperature are closely related, infrared causes you to feel warm. Near the long wavelength limit is the microwave region. Long wavelength infrared is very effective in heating food by making the water molecules (roughly 80% of all we eat) move faster.

Radio - ranging from centimeters to kilometers in length. All radio light arrives at the surface of the Earth.

A second wave property that we need to discuss is frequency, the number of complete events of a periodic phenomenon that occur in a certain time. The frequency and the period are reciprocals of one another.

These three properties of waves, the speed, the wavelength, and the frequency are related speed = wavelength times frequency. For light we can write c = lf. Using this relationship we see that since gamma rays have the shortest wavelength, they must also have the highest frequency. Radio waves have very long wavelengths and very low frequency.

Max Planck discovered that amount of energy carried by a particle of light (a photon). He found E = hf, h being Planck's constant. So the gamma rays have the highest frequency and the highest energy. Knowing the order of the electromagnetic spectrum will help in ordering the frequencies and the energies for various forms of light.

Our atmosphere is completely opaque to most short wavelength radiation (gamma rays, X-rays, and most UV). The strong block in the UV is due to ozone. Ozone forms naturally in the upper atmosphere due to UV light from the Sun. In fact it is this absorption of the UV by ozone that protects the surface of the Earth from this harmful form of radiation. Humans have been depleting the ozone layer by the uncontrolled use of CFC's. Recently, a global ban of CFC's has been in place that will eventually cure the problem.

Visible light, of course, reach the surface. In the infrared most of the light is blocked by water molecules. IR astronomy can be done from ground base at the high mountaintop. The premier facility for IR astronomy today is on Mauna Kea in Hawaii. The radio spectrum is very transparent and was the second region opened up to astronomy after the visible region. Light at very long wavelengths is reflected off of the ionisphere.

The fundamental instrument for our astronomical discoveries is the spectrograph. Light from the telescope passes through the:

· Slit - isolates the target and forms the image

· Collimating lens - make the light rays into parallel rays

· Dispersing element - breaks the light into the component wavelengths.

· Recording system - the eye, camera, or electric imaging system

Nature provides only three fundamental kinds of spectra:

· Continuous (incandescent light bulb) - all colors available, source is hot and dense

· Emission (candle flame) - only a few wavelengths (colors) are present, source is hot and tenuous

· Absorption (star) - continuous background with overlying dark lines, source has a hot, dense central region with light passing through cooler, tenuous gas

In order to understand what produces these spectra, we must probe the structure of the atom. Ernst Rutherford conducted the first meaningful experiments on the structure of the atom in the first decades of the 20^th century. His approach was to fire helium nuclei at the atoms and measure how they scattered. The result of his experiments was that the atom contained a very small, hard, positively-charged nucleus surrounded by a swarm of electrons. Soon models were developed for the hydrogen atom. There is very good reason to study hydrogen first. It is the simplest atom, containing a single proton in the nucleus and a single electron surrounding it. Hydrogen is also the most abundant element in the universe, comprising about 9 out of every 10 atoms.

The "Planetary Model" for hydrogen had a fundamental flaw. If the electron is orbiting the proton in a circular orbit, it must be accelerating. Nineteenth century physics showed that accelerating charges must radiate light, which carries energy. The electron must be losing energy but the radius of its orbit depends on the amount of energy it possesses. The electron must be moving closer to the nucleus. In fact, it should crash onto the nucleus, destroying the hydrogen atom. According to classical physics as then understood, atoms cannot exist.

Niels Bohr provided the next advance by proposing that there are stable orbits in which electron can reside and not lose energy. Jumps (transitions) between stable orbits are allowed so long as the electron ends up with the energy of the new orbit. Closer to the nucleus requires less energy, farther away, more energy. Emission spectra are now easily explained. As the electron jumps from a high energy state to one of lower energy, light is emitted. But only light that carries the energy difference between the two stable orbits. The energy of a photon is related to its frequency (E = hf), frequency can be converted to wavelength (l = c/f), and wavelength means color. Thus only certain colors are emitted by the atom, depending on the arrangement of the stable orbits.

Returning to hydrogen, we now see many families of transitions. Those transitions that terminate on the second stable orbit are members of the Balmer series and are seen in visible wavelengths. When the terminating orbit is the first one or the ground state, more energy is involved in the transition, and the resulting photons have shorter wavelength. Lyman discovered them in the UV part of the spectrum. When orbit three is the terminating level, the energies of the photons are in the IR.

Each atom has a different number of electrons and therefore, a unique set of stable orbits. Transitions between these stable orbits produce wavelengths of light unique to each element. The emission spectrum of a chemical element is like a fingerprint for identifying that element in an unknown sample. From the work of Bohr and others, we can know the chemical compositions of the stars.