Maxwell showed in the mid-1800's that light is energy carried in the form of opposite but supporting electric and magnetic fields in the shape of waves, i.e. self-propagating electromagnetic waves or electromagnetic radiation.

Optics:

Due to its wave-like nature, light has three properties when encountering, or passing through, a medium:

1) reflection
2) refraction
3) diffraction

When a light ray strikes a medium, such as oil or water, the ray is both refracted and reflected as shown below:

Diffraction is the constructive and destructive interference of two beams of light that results in a wave-like pattern. When two peaks of a wave intersect they combine to make a bigger wave. When a peak and a trough intersect, they cancel.

Doppler effect:

The Doppler effect occurs when an object that is emitting light is in motion with respect to the observer. If the object is moving towards the observer the light is ``compressed'', meaning that the wavelength of the light becomes smaller. Smaller wavelength means bluer light, so we say the object is blueshifted. If the object is moving away from the observer the light is ``expanded'', the wavelength is increased or redshifted.

Notice that the speed of light does not change, only the wavelength. It is a basic premise of the theory of relativity that the velocity of light never changes regardless of the motion of the observer.

Inverse Square Law:

The brightness of an object varies inversely as the square of the distance. This means that objects farther away are dimmer.

But notice that the dimming does not progress in a linear fashion (i.e. 1, 2, 3, 4 ...) but rather in an inverse square (i.e. 1/2, 1/4, 1/8, 1/16 ...).

Observational Astronomy:

Astronomy is a passive science, but observing phenomenon at different wavelengths has several advantages to overcome the lack of making experiments. There are different physics at different wavelengths, for example, high energy magnetic fields are seen in the x-ray, radiation from heat is seen in the infrared.

Atomic Theory:

The ancient philosopher, Heraclitus, maintained that everything is in a state of flux. Nothing escapes change of some sort (it is impossible to step into the same river). On the other hand, Parmenides argued that everything is what it is, so that it cannot become what is not (change is impossible because a substance would have to transition through nothing to become something else, which is a logical contradiction). Thus, change is incompatible with being so that only the permanent aspects of the Universe could be considered real.

An ingenious escape was proposed in the fifth century B.C. by Democritus. He hypothesized that all matter is composed of tiny indestructible units, called atoms. The atoms themselves remain unchanged, but move about in space to combine in various ways to form all macroscopic objects. Early atomic theory stated that the characteristics of an object are determined by the shape of its atoms. So, for example, sweet things are made of smooth atoms, bitter things are made of sharp atoms.

In this manner permanence and flux are reconciled and the field of atomic physics was born. Although Democritus' ideas were to solve a philosophical dilemma, the fact that there is some underlying, elemental substance to the Universe is a primary driver in modern physics, the search for the ultimate subatomic particle.

It was John Dalton, in the early 1800's, who determined that each chemical element is composed of a unique type of atom, and that the atoms differed by their masses. He devised a system of chemical symbols and, having ascertained the relative weights of atoms, arranged them into a table. In addition, he formulated the theory that a chemical combination of different elements occurs in simple numerical ratios by weight, which led to the development of the laws of definite and multiple proportions.

He then determined that compounds are made of molecules, and that molecules are composed of atoms in definite proportions. Thus, atoms determine the composition of matter, and compounds can be broken down into their individual elements.

The first estimates for the sizes of atoms and the number of atoms per unit volume where made by Joesph Loschmidt in 1865. Using the ideas of kinetic theory, the idea that the properties of a gas are due to the motion of the atoms that compose it, Loschmidt calculated the mean free path of an atom based on diffusion rates. His result was that there are 6.022x10²³ atoms per 12 grams of carbon. And that the typical diameters of an atom is 10^-8 centimeters.

Matter:

Matter exists in four states: solid, liquid, gas and plasma. Plasmas are only found in the coronae and cores of stars. The state of matter is determined by the strength of the bonds between the atoms that makes up matter. Thus, is proportional to the temperature or the amount of energy contained by the matter.

The change from one state of matter to another is called a phase transition. For example, ice (solid water) converts (melts) into liquid water as energy is added. Continue adding energy and the water boils to steam (gaseous water) then, at several million degrees, breaks down into its component atoms.

Atomic theory is the field of physics that describes the characteristics and properties of atoms that make up matter. The key point to note about atomic theory is the relationship between the macroscopic world (us) and the microscopic world of atoms. For example, the macroscopic world deals with concepts such as temperature and pressure to describe matter. The microscopic world of atomic theory deals with the kinetic motion of atoms to explain macroscopic quantities.

Temperature is explained in atomic theory as the motion of the atoms (faster = hotter). Pressure is explained as the momentum transfer of those moving atoms on the walls of the container (faster atoms = higher temperature = more momentum/hits = higher pressure).

Ideal Gas Law:

Macroscopic properties of matter are governed by the Ideal Gas Law of chemistry.

An ideal gas is a gas that conforms, in physical behavior, to a particular, idealized relation between pressure, volume, and temperature. The ideal gas law is a generalization containing both Boyle's law and Charles's law as special cases and states that for a specified quantity of gas, the product of the volume, V, and pressure, P, is proportional to the absolute temperature T; i.e., in equation form, PV = kT, in which k is a constant. Such a relation for a substance is called its equation of state and is sufficient to describe its gross behavior.

Although no gas is perfectly described by the above laws, the behavior of real gases is described quite closely by the ideal gas law at sufficiently high temperatures and low pressures (such as air pressure at sea level), when relatively large distances between molecules and their high speeds overcome any interaction. A gas does not obey the equation when conditions are such that the gas, or any of the component gases in a mixture, is near its triple point (see below).

The ideal gas law can be derived from the kinetic theory of gases and relies on the assumptions that (1) the gas consists of a large number of molecules, which are in random motion and obey Newton's deterministic laws of motion; (2) the volume of the molecules is negligibly small compared to the volume occupied by the gas; and (3) no forces act on the molecules except during elastic collisions of negligible duration.

While all the above conditions are not strictly true, (where they breakdown interesting things happen - such as friction) in general the behavior of matter is well described by this kinetic theory. Temperature is explained by atomic theory as the motion of the atoms (faster = hotter). Pressure is explained as the momentum transfer of those moving atoms on the walls of the container (faster atoms = higher temperature = more momentum/hits = higher pressure).

Thermodynamics:

The study of the relationship between heat, work, temperature, and energy, now encompassing the general behavior of physical system is called thermodynamics.

The first law of thermodynamics is often called the law of the conservation of energy (actually mass-energy) because it says, in effect, that, when a system undergoes a process, the sum of all the energy transferred across the system boundary--either as heat or as work--is equal to the net change in the energy of the system.

The measure of entropy must be global. For example, you can pump heat out of a refrigerator (to make ice cubes), but the heat is placed in the house and the entropy of the house increases, even though the local entropy of the ice cube tray decreases.

In a closed system, entropy never decreases. In open systems, entropy can decrease in local regions (e.g., the ice tray), but an increase in order in the open system is always paid for by a decrease in order (decrease in entropy) somewhere else (e.g., the outside room). In the growth of crystals, for example, the ordered arrangement of ions in a lattice produces heat which flows away to the nearby environment.

Classical or Newtonian physics is incomplete because it does not include irreversible processes associated with the increase of entropy. The entropy of the whole Universe always increased with time. We are simply a local spot of low entropy and our destiny is linked to the unstoppable increase of disorder in our world => stars will burn out, civilizations will die from lack of power.

The approach to equilibrium is therefore an irreversible process. The tendency toward equilibrium is so fundamental to physics that the second law is probably the most universal regulator of natural activity known to science.

The concept of temperature enters into thermodynamics as a precise mathematical quantity that relates heat to entropy. The interplay of these three quantities is further constrained by the third law of thermodynamics, which deals with the absolute zero of temperature and its theoretical unattainability.

Absolute zero (approximately -273 C) would correspond to a condition in which a system had achieved its lowest energy state. The third law states that, as this minimum temperature is approached, the further extraction of energy becomes more and more difficult.