Introduction to physics

Physics is the science of matter and its motion—the science that deals with concepts such as force, energy, mass, and charge. It is the general analysis of nature, conducted in order to understand how the world around us behaves.
In one form or another, physics is one of the oldest academic disciplines; through its modern subfield of astronomy, it may be the oldest of all. Sometimes synonymous with philosophy, chemistry and even certain branches of mathematics and biology during the last two millennia, physics emerged as a modern science in the 17th century and these disciplines are now generally distinct, although the boundaries remain difficult to define.
Advances in physics often translate to the technological sector, and sometimes influence the other sciences, as well as mathematics and philosophy. For example, advances in the understanding of electromagnetism have led to the widespread use of electrically driven devices (televisions, computers, home appliances etc.); advances in thermodynamics led to the development of motorized transport; and advances in mechanics led to the development of the calculus, quantum chemistry, and the use of instruments like the electron microscope in microbiology.
Today, physics is a broad and highly developed subject. Research is often divided into four subfields: condensed matter physics; atomic, molecular, and optical physics; high energy physics; and astronomy and astrophysics. Most physicists also specialize in either theoretical or experimental research, the former dealing with the development of new theories, and the latter dealing with the experimental testing of theories and the discovery of new phenomena. Despite important discoveries during the last four centuries, there are a number of open questions in physics, and many areas of active research.

Absorption spectra of elements

A related phenomenum to the emission spectra of elements is the absorption spectra. Imagine that we have an electron in a lower energy state, and a photon comes along. If this photon has just the right amount of energy, it can be absorbed, causing the electron to make a transition to a higher energy state.

This will only occur though if the photon has an energy equal to the energy difference between the two levels involved in the transition - if not, the photon will just pass through.
Such an effect can be seen by shining light of all different frequencies through a gas of a particular element. Some of the photons will be absorbed in the gas, and so will be missing in the light that emerges from the gas. What one will therefore see from the emerging light is an almost continuous band of frequencies, but with some frequencies missing corresponding to the absorbed photons.

As with the emission spectrum, each element has its own unique absorption spectrum, as the energy levels of different elements differ. Measuring the absorption spectrum can therefore be used to identify elements in an unknown substance by comparing it to the spectrum of known elements. Among other areas, this is a major technique used to identify elements in gaseous clouds in galaxies.

Alpha scattering experiment

Experiment to Scatter Alpha Particles

In 1909 Hans Geiger and Ernest Marsden observed that occasionally alpha particles are deflected through large angles when they strike a thin leaf of gold, even though most had passed through the leaf with little or no deviation. This scattering experiment led to Ernest Rutherford's nuclear theory of atomic structure, which was the first to describe the atom's positive charge as being concentrated in a dense nucleus around which negatively charged electrons circle.