Electromagnetic Radiation

Electromagnetic radiation is classified into several types according to the frequency of its wave; these types include (in order of increasing frequency and decreasing wavelength): radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

Electromagnetic Radiation

A small and somewhat variable window of frequencies is sensed by the eyes of various organisms; this is what is called the visible spectrum.

Electrons and Light

By 1900, scientists knew that light could be thought of as moving waves that have given frequencies, speeds and wavelengths.

Electrons and Light

A quantum is the minimum quantity of energy that can be lost or gained by an atom.

A photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation.

It is also the force carrier for the electromagnetic force.

Electrons and Light

In empty space, light waves travel at

2.998 x 108 m/s

The speed of light is often represented by c

c = 3 x 108 m/s

That means that the light from the sun only takes 500 s to travel the 150 million kilometers between the sun and the Earth.

Electrons and Light

The wavelength is the distance between two consecutive peaks or troughs of a wave which is measured in meters.

Electromagnetic Radiation

Frequency (ν) is defined as the number of waves that pass a given point in a specific time, usually one second.

One wave per second is called a hertz (Hz), named for Heinrich Hertz, who was a a pioneer in the study of electromagnetic radiation.

The Photoelectric Effect

The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal.

The Hydrogen-Atom Line Emission Spectrum

When investigators passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteristic pinkish glow.

The Hydrogen-Atom Line Emission Spectrum

This was expected.

Attempts to explain this observation led to an entirely new theory of the atom called quantum theory.

The Hydrogen-Atom Line Emission Spectrum

When a narrow beam of the emitted light was shone through a prism, it was separated into a series of specific frequencies (and therefore specific wavelengths, c = λν or λ = c/ν) of visible light.

Light as Particles

Einstein explained the photoelectric effect by proposing that electromagnetic radiation is absorbed by matter only in whole numbers of photons. In order for an electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the minimum energy required to knock the electron loose.

The Hydrogen-Atom Line Emission Spectrum

When current is passed through a gas at low pressure, the potential energy of some of the gas atoms increases.

The lowest energy state of an atom is its ground state.

The Hydrogen-Atom Line Emission Spectrum

A state in which an atom has a higher potential energy than it has in its ground state is an excited state.