The height of a wave from its zero crossing to its maximum. The square of the amplitude of an lectromagnetic wave is directly proportional to its intensity. Thus, the amplitude of a light wave is directly responsible for how bright a light source appears.


A means for transmitting and receiving electromagnetic waves. The function of an antenna is based on the principle of periodic oscillation of free electrons in matter. Voltage applied causes movement of electrons in the antenna so that radiation is transmitted. Alternatively, electrons are moved in the antenna by incoming radiation, which can be further processed as an electrical signal.


Atoms are the building blocks that make up all matter. They are composed of a nucleus and an atomic shell. The nucleus, with a diameter of only a few femtometers, is ten thousand to one hundred thousand times smaller than the whole atom, but contains more than 99.9% of the atomic mass. It consists of protons and neutrons. The atomic shell, consisting of electrons, surrounds the core. This determines the size of the atom. The positive core and negative shell are bound to each other by electrostatic attraction. Atoms can form larger structures called molecules.


A unit of length equal to 10−10 m (one ten-billionth of a meter). This is approximately the size of an atom and is often used as a unit of length in atomic physics. Named after the Swedish physicist Anders Jonas Ångström.

Charge (electric)

A characteristic of particles that determines how much they participate in electromagnetic interaction. There are only two possible electric charge settings: a negative charge, as electrons have; and a positive charge, as protons have. The number of electrons and protons an object contains determines its total charge. An electric charge creates an electric field around itself. At the same time, a particle needs a charge to be able to sense electric fields. Electric charge is represented by the unit Coulomb [C].


One of the fundamentally important liquids in the universe and one of the greatest discoveries of mankind. It contains caffeine, which keeps scientists happy and performing capably around the clock. Many research milestones were made possible thanks to sufficient coffee consumption. Together with chocolate, coffee is responsible for the functioning of the modern world.


Wave trains that have a clearly defined phase relationship to each other are referred to as coherent. In contrast, a light bulb emits incoherent light: individual light waves are emitted from different atoms, and these light emissions have no relation to one another. They can vary in wavelength, direction or polarization. The coherence of wave trains in a laser makes it possible to perform repeatable and verifiable experiments. Additionally, stationary interference phenomena can only be observed with coherent light.


Color is an interpretation of different wavelengths of light by human eyesight. Through evolution, our sense of sight has become specialized in the wavelength range that is emitted most intensely by the sun. Light with a wavelength of 400 nm we see as blue, while a wavelength of 800 nm appears as red. All colors visible to us fall within the wavelength range between these.

Coulomb force

The force that attracts opposite electrical charges, and causes similar electrical charges to repel each other. The strength of this force follows the inverse square law: for example, if you double the distance between the charged particles, the force reduces by a factor of four.

Electric field

The area around an electrically charged object. An electric field shows how an electric charge influences its surroundings. A point charge, such as an electron, is the center of a radial electric field. This is comparable to the way in which the earth is the center of a gravitational field, and exerts a force on all masses.

Electromagnetic interaction

One of the four fundamental interactions of physics (strong, weak and electromagnetic forces, and gravity). Electromagnetic interaction determines the interaction of all electrical charges and all magnets. For example, the Coulomb force is a direct result of electromagnetic interaction. Photons are the exchange particles of this interaction: i.e. in particle physics, the communication between individually charged particles can be imagined as an exchange of photons between those particles.


A particle with a single negative charge – one of the three basic building blocks of matter. Depending on the type of element, varying numbers of electrons form an atomic shell (also known as an electron shell) around the nucleus of an atom. Electrons are about 2000 times lighter than protons or neutrons. Electrons are also responsible for the chemical bonds that form atoms into molecules. Electrons can also occur as free particles, not bound to atoms, which transfer energy and information. The collective motion of free electrons is known as electric current.

Electron shell

Also known as an electron cloud, this describes a set of electrons orbiting a nucleus. The term electron cloud illustrates the fact that while it is not possible to determine the precise position of individual electrons, we can describe the probable distribution of electrons relative to the nucleus.


An element is the generic term for all atoms with the same number of protons. Each atom is associated with an element. Atoms with one proton in their nucleus are known as hydrogen, those with two are known as helium, and so on, up to the heaviest stable atoms, such as uranium, which has 92 protons.

Elementary charge

The smallest possible charge that a free particle can have. Electrons and protons each carry a negative or positive elementary charge, both of which are exactly the same magnitude. Every greater charge is an integer multiple of this elementary charge. The magnitude of this charge was first measured by Robert Millikan in 1909, and comes to 1.602 * 10-19 Coulomb [C].


The ability to perform work. Energy can happen in different ways, for example as kinetic energy (motion energy), as potential energy (such as high/potential energy in a gravitational field or electric-potential energy of charges in electric fields), chemical energy or thermal energy. Energy can never be created or destroyed – it takes place in physical processes, where one form of energy converts into another. The unit of energy is the joule [J].


The number of particular events per unit of time. In optics, light is usually described using the term frequency. Here, frequency refers to the number of oscillations of a light wave per second. For green light with a 500 nm wavelength, this is 6 * 1014 Hz. Hz (Hertz) is defined as one cycle per second.


Electromagnetic radiation with a wavelength between about 800 nm and 1 mm. Infrared radiation has a longer wavelength than visible light. It is not visible to us, but we perceive it as heat. Wavelengths of more than 1 mm are referred to as microwave radiation.


Intensity is defined as power per unit area and is represented by the unit W/m2. In optics, the intensity of an electromagnetic wave is calculated alongside some constants from the square of the amplitude of the electromagnetic field.


The overlapping of several waves together. According to the superposition principle, the amplitudes of these individual waves add together. Depending on the phase of the wave, interference can amplify or cancel the waves. Static interference patterns can only form from coherent light.


An atom in which less electrons are present in the shell as there are protons in the nucleus. Ions are thus charged atoms. An excess of electrons leads to a negative charge, an excess of protons leads to a positive charge.

Laser pulse

A short packet of light/energy emitted by a laser. Some lasers send out a continuous beam, while others are pulsed in order to deliver as much energy as possible in a very short time. At present, it is possible to produce laser pulses with a duration of less than one hundred attoseconds.


In the strictest sense, light includes only that within the electromagnetic spectrum which is visible to the human eye (approx. 400-800 nm). However, other, non-visible types of radiation existing outside of this range (for example, in the infrared or ultraviolet range, or electromagnetic radiation in general) are also referred to as light.

Light can be considered as consisting of particles (photons), or as a wave of electric and magnetic fields. The characteristics of light are determined by the experiment with which the light is observed.

Light Amplification by Stimulated Emission of Radiation.

A light source emitting a coherent, collimated light beam by the stimulated emission of photons. A laser usually consists of a laser medium, an energy source to excite the medium, and two mirrors between which the medium is located. The energy source (a flash lamp, electricity source, or another laser) excite the atoms or molecules in the laser medium. When an atom sends out light in the direction of one of the mirrors, this is reflected back to the medium, from which other atoms emit light in the same direction and with the same characteristics. This is called stimulated emission. Since the energy source constantly excites new atoms, this process can be repeated many times, so that the intensity of light between the two mirrors in the medium becomes increasingly stronger. One of the two mirrors allows a fraction of the light to shine through: this is the usable, coherent laser beam.


Electromagnetic radiation with a wavelength range of approximately 1-300 mm. Shorter wavelengths are referred to as infrared radiation, and longer wavelengths as radio waves. Microwaves are particularly suitable for vibration excitations in water molecules, which is why they can be used to warm water-containing matter such as food and beverages.


A stable union of several atoms. This can range from small molecules of two hydrogen atoms (H2) to molecular chains, like our DNA, with about 100 billion atoms. The bond between the atoms is determined by electrons, which are also responsible for the chemical reactions of the individual molecules with each other.


Electrically neutral particles that – together with protons – form the building blocks of atomic nuclei. Neutrons can also occur as free particles. However, they are then unstable, and decay within a half-life period of about 10 minutes into a proton, an electron and a neutrino (or more precisely, an electric antineutrino). Due to strong interaction, protons and neutrons are bonded to each other in the nucleus and can thus compensate for the electrostatic repulsion of protons.


At the center of an atom is the nucleus, surrounded by the electron shell. The nucleus consists of protons and neutrons. The number of protons and neutrons are very different in different elements. For example, in hydrogen, the nucleus consists of only one proton. Where numbers of protons are low, atomic nuclei have roughly equal numbers of neutrons and protons; the larger the nuclei are, the more neutrons keep the core together. Uranium-238, for example, consists of 92 protons and 146 neutrons. The number of protons determines the name of each element. Atomic nuclei of a particular element can have slightly differing amounts of neutrons, yet remain stable.


The theory of light. The term optics includes the nature of light itself, and experiments and physical phenomena related to light. This term is used very universally, ranging from the description of the pure geometric flight path of a light beam, to the technical implementation of modern, laser-driven quantum optics.


Particles that make up light. Light can be thought of as a stream of many photons that individually determine its parameters, such as color or polarization. The intensity of light is directly dependent on the number of photons. Photons are massless and thus move at the speed of light. In opposition to this image of particles is the wave-like nature of light, in which light is described not as particles, but as an electromagnetic wave. Whether light behaves like a wave or a particle depends on the experiment being conducted.

Physics abbreviations
  • atto (a): prefix denoting a factor of 10−18, i.e. 0.000000000000000001.
  • femto (f): prefix denoting a factor of 10−15, i.e. 0.000000000000001.
  • pico (p): prefix denoting a factor of 10-12, i.e. 0.000000000001.
  • nano (n): prefix denoting a factor of 10−9, i.e. 0.000000001.
  • micro (μ): prefix denoting a factor of 10−6, i.e. 0.000001.
  • milli (m): prefix denoting a factor of 10−3, i.e. 0.001.
  • kilo (k): prefix denoting a factor of 103, i.e. 1,000.
  • mega (M): prefix denoting a factor of 106, i.e. 1,000,000.
  • giga (G): prefix denoting a factor of 109, i.e. 1,000,000,000.
  • tera (T): prefix denoting a factor of 1012, i.e. 1,000,000,000,000.
  • peta (P): prefix denoting a factor of 1015, i.e. 1,000,000,000,000,000.

Describes the orientation of a wave in space. In the case of light, polarization indicates the plane in which the electromagnetic fields oscillate. Light can have a linear polarization, which means that the electric field oscillates up and down in a plane. In so-called circular (or elliptical) polarization, the oscillation of the field is not limited to a plane, but moves spirally. This can also be interpreted as a superposition of two linear polarizations.


An amount of energy per unit of time. The unit of power is a watt [W], which corresponds to energy of one joule per second [J/s].


Positively charged particles that – together with neutrons – form the building blocks of atomic nuclei. Unlike neutrons, protons can occur as stable, free particles.

Radio waves

Electromagnetic radiation with a wavelength of more than about 300 mm. Shorter wavelengths are referred to as microwave radiation.

Refractive Index

An index of measurement that determines how much light is refracted as it enters a transparent medium. The refractive index also determines the velocity of light in the medium. For example, in water, which has a refractive index of approximately 1.3, light is about 25% slower than it is in air, or in a vacuum.


Electromagnetic radiation with a wavelength between approximately 10 nanometers and a picometer. These limits, and the definition of the term, thus the definition of X-rays themselves, differ depending on the field in which the X-rays are being applied. Shorter wavelengths become gamma radiation, and longer wavelengths join the ultraviolet range. Due to the different absorption of X-rays in bone and surrounding tissue, they are often used for medical purposes.

Speed of light

The speed at which light propagates is about 300,000 km per second. This term refers to its speed in a vacuum, if not otherwise stated. The speed of light will vary, depending on the refractive index of material the light interacts with. However, this will always be slower than the speed of light in a vacuum. The speed of light in a vacuum is also the maximum velocity at which information can be transmitted.


Electromagnetic radiation with a wavelength between about 10 and 400 nm. These limits, and the definition of the term, differ depending on the field of use. Shorter wavelengths are called X-rays, and longer wavelengths join the visible light range.


The spatial extent of an entire wave oscillation. The wavelength shows the complete length of a wave without any identical repetition. In light, the wavelength is inversely proportional to the energy (i.e. longer wavelengths result in lower energy). The wavelength determines which color our eye assigns light.