Speed of light (c)

The speed of light (symbol: "c" from Latin "celeritas" = quickness) is the speed of propagation of light and other electromagnetic waves. In a vacuum it is 299,792,458 meters per second. This value is seen today as a physical constant.

The fact that the speed of light in a vacuum is a constant has far reaching consequences for the understanding of time and space in physics and is one of the basic suppositions of the theory of relativity. In "dielectric" media the speed of light is less and usually depends on the frequency of light.

Luminous flux (lumen)

Luminous flux is the photometric equivalent of radiated power (also known as: radiant flux, unit: Watts) from the field of radiometry.

It is therefore the photometric measure of power. SI unit: lumen, symbol of units: lm.

Luminous energy (lumen seconds)

Luminous energy is the term used in photometry for the weighted radiant energy. The luminous energy is given in the units of lumen seconds (lm s) or talbot, previously known as lumberg.

Luminous intensity (candela)

Luminous intensity "I" (units: candela, symbol of units: cd) is the radiant power of a light source per solid angle, weighted with the spectral sensitivity of the eye. Measuring the luminous intensity in candela through the entire solid angle in steradian will give the luminous flux in lumen.

Luminance (candela/m²)

Luminance "L" is the photometric dimension for brightness. It expresses how a light source appears brighter the smaller its area is in comparison to its luminous intensity "I". The luminance is what people perceive as brightness.

Characteristic luminance levels

The brightness of the sky

Average clear sky: 8000cd/m²
Average overcast sky: 2000cd/m²
Night sky with full moon: 0.1cd/m²
Starlit night sky: 0,001cd/m²
Cloudy night sky: 1...100 · 10-6cd/m²
Limit of perception: 3 · 10-6cd/m²

The area brightness of radiant bodies

The Sun at midday: 1600 · 106cd/m²
The Sun on the horizon: 6 · 106cd/m²
Frosted 60W light bulb: 120 · 103cd/m²
T8 cold-white fluorescent tube: 11 · 103cd/m²
Surface of the Moon: 2.5 · 103cd/m²
Green electroluminescent light source: 30cd/m²

Illuminance (Lux)

Illuminance (symbol E, SI-unit: Lux or lux, symbol of units: lx) is the photometric equivalent of irradiance E (units: Watts/square metre, i.e. W/m²) in radiometry. E is therefore the quotient of the incident luminous flux per unit of receptor surface, i.e. the radiant power per surface area. The illuminance is therefore a pure receptor value.

Light pressure (Newton seconds)

Radiation pressure or light pressure (also pressure of light) is the force exerted on a body by absorbed, (re-)emitted or reflected light. In accordance with the description of light as photons, light does not just have energy but also has an impulse.

Light colour (Kelvin)

Light colour is the term given to the spectral constitution of light as emitted from a light source or reflected from a body. In contrast to body colours, light colours can be directly measured and perceived. They follow the principle of additive colour mixing.

The opposite of light colours are the body colours, which obey the principles of subtractive colour mixing. The body colour of an object is the light colour of the light reflected back from that body.

The light emitted by lamps has its own intrinsic colour, known as the light colour. It is determined by the colour temperature (TCP) in Kelvin (K).

The higher the temperature, the whiter the light colour.

The light colours of lamps are divided into three groups:

warm white (ww) < 3,300K

Warm white light is perceived as comfortable and cosy.

neutral white (nw) 3,300 to 5,300K

Neutral white light evokes a more clinical mood.

daylight white (dw) > 5,300K

Daylight white light is only suitable for interior rooms with an illuminance of 1,000 Lux or more.

The light from lamps of the same light colour can have different colour rendering properties. The reason for this is the different spectral constitution of the light colour. It is therefore not possible to draw any conclusions about the quality of a lamp's colour rendition from its light colour.

The light colour and the colour rendering property can be changed by special attachments that direct, filter or colour the light. The quality of the colour rendering property is given by the colour rendering index (Ra).

Conversion of units

Length conversion (Metric - Imperial)

Length conversion (Imperial - Metric)

Weight conversion

Temperature conversion

Light year (Ly, ly)

A light year is the distance covered by an electromagnetic wave, such as light, in a vacuum in one tropical year. It is about 9.5 trillion (9.5 x 1012) kilometres.

A light year is an astronomic unit of length and not, as the name might imply, a unit of time.

The light year has ceased to be an accepted unit in the International System of Units (SI) since 1978. It is nevertheless seen as indispensable in astronomy and is still used.

The symbol of units for a light year is Ly or ly. The usual symbol is d (as the distance from the earth) or l (as the length of an object).

Analogous to the light year, there are also the units of: light second Ls (just under 300,000km); light minute Lm (approx. 18 million km) and light hour Lh (approx. 1.08 billion km).

One light year (1 Ly) is: 9.460528 x 1015m, i.e. almost 9.5 billion km, 63240AU (Astronomic Units) or 0.3066pc (parsecs or parallax seconds).

Examples

On average, our Earth is about 500 light seconds or 8 light minutes away from the Sun.

Our solar system measures approximately 150 light hours in diameter.

The diameter of our galaxy, the Milky Way, is approximately 100,000 light years.

The nearest star to the Sun, Proxima Centauri, is 4.22 light years away.

The distance to the Andromeda Nebula is approximately 2.7 million light years.

One light second in a vacuum is exactly 299,792.458km. A light year is the distance covered by an electromagnetic wave, such as light, in a vacuum in one tropical year. It is about 9.5 trillion (9.5 x 1012) kilometres.

Brightness

Brightness is an umbrella term for subjective and objective, measured values for the intensity of a visible entity measured in terms of space and colour.

Brightness as a sensory impression

The word brightness is usually used for the subjective impression of light elicited through the eye of the observer. In physics, this sensory impression approximately proportional to the logarithm of the stimulus (see Weber-Fechner Law), but it can vary from person to person. It depends in particular on the spectral sensitivity of the photoreceptor cells, which in most people is highest at the wavelength of 0.47­µm (green-yellow) (maximum level of sunlight). In many animals this shifts to other colours (e.g. in cats or bees).

The human eye works across a very wide brightness range, which corresponds to light intensities of 1 : 10 billion (visual threshold: 10-13 lumen). We can however perceive different brightness levels differently, as soon as their luminous energy differs by more than 10%. It was based on this that the astronomer Friedrich Argelander developed his photometric "incremental method" for apparent brightness levels at around 1840. 

Solar radiation

Solar radiation is the radiant energy emitted by the Sun. The electromagnetic radiation reaches its maximum value in visible light, but also includes other electromagnetic waves ranging from X-ray and UV-radiation to radio waves. The particle radiation (also known as solar wind) mainly contains fast ions, which due to the Earth's magnetic field hardly ever reach the Earth surface.

In addition to solar radiation, the following are also referred to as sunlight:

> the visible light (light spectrum) of the Sun;

> the solar radiation filtered by the Earth's atmosphere.

Laws and types of radiation

The maximum radiation is in yellow-green light. This is derived from the Sun's temperature of approx. 6000 °C and the radiation laws of Planck and Wien.

Together with the red and blue components of light, our eyes perceive this visible photo-radiation as white, whereas the invisible radiation is largely accounted for by infrared. The heat receptors in the skin react to the effect of infrared. Of the shorter-waved radiation, some of the UV-light does get through to us, but the individual X-ray emissions from eruptions on the sun (solar flares) and the cosmic radiation (e.g. ionising radiation) are blocked by the atmosphere.

Polar light

Polar light (also known as Aurora Borealis or the Northern Lights and Aurora Australis, the Southern Lights) is a luminous phenomenon in the sky caused by particles of solar wind hitting the Earth's atmosphere.

Polar lights arise when electrically charged particles of solar wind (mainly electrons, though also protons, alpha particles and some heavy ions) hit the upper layers of the Earth's atmosphere, where they agitate the air molecules present there into a state of illumination. The impact of a particle in the molecule/atom causes an agitation commensurate with the thus caused altered electron configuration. When the next agitation occurs shortly afterwards, light is emitted (generally referred to as fluorescence). Atomic weapons testing also causes such phenomenon in the higher layers of the atmosphere (400km) - as was observed for instance at the USA's Starfish Prime Test on the 9th of July 1962.

Polar lights mainly arise in the polar regions because the solar wind particles - an electrically charged plasma having an average speed of approx. 500-833km/s (3,000,000km/h max.) and a density of approx. 5 x 106 particles per m3 when it reaches the Earth - are steered along the magnetic field lines to the poles by the Earth's magnetic field. At the poles the magnetic field becomes perpendicular to the Earth surface and the particles can enter into the atmosphere. Before entering the atmosphere, the plasma will have taken around two to four days to complete the approximately 150-million-kilometre journey from the Sun to the Earth.

Polar lights occur both at Northern latitudes and in the Southern hemisphere, and are known respectively as Northern Lights or Aurora Borealis and as Southern Lights or Aurora Australis. This phenomenon has also been observed on other planets of the solar system.

Even though the polar lights are sometimes mentioned in the weather proverbs of some countries, they have nothing to do with the weather. They occur at altitudes of 60km and above, far above the troposphere, the layer in which the weather happens, which only reaches to an altitude of about 15km.