Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. Visible light, which largely coincides with the Photosynthetically Active Radiation (PAR) is the spectral range which plants use for photosynthesis. PAR is quantified in units of micromole per meter squared per second (µmol m-2 s-1) and is designated as the Photosynthetic Photon Flux (PPF), also known as Photosynthetic Photon Flux Density (PPFD). is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400-700nm, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).

The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Some species of animals generate their own light, called bioluminescence.

Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum. Insolation is the total amount of solar radiation energy received on a given surface area during a given time. It is also called solar irradiation and expressed as "hourly irradiation" if recorded during an hour or "daily irradiation" if recorded during a day. Practitioners in the business of solar energy use the unit watt-hour per square metre (Wh/m2).

Light is a fundamental requirement for plant growth. On Earth, plants have evolved for the full spectra and intensity found at the surface, about 1200 W/m2. The PI (photosynthesis-irradiance) curve is a graphical representation of the empirical relationship between solar irradiance and photosynthesis, which shows the generally positive correlation between light intensity and photosynthetic rate. Plotted along the x-axis is the independent variable, light intensity (irradiance), while the y-axis is reserved for the dependent variable, photosynthetic rate.Plants have also evolved specifically for various Earth climates and seasons, so will have varying degrees of success in a closed system depending on the plant and the light conditions (photoperiod, intensity, and spectra).

Humans and many other animals also require light to maintain both physiological and mental health.

In the Inner Solar System, at least, light from is also the primary source of energy, dictating the equilibrium thermal conditions of a habitat.

Light can be converted directly to electricity with photovoltaics.

One of the first people to experiment with measuring the amount of light hitting the earth is John Herschel. (7 March 1792 – 11 May 1871). He invented the actinometer in 1825 to measure the direct heating power of the sun's rays and his work with the instrument is of great importance in the early history of photochemistry.

An actinometer is a device that is used to measure the intensity of solar radiation. It is a chemical system that determines the number of photons by measuring the rate of change of photoinduced responses in a chemical system. It basically worked by...

  • The gas of interest is filled into a photolysis reactor.
  • The actinometer is exposed to heat radiation.
  • The photochemical rate is measured.


John Herschel was a photographer in his lifetime. There is no surprise he would make one of the first exposure meters. He was also interested in plants and their growth. He pointed his actinometer to plants. He wanted to know how how light influenced plant growth. Today's unit for the relation between light energy and plant growth is PAR, µE m−2s−1 (Einstein units) and µmol/J (which is PAR watts); It is the solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.

Plants really use blue and red light for photosynthesis. Green light is reflected off the leaves; hence, that is why leaves are green. The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reaction

6H2O + 6CO2 + energy → C6H12O6 + 6O2

"energy" is the spectrum light. C6H12O6 is glucose (which is subsequently transformed into other sugars) It takes about 10 photons to convert 1 CO2 molecule. The efficiency of plants to make CO2 or O2 depends on the light source and its intensity; hence, PAR units. Most lamps are categorized in units of degrees Kelvin. This is the color spectrum of light from a tungsten lamp where the lamp is burning at a temperature measured in degrees Kelvin.

Created by admin. Last Modification: Tuesday 17 of November, 2015 12:18:27 CST by Pholowko.