What is Light? This Question is Relevant in
Plant Growth and Development

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What is light? What may be your answer when you are suddenly confronted with this question? Surely it would not be a surprise if the question elicits a shocking, caught-in the-act, stammering realization to many persons.

Oftentimes the automatic answer is the funny “Well, light is light!” But this answer should not be a surprise too. Light has always been there, and we live day after day without finding the necessity to know further what is light or what is the definition of light.

It’s quite different in studying the science of crop production. To find ways to improve crop productivity, it is important to be able to acquire sufficient understanding of photosynthesis, as well as other photo-processes. It means understanding also the different factors that will promote efficient photosynthesis. The root word ‘photo’ means light.

Light performs a major function in plant growth and development. It is essential in the production of energy that sustains life through the process of photosynthesis. In addition, it influences or controls other plant growth processes such as the synthesis of the different photoreceptors (e.g. chlorophyll, other pigments), photomorphogenesis (organ formation and development), phototropism (plant response to unilateral light), photoperiodismtranslocationmineral absorption, and transpiration. To better understand these functions, it is necessary to acquire sufficient background on what is light including its various properties.

What is Light: Definitions / Meaning

Various sources provide many meanings or definitions of light.  Here are some entries:

  • Cambridge Dictionary of Science and Technology: (Phys.). Electromagnetic radiation capable of inducing visual sensation, with wavelengths between 400 and 800 nm (Walker 1988).
  • Botany book: Light is the part of the electromagnetic spectrum having wavelengths visible to the human eye (about 390-760 nm) (Moore et al. 2003).

The herein author of this and companion pages prefers the definition of light given by Hopkins (1999). It is a more general definition and provides also an idea on the wave and particle-like properties of light.

According to Hopkins (1999) in relation to photobiology which includes the study of photomorphogenesis, photoperiodism and photosynthesis, light is a form of electromagnetic energy that has dual attributes of continuous wave and discrete particles. For a more relevant introduction, it would be appropriate also to add the clause “and which controls or influences plant growth and development processes”.


Energy is the ability to do work, that is, to bring about change or to move matter against an opposing force. There are various forms of energy: potential and kinetic; chemical, gravitational, nuclear, mechanical, electrical, sound, radiant (electromagnetic), and thermal (heat) energy.

The radiant energy from the sun, or solar energy, is transformed into chemical energy through the process of photosynthesis. It is stored in the bonds that hold atoms together, as in food that provides the energy to perform mechanical work. In these cases there is convertion from one form of energy to another in accordance with the first law of thermodynamics: Energy can be converted from one form to another, but it cannot be created or destroyed.


However, most of the solar energy is absorbed or reflected back to the atmosphere and becomes heat. The following summary of the fate of the solar energy by Mader (1990) further provides more elaboration on what is light:

(1) Only 42% reaches the surface of the earth; (2) of this, most becomes heat and only about 2% is finally utilized by plants; (3) of this 2%, only 0.1-1.6% is incorporated into plant tissues; (4) of this, only 20% is eaten by herbivores, of which 30% is eaten by carnivores.

When used by organisms, the energy in food is ultimately converted to heat upon death and decay and released to the atmosphere. A large proportion of the energy in plant and animal tissues is stored in the fossil fuels that are burned to provide heat energy.

(Ben G. Bareja Sept. 2012)

More reading: The Wave-Particle Properties of Light

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