what is light? this question is relevant in reviewing photosynthesis
and plant growth and development

What is light? In particular, what is light from sun? What may be your answer when you are suddenly confronted with such 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.

What is light?This light is visible, but there's also that invisible light.

Many of us may just say that it is something from the sun which makes dark undark, or makes us see things more clearly, or that which makes day a day instead of night.

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).
  • 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. 

The herein author of this and companion pages prefers the definition of light given by Moore et al. (2003) as an introductory description of light. It clearly describes light as visible to the naked eye, one which even primary graders can easily comprehend. This light is also associated with colors that humans can perceive, like those in rainbows.  

On the other hand, it would appear that entirely equating "light" with visible light will tend to confuse one who digs deeper into its functions in plant physiological processes. This is so because light has become a generic term used in referring to the part (the wavelengths) of the electromagnetic spectrum with effect or influence on plant processes. Thus there is the use of "ultraviolet light" and "infrared light." Such terms are indeed confusing because both the ultraviolet and infrared wavelengths are invisible and yet these two are combined with "light" which is visible according to the above two definitions. Both these wavelengths are also included in the study of "photo" prefixed processes referred to by Hopkins (1999).

To resolve, let it be therefore that from hereon light should refer to both the visible and invisible wavelengths that influence plant growth and development. That of Hopkins (1999) should be made the reference definition.


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 thermodynamicsEnergy 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)

Climatic Factors  <<<   >>> Properties of Light

>>> What is Photosynthesis?

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