The properties of light with marked influence on plant growth and development are **light quality**, **light intensity,** and **light duration or photoperiod**.

A substantial understanding of light in relation to plant growth and development is essential in crop production.

**Visible light is just a small fraction of the entire electromagnetic spectrum**

But first, its* dual attributes of continuous wave and discrete particles *as described by Hopkins (1999).

## The Wave-Like Property of Light

Light is described as **electromagnetic radiation** that consists of two waves, the electric and magnetic waves, which are perpendicular to each other in the direction that the wave is travelling.

As a wave, the properties of light are characterized according to **speed**, **wavelength,** and **frequency**.

The light that hits the Earth travels an average distance of about **150 million kilometers** (149,476,000 km or 92,900,000 miles; Davis 1977) from its source, the **sun**.

It has a speed of **299,792,458 m s ^{-1}** or 2.99792458 x 10

^{8}m s

^{-1}(

**s**is another form of writing

^{-1}**/s**which means

**per second**).

For convenience, the speed of light is rounded off to **300 million meters per second** (3 x 10^{10} m s^{-1}) which is equivalent to 300 thousand kilometers per second (186,420 mi s^{-1}) or 18 million km per minute (28,966,849 mi min^{-1}) or 1.08 billion km per hour (671.1 million mi hr^{-1}).

By dividing the sun-earth distance by the speed of light (**150 M km / 18 M km min ^{-1}**), light will have reached the earth through empty space in about

**8.3 minutes**.

One of the important properties of light has reference to certain wavelengths that reach an object, or the plant.

This determines light quality and also affects light intensity.

Understanding of this property of light has led to improved practices in crop production particularly the use of colored lights.

The **wavelength**, denoted by the Greek letter *lambda* (**λ**), is the distance in space from one wave crest to another.

*Wave crest* is that uppermost level of a wave as in the ocean, with reference to the wave-like vibrational movement of a photon of light.

**Frequency**, denoted by the Greek letter *nu* (**ν**) and measured in **hertz**, is the number of wave crests passing a point in space in one second.

Frequency is inversely proportional to wavelength.

The entire **electromagnetic spectrum** of the solar radiation, in order of increasing wavelength or decreasing frequency, consists of the gamma rays, X-rays, ultraviolet (100-390 nm), visible light (390-760 nm), infrared (760-1000 nm), microwaves, and radio waves.

However, the ranges are not exacting, particularly the boundaries of the different regions of visible light.

The ranges here provided (in parenthesis) are derived from Devlin (1975).

The **light** that is visible to the human eye is just a small fraction of the electromagnetic spectrum of radiation that is continuously emitted by the sun.

However, “light” is generally used to refer to a broader range of electromagnetic spectrum that includes the invisible wavelengths, the **ultraviolet light** and **infrared light**.

## The Particle-like Property of Light

As to the **particle-like property of light** that is emitted from a source or absorbed by any matter, it behaves as though the energy that it carries is divided into tiny packets or particles called **photons**.

The energy that a photon carries is called a **quantum** (pl. quanta), indicating that the energy can be *quantized* or divided into multiple units.

The **energy content** (**Eq**) of a photon is inversely proportional to the wavelength and directly proportional to the frequency.

In accordance with its relationship to wavelength, it is calculated using the formula:

**Eq = hc/ λ** or, based on frequency, **Eq = hν**where **h** is the **Planck’s constant** (h = 6.62 x 10^{-34} J s photon^{-1}; **J s** is the abbreviation of the unit *Joule second*) and c is the speed of light (3 x 10^{8} m s^{-1}).

For convenience, the result is adjusted to a larger value by multiplying with the **Avogadro’s number** (N = 6.023 x 10^{23} photons mol^{-1}).

To illustrate, the energy content of **red** light with representative wavelength of 650 nm (6.5 x 10^{-7} m) is derived through the following calculation:

**Eq**= [(6.62 x 10^{-34}J s photon^{-1})(3 x 10^{8}m s^{-1}) / 6.5 x 10^{-7}m] x [6.023 x 10^{23}photons mol^{-1}]

= (3.055 x 10^{-19}J photon^{-1})(6.023 x 10^{23}photons mol^{-1})

= 18.4002 x 10^{4}J mol^{-1}≈**184 kJ mol**^{-1}

Following the same mathematical procedure, a mole of photon of **blue** light with a representative wavelength of **470 nm** (4.7 x 10^{-7} m) carries an energy of **254 kJ mol ^{-1}**.

With shorter wavelengths the energy content will be much higher.

For example, the energy content of a mole of photon of **UV-C** with a specific wavelength of **200 nm** (2 x 10^{-7} m) is **598 kJ mol ^{-1}**;

**100 nm**will have an energy content of about

**1,196 kJ mol**.

^{-1}For comparison, the following conversion equivalents of **energy units** are given: **1kJ** = 1 x 10^{10} ergs = 0.24 calories (nutritional) = 0.28 watt-hr = 2.4 x 10^{-7} ton TNT explosion equivalent.

**REFERENCES**

DEVLIN R. 1975. Plant Physiology. 3rd ed. New York, NY: D. Van Nostrand Company. 600 p.

DAVIS TN. 1977. Sun-Earth distance. Alaska Science Forum (Feb. 16, 1977). Retrieved April 17, 2011 from http://www2.gi.alaska.edu/ScienceForum/ASF1/142.html.

HOPKINS WG. 1999. Introduction to Plant Physiology. 2nd ed. New York, NY: John Wiley & Sons, Inc. p. 125-141.