sy b.sc. electromagnetic spectrum

8/13/2019 SY B.sc. Electromagnetic Spectrum

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To understand how the thylakoids of chloroplasts transform light energy into thechemical energy of ATP and NADPH, it is necessary to know some important properties of light.

1. The nature of sunlight

Sunlight is electromagnetic energy.

The quantum mechanical model of electromagnetic radiation describes light as having a behavior that is both wavelike and particlelike.

a. Wavelike properties of light- Electromagnetic energy is a form of energy that travels in rhythmic waves which

aredisturbances of electric and magnetic fields.- A wavelength is the distance between the crests of electromagnetic waves. - The electromagnetic spectrum ranges from wavelengths that are less than a

nanometer (gamma rays) to those that are more than a kilometer (radio waves).- Visible light, which is detectable by the human eye, is only a small portion of the electromagnetic spectrum and ranges from about 380 to 750 nm. The wavelengthsmostimportant for photosynthesis are within this range of visible light.

b. Particlelike properties of light- Light also behaves as if it consists of discrete particles or quanta called photons.- Each photon has a fixed quantity of energy which is inversely proportional to the

wavelength of light (the shorter the wavelength the more energy it has). For example,aphoton of violet light has nearly twice as much energy as a photon of red light.

The sun radiates the full spectrum of electromagnetic energy.- The atmosphere acts as a selective window that allows visible light to passthroughwhile screening out a substantial fraction of other radiation.

- The visible range of light is the radiation that drives photosynthesis.- Blue and red, the two wavelengths most effectively absorbed by chlorophyll, are

the colors most useful as energy for the light reactions.

the vast majority of life on earth depends on sunlight for energy.Living things require energy to stay alive.The main energy input to planet earth is from the sun.The sun produces a vast amount of

energy in many different forms. The main form of energy from the sun is in the form ofelectromagnetic radiation, although it also produces vast quantities of subatomic charged particles

into the space around it. The electromagnetic radiation from the sun can be shown in a diagram:


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Of the many different wavelengths of e-m radiation hitting the earth, very little passesthrough the atmosphere. X rays are absorbed in the Van Allen belt, high in the atmosphere.UV rays are also reduced by the gas ozone, although pollution from CFCs and other gaseshas damaged the ozone layer and permitted more biologically dangerous UV light to passthrough to ground level. Infrared energy is trapped by the atmosphere the so-calledgreenhouse effect that keeps the temperature of the planet warm and stable.Visible light passes readily through the atmosphere, and it is these wavelengths (between

400nm and 700nm) that photosynthetic organisms use.The sun produces many wavelengths of electromagnetic radiation, but only visible radiation is used

for photosynthesis.

The energy produced by the sun reaches the earth as electromagnetic radiation. Light and other

forms of electromagnetic radiation are considered to have both a wave natureand a particle

nature. Particles or packets of light (its particle nature) are known as photons- the smallest

divisible units of light. The brightness of light depends on the number of photons absorbed per unit

time. Each photon carries a fixed amount of energy which determines the amount that the photon

vibrates. The distance moved by a photon during one of it vibrations is referred to as its

wavelengthand is measured innanometres.
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Electromagnetic radiation spans a

broad range of wavelengths. At the

one end of the spectrum of

electromagnetic radiation there are

gamma rays which have a wavelength

of 10-5

nm and at the other end, radiowaves which have a wavelength of

1012nm. A very small part of this

spectrum can be seen by the human

eye i.e. between the wavelengths 380

and 750 nm. This part of the

electromagnetic spectrum is called

visible light. Almost all life depends

ultimately on this part of the

spectrum for its energy. Humans

perceive the different wavelengths of

visible light as different colours.

Within the spectrum the longer the

wavelength of the radiation, the

slower the vibration of the photons

and the less energy each photon

contains. Thus photons of ultraviolet

light, at the blue end of the visible

spectrum, have shorter wavelengths

and contain more energy than red light

and infrared radiation.

Sunlight contains 4% ultraviolet

radiation, 52% infrared radiation and 44% visible light.

Why is only visible light used by plants ?

Light and photosynthesis.

Chlorophylldoes notabsorball the wavelengths of visible light equally. Chlorophyll a, the most

important light-absorbing pigment in plants, does not absorb light in the green part of the

spectrum. Light in this range of wavelengths isreflected.This is the reason why chlorophyll is

green and also why plants (which contain a lot of chlorophyll) are also green. Note in the graphabove that the absorption of light by chlorophyll a is at a maximum at two points on the graph 430

and 662 nm. The rate of photosynthesis at the different wavelengths of visible light also show two

peaks which roughly correspond to the absorption peaks of chlorophyll a. Plants do not depend only

on chlorophyll a in their light harvesting machinery but also have other pigments (accessory

pigments)which absorb light of different wavelengths.
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II. Properties of Light

A. Light is part of theelectromagnetic spectrum-- Wave propertieso 1. Wavelength= the distance between crests of the wave

The Spectrum

a. TV waves are very long wavelengths -> Infra-red (IR) (appear black)b. Ultraviolet (looks black) -> X-rays wavelengths are very short

c. Visible light are the colors you see (each color has a different

wavelength)

X-ray---UV--380nm-------------------------760nm---IR---TV

appears black violet blue green yellow orange red black

tanning heat lamps

(nanometer - nm = 1 billionth of a meter)

B. Particle nature of radiation

o 1.Photon- a quantum of light; a unit of light energy

2. Energy in a photon:

E= hc/lambda (violet has more energy)

where h =Plank's constant,c = speed of light (constant), lambda=

wavelength

3. As wavelength increases, energy per photon decreases. Photons of violet light

have more energy than photons of red light

NOTE: A photon (as discussed here) is different from aphoton torpedo.
http://covis.atmos.uiuc.edu/guide/optics/html/refr-prism.htmlhttp://covis.atmos.uiuc.edu/guide/optics/html/refr-prism.htmlhttp://covis.atmos.uiuc.edu/guide/optics/html/refr-prism.htmlhttp://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.htmlhttp://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.htmlhttp://zebu.uoregon.edu/~soper/Light/photons.htmlhttp://zebu.uoregon.edu/~soper/Light/photons.htmlhttp://zebu.uoregon.edu/~soper/Light/photons.htmlhttp://scruffy.phast.umass.edu/a114/lectures/lec08/node1.html#SECTION00010000000000000000http://scruffy.phast.umass.edu/a114/lectures/lec08/node1.html#SECTION00010000000000000000http://oak.cats.ohiou.edu/~ms370794/photorps.htmlhttp://oak.cats.ohiou.edu/~ms370794/photorps.htmlhttp://oak.cats.ohiou.edu/~ms370794/photorps.htmlhttp://oak.cats.ohiou.edu/~ms370794/photorps.htmlhttp://scruffy.phast.umass.edu/a114/lectures/lec08/node1.html#SECTION00010000000000000000http://zebu.uoregon.edu/~soper/Light/photons.htmlhttp://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.htmlhttp://covis.atmos.uiuc.edu/guide/optics/html/refr-prism.html

sy b.sc. electromagnetic spectrum

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