3. nature of ligt

7/27/2019 3. Nature of Ligt

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NATURE OF LIGHTRanjan Das

Department of Crop Physiology

Light s the form of energy and it travels as a stream of tiny particles. Quantum

represents the energy contained in an individual proton. Light represents only one part of

electromagnetic radiation. And cosmic rays, X-rays, UV rays, infra-red radiation and radiowaves. A visible right has seven separated groups of more or less complete absorption. In a

spectrum of sunlight, bands of blending colors are seen dark red at one end running through

red orange yellow green blue indigo violet portion of spectrum are about 400 millimicrons in

length and at other end of spectrum-the red portion-are much longer about 730 millimicrons.

In other words, visible light lies between wavelengths of ultra-violet and infrared. The most

efficient wavelengths of light effective in photosynthesis are those or red light. Green light is

least effective and has inhibitory effect. However art of blue and red wavelengths, blue light

carry more energy.

ABSORPTION AND ACTON SPECTRA

The curve representing the light absorbed at each wavelength by pigment is called

absorption spectrum. Curve showing rate of photosynthesis at different wavelengths of light

is called action spectrum. Absorption spectrum is studied with the help of spectrophotometer.

The absorption spectrum of chlorophyll a and chlorophyll b indicate that these pigments

mainly absorb blue and red lights. Action spectrum shows that maximum photosynthesistakes place in blue and red regions of spectrum.

In this case actual rate of photosynthesis in terms of oxygen evolution or carbon

dioxide utilization is measured as a function of wavelength. Both chlorophyll a and b show an

absorption maximum in blue and red regions.

ACTION SPETRA

A plot of the intensity of rate of photosynthesis as a function of different wavelength.

ABSORPTION UTILIZATION OF LIGHT ENERGY BY PHOTOSYNTHETIC PIGMENTS

Fig: utilization of light energy

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What happens to the light that strikes a leaf?

1. Absorbed

85 to 90% of the PAR, 60% of total energy2. Reflected

0 to 8% of the PAR3. Transmitted (passes through leaf)

0 to 8% of PAR

Photosynthetic pigment absorbs light energy only visible part of the spectrum ranging between 400-

700 nm. Such radiation is called photosynthetically active radiation.

Chlorophyll chiefly absorb in the violet blue and red part of the spectrum. The

absorption band shown by the chlorophyll in violet blue region is called as soret band.

There is very weak absorption by pigments in green part of the spectrum and hence the

chloroplasts appear in green plants

Transfer of light energy absorbed by accessory pigment to chlorophyll a :

a) All the pigments except chlorophyll a are called as accessory pigments or antenna

pigments

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b) Almost all (95-99%) the light energy absorbed by accessory pigments is transferred to

chlorophyll a by a resonance

c) Chlorophyll molecules also absorb energy and directly alone take part inphotosynthesis.

FLUORESCENCE AND PHOSPHORESCENCE

a) The normal state of the molecules or atom is called as ground state or singlet state.

When a molecule or electron absorb quantum of light it is raised to a higher energylevel which is called as excited second singlet state. This is unstable with a half-life of

10-12 second.

b) The electron comes to the next higher energy level by the loss of some of its extra

energy in the form of heat. This is called as excited first singlet state and is also

unstable also with a half-life of 10-9 second.

c) From the first singlet state the exited electron may return to the ground state in

two ways:

i. Either losing its remaining extra energy in the form of heat

ii. By losing extra energy in the form of radiation energy.

The later process is called fluorescence. The substances, which show this property of

fluorescence, emit fluorescent light only during the period they are exposed to incident light.

It is because some energy is lost during the change of excited second singlet state to excited

first singlet state.

d)

The excited molecule or the atom may also loss its electron excitation energy by internal

conversion and comes to another excited state called as triplet state which is metastable with

half life of 10-3 second.. In this excited state the electron carrying extra energy can also takepart in further reaction.

e) From the triplet state the exited molecule on the atom may return to the ground

state in by three ways

i. Either losing its remaining extra energy in the form of heat

ii. By losing extra energy in the form of radiation energy. The later process

is called phosphorescence. The substances, which show this property of

phosphorescence, emit phosphorescent light even after the incident radiant light is cut-

off. The phosphorent is of higher wavelength than the incident light and also

fluorescent light.iii. The electron carrying the extra energy may be expelled from the

molecule and is consumed in some further chemical reaction and fresh normal electronreturn to the molecule or extra energy is utilized in the reaction. This is what exactly

happens with excited triplet state of chlorophyll-a molecule, which takes part in

photochemical reaction in photosynthesis.

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3. nature of ligt

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