LIGHT AND PIGMENTS

 

THE PHYSICAL NATURE OF LIGHT

 

   LIGHT IS A FORM OF RADIANT

   ENERGY WITHIN THE CONTINUOUS

   ELECTROMAGNETIC SPECTRUM

   RADIATED FROM THE SUN

 

   LIGHT DESCRIBES THE VISUAL

   PORTION OF THE SPECTRUM

   (400 TO 700 nm)

 

 

400      500      600      700     800

____________________________________________

 

VIOLET BLUE GREEN YELLOW ORANGE RED  INFRARED

 

 

LIGHT AS A WAVE PHENOMENON

 

   1.   ELECTROMAGNETIC RADIATION

      CONSISTS OF TWO WAVES

 

      a.   ONE ELECTRICAL

 

      b.   ONE MAGNETIC

 

   2.   WAVES PROPERTIES OF LIGHT

      CHARACTERIZED BY;

 

      a.   WAVELENGTH (  )

 

         DISTANCES BETWEEN CRESTS

         OF THE WAVES

 

         1.   EXPRESSED IN UNITS OF

            NANOMETERS (nm)

 

            1 nm = 10-9m

 

      b.   FREQUENCY (v)

 

         1.   THE NUMBER OF WAVE

            CRESTS THAT PASS A

            POINT IN SPACE IN ONE

            SECOND

 

FREQUENCY AND WAVELENGTH ARE

RELATED TO EACH OTHER

 

      v = c/ 

      c = SPEED OF LIGHT

          (3 X 108 m s-1)

 

LIGHT OF IMPORTANCE TO BIOLOGISTS

 

   1.   ULTRAVIOLET

 

   2.   VISIBLE

 

   3.   INFRARED

 

LIGHT AS A STREAM OF PARTICLES

 

   LIGHT IS EMITTED FROM A SOURCE

   IN DESCRETE PACKETS KNOWN AS

   PHOTONS

 

   ENERGY CARRIED BY A PHOTON IS

   CALLED A QUANTUM

 

THE ABSORPTION AND FATE OF LIGHT ENERGY

 

ENERGY OF LIGHT CAN BE ABSORBED BY MOLECULES CALLED PIGMENTS

GOTTHAUS-DRAPER PRINCIPLE - ONLY LIGHT THAT IS ABSORBED CAN BE ACTIVE IN PHOTOCHEMICAL PROCESSES (PHOTOBIOLOGICAL PROCESSES CAN BE INCLUDED HERE)

 

PIGMENTS IN PLANTS ABSORB LIGHT

 

   1.   CHLOROPHYLLS - GREEN

     

      ABSORBS LIGHT ENERGY USED       IN   PHOTOSYNTHESIS

 

   2.   ANTHOCYANINS

 

   3.   PHYTOCHROME

 

EVENTS IN PIGMENT ABSORPTION

 

   1.   LIGHT IS ABSORBED IN A

      FEMPTOSECOND (10-15s)

 

   2.   ENERGY OF THE ABSORBED

      PHOTON IS TRANSFERRED TO

      AN ELECTRON IN THE PIGMENT

      MOLECULE

   3.   THE ENERGY OF THE ELECTRON

      IS ELEVATED FROM A LOW

      ENERGY STATE (GROUND STATE)    TO A HIGH ENERGY (SINGLET         OR EXCITED) STATE

 

EINSTEIN-STARK LAW OF PHOTO-

CHEMICAL EQUIVALENCE SAYS THAT A SINGLE PHOTON CAN EXCITE ONLY ONE ELECTRON

     ____

     ____

S2   ________

 

 

 

 

     ____

     ____

S1   ________

                 

   2    1        F           ____T1

 

                            P

S0    _________________________

         GROUND STATE

A PHOTON CAN BE ABSORBED ONLY IF

ITS ENERGY CONTENT MATCHES THE ENERGY REQUIRED TO RAISE THE ENERGY OF ONE ELECTRON TO ONE OF THE HIGHER, ALLOWABLE ENERGY STATES

 

CHLOROPHYLL, A COMPLEX PIGMENT, HAS MANY DIFFERENT ELECTRONS AND EACH MAY ABSORB A PHOTON OF A DIFFERENT ENERGY LEVEL (WAVELENGTH)

 

AN EXCITED MOLECULE HAS A VERY SHORT LIFESPAN. ANY EXCESS ENERGY MUST BE DISSIPATED

 

   1.   THERMAL DEACTIVATION -

      MOLECULE LOSES EXCITATION

      ENERGY AS HEAT

 

   2.   FLUORESCENCE - EMISSION OF

      A PHOTON OF LIGHT BY AN

      EXCITED MOLECULE

 

      a.   FOLLOWS THERMAL                      DEACTIVATION TO THE

         LOWEST SINGLET STATE

 

      b.   THIS GIVES THE EMITTED

         PHOTON A DIFFERENT WAVE-

         LENGTH (GREEN --> RED)

 

   3.   INDUCTIVE RESONANCE -

      ENERGY TRANSFER BETWEEN

      PIGMENT MOLECULES

 

   4.   TRIPLET STATE OR METASTABLE

      STATE - PHOTO-OXIDATION

 

ABSORPTION AND ACTION SPECTRA

 

 

 

 

 

 

 

 

 

 

MEASURING LIGHT

 

   3 IMPORTANT PARAMETERS

 

   1.   LIGHT QUANTITY - HOW MUCH

         LIGHT RECEIVED

 

   2.   LIGHT QUALITY (SPECTRAL

         COMPOSITION)

 

   3.   TIME (DURATION AND                     PERIODICITY)

 

QUANTITY

 

FLUENCE - QUANTITY OF RADIANT ENERGY FALLING ON A SMALL SPHERE DIVIDED BY THE CROSS SECTION OF THE SPHERE

 

   1.   PHOTON FLUENCE (mol m-2) =

      TOTAL NUMBER OF PHOTONS

      FALLING ON A SPHERE

 

      PHOTON FLUENCE RATE =

      mmol m-2 s-1

   2.   ENERGY FLUENCE (J m-2) =

      AMOUNT OF ENERGY FALLING ON

      A SPHERE

 

      ENERGY FLUENCE RATE =

      (mol m-2 s-1)

 

QUALITY

 

   MEASURED USING A    SPECTRORADIOMETER

 

      SPECTRAL PHOTON FLUENCE                RATE =

       (mol photons m-2s-1nm-1)

 

      IS PLOTTED AGAINST WAVE-

      LENGTH

 

   TERMS NO LONGER USED TO

   SCRIBE LIGHT QUANTITY

 

   1.   LUX

   2.   FOOT CANDLES

 

 

      BOTH ARE PSYCHOPHYSICAL

      MEASUREMENTS (RELATED TO

      HUMAN VISION)

 

      1 LUX = 10 FT. CANDLES

 

OTHER TERMS USED IN PHOTOBIOLOGY

 

   INTENSITY = QUANTITY OF RADIA-

      TION EMITTED FROM A SOURCE

 

   DOSE = ENERGY THAT IS ABSORBED