Re: When my teacher shined a light on a vial of green chloroplast it turned red

Congratulations John, you have observed natural fluorescence.  The 
chloroplasts were giving off red light.  Not a trivial thing to see, in 
fact my research uses fluorescence of the chloroplast as an important tool 
in studying the activity of photosynthetic organisms.  

WHAT IS COLOR?
	Before I explain what happened, just a reminder about COLOR.  If 
you know about how we see color, skip to the lower section.  COLOR IS 
LIGHT.  We see color because our eyes can detect (and tell apart) a range 
of radiation in the so-called "visible" spectrum (which contains radiation 
with wavelengths of approximately 400-700 nm.).  They can only detect light 
or color if radiation reached the eyes.  An image is formed after light is 
reflected and refracted from  an object in a way that depends on its shape 
and "color".
	When all the wavelengths of light are combined they form white 
light.  For us to see an individual color, two things can happen.  We can 
see a color if only one wavelength of light it shining toward our eyes or 
if white light bounced off an object and all the OTHER colors were absorbed 
by the object.  So your red shirt is not red because there is something 
inherently red in the fabric.  It is because there is a pigment that 
absorbs all the blue, violet, green ... etc. light and all that is left to 
be reflected back to your eyes is the red light.
	
LIGHT CONTAINS ENERGY
	There are many other kinds of radiation.  Some have shorter 
wavelengths (X-rays and ultraviolet rays), some have longer wavelengths 
(infrared, rays TV, radio and microwaves).  Think about the fact that 
somewhere between the X-rays that pass through flesh and microwaves that 
cook flesh, in visible light.  The energy it contains is significant.
	 Of the colors we see, there is a range of wavelengths.  This is 
important because the wavelength and the amount of energy contained in the 
light are related.  Light with shorter wavelengths (blue) has more energy 
than light with longer wavelengths (red).  X-rays have very short (less 
than 1 nm), high energy wavelengths, which is why they can penetrate flesh. 
Light also travels in packets of energy called PHOTONS.  It is a bit 
difficult to imagine, but this is what gives light the properties of both 
waves and particles.  So a "packet" of blue light contains more energy than 
a "packet" of red light.

PLANTS ABSORB LIGHT ENERGY
	Now... the way plants interact with light is different from us.  
Plant chloroplasts contain pigments that actually ABSORB light (rather than 
passively detecting it).  These pigments absorb a lot of colors of light, 
in fact there are pigments in plants that absorb every color, but the color 
that plant pigments absorb least is ... green.  The energy contained in 
that light is what drives photosynthesis.  It drives photosynthesis because 
plants have PHOTOSYSTEMS that change light energy into chemical energy.  
Photosystems can only use RED LIGHT. 
	When  light strikes a pigment, the energy "excites" the molecule.  
Molecules tend to return to their stable state, which means the energy must 
be given off.  When pigments give off energy it can be in the form of light 
-- but ALWAYS at a LONGER WAVELENGTH.  Pigments cannot give off higher 
energy light than they receive.  So through a series of steps and pigments, 
all the different colors of light absorbed are changed to lower-energy RED 
LIGHT. 
	How coincidental that plant photosystems use only red light!  This 
is because ALL photosystems contain the same single pigment -- 
chlorophyll-a.  All photosynthetic organisms -- from blue-green algae 
(cyanobacteria) through algae and seaweeds, all the way up to the great 
trees -- contain photosystems with chlorophyll-a.  The other pigments 
capture energy from many colors of light, but must transfer it to the 
photosystems in the form of red light.  These pigments are called "antenna 
pigments". 

FINALLY,  HERE IS THE ANSWER
	Imagine this...  A plant cell is exposed to very bright light, much 
brighter than usual.  It absorbs that light energy using the many pigment 
molecules it contains.  All those pigments work to transfer all that energy 
to the photosystems.  But now there is too much light.  All those pigments 
are absorbing white light and shifting it down to red light, but there 
aren't enough photosystems to absorb all that red light.  The pigments must 
get rid of the energy, but it can't be transferred to the "full" 
photosystems.  So they give off the excess energy as red light.
	  The light from a projector is controlled in a narrow beam. The 
parts of the vial in the projector beam received too much light and 
fluoresced red.  The other areas remained green because they did not 
receive too much light.  Plants do this all the time!  It is common for 
plants to receive too much light during midday of sunny days.  We cannot 
see the fluorescence because the red light is swamped by all the white 
light around us.  You could see the fluorescence because the white light of 
the projector did not interfere.  It did not interfere with your eyes 
ability to detect the red fluorescence because that narrow beam was shining 
in a different direction, not into your eyes.  But the pigments were giving 
off red light is ALL directions.  Some of that red fluorescence was 
radiating in the direction of your eyes.  That is why you saw red.