MadSci Network: General Biology

Re: How many fireflies does it take to produce enough light to read a book?

Date: Wed Apr 12 08:12:59 2006
Posted By: Tye Morancy, Staff, Medical Physicist/Dosimetrist, Beth Israel Deaconess Medical Center
Area of science: General Biology
ID: 1143425774.Gb

Thank you Amber for the question,

Fireflies or lightning bugs make light within their bodies.  This process 
is called bioluminescence and is shared by many other organisms, mostly 
sea-living or marine organisms.  To do this, the fireflies contain 
specialized cells in their abdomen that make light. 

The cells contain a chemical called luciferin and make an enzyme called 
luciferase.  To make light, the luciferin combines with oxygen to form an 
inactive molecule called oxyluciferin.  Luciferin, a heat resistant
substrate, is the source of light; luciferase, an enzyme, is the trigger; 
and oxygen is the fuel.  A body chemical, ATP (adenosine triphosphate), 
converts to energy and causes the luciferin-luciferase mixture to light 

The wavelength of light given off is between 510 and 670 nanometers, which 
lies in the visible range of what our eyes can detect, bascially a pale 
yellow to reddish green color.  The cells that make the light also have 
uric acid crystals in them that help to reflect the light away from the 

The amount of light is a little tricky as it depends on the firefly.  So 
let's use a good example of one to arrive at an answer.  The Pyralis 
firefly (also known as the lightning bug) is a common firefly in North 
America.  This partly nocturnal, luminescent beetle is the most common 
firefly in the USA.  

The brightness of a single firefly is 1/40 of a candle.  Males flash about 
every five seconds; females flash about every two seconds.  

Lets' do some definitions now……

We're in the United States, so we are going to talk about units of 
measurement that concern distance in feet and inches.  We're going to talk 
about "foot-candles".

As asimple example, get a birthday cake candle, a ruler, and stick the 
candle on one end of the ruler.  Light the candle and turn out the lights. 
One foot-candle of light is the amount of light that birthday cake candle 
generates one foot away.  So, to go further, say you have a lamp.  You are 
told it produces 100 foot candles of light.  That means at one foot from 
the lamp, you will receive 100 foot candles of light.

But here's where it gets tricky. The further away you move the light from 
what you want to illuminate, the less bright the light seems.  If you 
measure it at the light, it's just as bright.  But when you measure at the 
object you want illuminated, like the book you want to read, there is less 

Now, lets look at another unit of measure called a "lumen".

It measures light much the same way.  Remember, a foot-candle is how 
bright the light is one foot away from the source.  A lumen is a way of 
measuring how much light gets to what you want to light up.  A lumen is 
equal to one foot-candle falling on one square foot of area.

So, to continue the candle example, we will now take the candle and ruler 
and place a book at the opposite end for the ruler from the candle.  If 
that book happens to be one foot by one foot, it's one square foot.  Now, 
all the light falling on that book, one foot away from your candle equals 
both…….1 foot candle and 1 lumen.

Now, I am assuming that, though it might be tough, you could read that 
book with the candle a foot away.  Thus, if we take that measurement of 
1/40 of a candle of light being put out by a common firefly, then it would 
take 40 of them to produce enough light to read the book at any one time.

I hope this answered your question.


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