Photobiology for Kids


What is Photobiology?

Photobiology is the study of the interactions of light with living organisms. Photobiologists study animals, plants, fungi, and microbes that respond to light.
Vision sense of sight, mediated by the eye and brain of animals.
Photosynthesis conversion of solar energy into biochemical energy by green plants and certain microbes.
Ultraviolet Effects biological effects of ultraviolet radiation, such as damage of DNA.
Circadian Rhythms biological rhythms that have periods of about 24 hours and can be altered by exposure to light.
Bioluminescence emission of light from biochemical reactions within living organisms, such as the flash of a firefly.

It seems like light has lots of different effects on different organisms. But what exactly is light?

Light can be defined as "electromagnetic radiation that is visible". But now we need to define electromagnetic radiation!

This is a bit more difficult. Let's start by saying that light from the sun or a simple light bulb is called "white" light. When a beam of white light passes through a simple prism, it is dispersed into different colors, called the color spectrum. A rainbow shows the same dispersion of colors.
      Rainbow
Left, Light beam passing through a prism (from Exploratorium). Right, Rainbow.
The colors in the spectrum always appear in exactly the same order. An easy way to remember the order of colors is the name ROY G BIV. Each letter in this name is the first letter of a color in the spectrum.
RED - ORANGE - YELLOW - GREEN - BLUE - INDIGO - VIOLET

Why does a prism or a rainbow separate white light into different colors?

The white light from the sun or a light bulb is really a mixture of many different colors or wavelengths of light. In fact, light has many of the same characteristics as waves of water.
     
Just as the crests of water waves can be close together or far apart, the crests of light waves can be close together or far apart. The wavelength is simply the distance in which a wave repeats itself. Red light, which is only moderately refracted (deflected) by a prism, has a long wavelength. Violet light, which is highly refracted, has a short wavelength.
       

OK. Now can you explain what you mean when you say that light is "visible electromagnetic radiation"?

Sure. Visible electromagnetic radiation has a wavelength between about 0.000000400 meters (400 nanometers, nm) and 0.000000700 meters (700 nm). The wavelength of light is typically expressed in nanometers simply because it's much easier to write "400 nm" than "0.000000400 meters". Light with a wavelength of 400 nm is violet light and light with a wavelength of 700 nm is red light.

The wavelength of light can also be expressed using "scientific notation". If you are not familiar with scientific notation, click HERE

But electromagnetic radiation can have a wavelength shorter than 400 nm and longer than 700 nm, even though we cannot see outside this range. As shown in the picture below, visible light constitutes only a very small part of the overall electromagnetic spectrum.


The Electromagnetic Spectrum (adapted from NASA)
But electromagnetic radiation can have a wavelength shorter than 400 nm and longer than 700 nm, even though we cannot see outside this range. As shown in the picture below, visible light constitutes only a very small part of the overall electromagnetic spectrum.

This is a very detailed picture. Can you explain it for me?

Sure! Let me explain it row-by-row.

The TOP TWO ROWS show the wavelengths that penetrate the earth's atmosphere. You can see that radiowaves and visible light easily penetrate the earth's atmosphere. Microwaves and Infrared radiation also penetrate the atmosphere, although to a lesser extent. You can also see that light (visible electromagnetic radiation) constitutes a very small part of the electromagnetic spectrum.
The SECOND TWO ROWS show some common objects of different sizes within this range. A large office building can be 102 m (100 meters) high. A bacterium is about 10-6 m (0.000001 meters) in diameter. The nucleus of an atom is about 10-12 m (0.000000000001 meters) in diameter.
The BOTTOM ROW shows the frequency that is associated with each wavelength in the top row. The frequency is simply the number of wavelengths that pass a point in a second and is expressed in Hertz (Hz), in honor of the German physicist Heinrich Rudolph Hertz (1857-1894). One MegaHertz (MHz) is equivalent to one million Hertz (1 Mhz = 106 Hz).
You can see that frequency increases from left to right, whereas the wavelength decreases from left to right. In fact, since the speed of radiation is known (299,792,458 meters per second, or 186,282.397 miles per second), you can always determine the frequency if you know the wavelength. Just use this equation:

Speed of Light / Wavelength = Frequency
or more simply: c / λ = f

Activity

Use the Frequency Calculator (javascript) to calculate the frequency for different wavelengths of light.
In addition, it is easy to determine the energy of radiation if you know the frequency, from the equation:

Energy = Planck's constant × Frequency
or more simply: E = h × f
where h = 6.626 × 10-34 Joule sec (Planck's constant, in honor of German physicist Max Planck)

Activity

Use the Energy Calculator (javascript) to calculate the energy for different frequencies of light.

Thanks! Now I have a much better understanding of light and electromagnetic radiation. How can I learn more about the specific topics studied by photobiologists?

Try visiting these external web sites:
Photobiological Sciences Online [advanced]:
(free internet-based textbook on photobiology, sponsored by the American Society for Photobiology)

Optical Illusions & Visual Phenomena [intermediate]
("Illusions of the senses tell us the truth about perception" - Jan Purkinje)

Photosynthesis [elementary]
(how plants make chemical energy from sunlight)

Skin Cancer Foundation [elementary]
(protection from sun burn and skin cancer)

Biological Rhythms [elementary]
(simple classroom experiments on circadian rhythms)

The Bioluminescence Web Page [intermediate]
(lots of info with many photos and videos)