A spectroscope separates light from a source into its component colors. Light is a form of electromagnetic (EM) radiation whose color depends on its wavelength. Although the speed of light is a constant, the wavelength, frequency and photon energy are not.
The distance (meters) between wave crests is its wavelength. The speed (hertz) at which the same point on the wave passes by a point is the frequency. Photon energy (electron-volts) is not surprisingly a measure of how much energy per photon is in the light.
Out of the visible colors, red has the longest wavelength (the length between the top of each wave), the slowest frequency (how often the wave hits the top), and the least amount of photon energy. These values shift as a gradient through the different colors of red, orange, yellow, green, blue, and finally, violet which has the shortest wavelength, highest frequency, and most amount of photon energy out of the visible colors¹.
Approximate values by color²:
Red 625-740 nm
Orange 590-624 nm
Yellow 570-589 nm
Green 495-569 nm
Blue 450-494 nm
Violet 380-449 nm
What color a light emits is important to plants in part because photosynthesis is important to plants and it is powered best by light with wavelengths from peaks at the red and blue bands. Plants don’t use a whole lot of green light, which is why most of it is reflected³.
While sunlight provides a full spectrum of colors, grow lights show distinct spikes that show as color bands that are observable once the colors are separated. The unassisted human eye is not particularly well suited to discriminating exactly which colors are being emitted by a given light source. Fortunately, a primitive spectroscope can be made with a prism or a diffraction grating, and some creativity.
To see color separation with a homemade spectroscope:
- The source light must pass through a narrow slit into relative darkness. This can be a slit in a cardboard tube or box, or a door just barely cracked open enough to let a sliver of light pass. This ensures that only parallel light waves pass onto the next step.
- The light is then passed from the slit through a prism or diffusion grating.
- After the light has been separated into its component colors by the prism or diffusion grating, the different colors can be observed.
Although there are more sophisticated spectroscopes that can be purchased or made, they all work on a variation of this primitive example.
- There are electromagnetic waves with a longer wavelength than red (infrared, microwave, radio waves, and low-frequency waves) and shorter than violet (ultraviolet, x-ray and gamma rays).
- These values are approximate as the colors are along a gradient and there is disagreement as to exactly where one color stops and the next starts.
- Most green light is reflected by green plants, which is why they appear green.