• If you are citizen of an European Union member nation, you may not use this service unless you are at least 16 years old.

  • You already know Dokkio is an AI-powered assistant to organize & manage your digital files & messages. Very soon, Dokkio will support Outlook as well as One Drive. Check it out today!

View
 

The wave model (2013)

Page history last edited by Matt Harrington 9 years, 12 months ago Saved with comment

Working Content

As a wave, light can be described by its wavelength, speed and frequency.  These are  related by 

c = λ f

with c being the velocity in meters per second, λ being the wavelength in meters and f being the frequency in Hertz (1 Hz = 1 wave cycle per second).


Wavelengths

Light is part of the electromagnetic spectrum which includes everything from gamma rays to infrared radiation.  For biological applications, we typically think about light as that in the visible part of the spectrum.  Visible light has wavelengths of 400 to 700 nm.  However, visible light is very much defined based on our human biases since humans can not detect light shorter than 400 nm or longer than 700 nm.  By contrast, many organisms, from birds to bees, have the ability to detect light in the ultraviolet part of the spectrum between 340 and 400 nm.  In addition, some snakes can detect infrared light using thermal sensitive pits on their faces.  So the range of wavelengths that animals use is broader than we typically define as “visible”.


Light wavelengths are quite small compared to those of sound.  A typical sound frequency would be that of the note that an orchestra uses for tuning, A 440 (A with a frequency of 440 Hz).  The wavelength corresponding to A440 is 0.77 m.  This wavelength is 1-2 million times larger than that of visible light.


Speed

One of the nice absolutes in life is that the speed of light is a constant and nothing can go faster than light.  In a vacuum, the speed of light is 2.998 x 108 m/s.  However, light does actually slow down when it moves through matter.  The decreased speed in air is quite negligible but it becomes more significant in water or solids.  The ratio of the speed of light in a vacuum to that in a material is called the index of refraction, n.

            n = c / v

where c is the speed of light in a vacuum and v is the speed in the material.  A typical value of the index of refraction for water is 1.33 and for glass is 1.55.  So the speed of light in water is only 75% of what it is in a vacuum while in glass it moves only 65% as fast.

 

Compared to sound, light moves very fast.  In room temperature air, sound moves at 340 m/s.  Light is moving about a million times faster than sound.  One interesting difference between sound and light is the change in speed on entering a material.  Sound is a pressure wave.  When the molecules get closer together, such as in a liquid as compared to air, sound actually speeds up.  This is opposite to light which slows down.


Frequency

Light frequencies are determined by the speed and wavelengths from ν = c / λ. For visible wavelengths of 350 and 700 nm, the frequencies are 8.6 x 1014 Hz and 4.3 x 1014 Hz. These numbers are quite large and unwieldy and that is why we typically describe light by wavelength and not frequency.  Sound on the other hand is frequently described in terms of frequency.  Light frequencies are 1012 times larger than those in the middle range of a piano (200-800 Hz).

Comments (0)

You don't have permission to comment on this page.