Rainbows show how visible light is a combination of many colors.
UCAR
Visible light is one way energy moves around. Light waves are the result of vibrations of electric and magnetic fields, and are thus a form of electromagnetic (EM) radiation. Visible light is just one of many types of EM radiation, and occupies a very small range of the overall electromagnetic spectrum but because we can see light with our eyes, it has special significance to us.
Light waves have wavelengths between about 400 and 700 nanometers (4,000 to 7,000 angstroms). Our eyes perceive different wavelengths of light as the rainbow hues of colors. Red light has relatively long waves, around 700 nm long. Blue and purple light have short waves, around 400 nm. Shorter waves vibrate at higher frequencies and have higher energies. Red light has a frequency around 430 terahertz, while blue's frequency is closer to 750 terahertz. Red photons of light carry about 1.8 electron volts (eV) of energy, while each blue photon transmits about 3.1 eV.
Visible light's neighbors on the EM spectrum are infrared radiation on the one side and ultraviolet radiation on the other. Infrared radiation has longer waves than red light, and thus oscillates at a lower frequency and carries less energy. Ultraviolet radiation has shorter waves than blue or violet light, and thus oscillates more rapidly and carries more energy per photon than visible light does.
Light travels at a speed of 299,792 kilometers per second (about 186,282 miles per second). At this speed, light could circle Earth more than seven times in one second! The lowercase letter "c" is often used to represent the speed of light in equations, such as Einstein's famous relation between energy and matter: E = mc2. All forms of electromagnetic waves, including X-rays and radio waves and all other frequencies across the EM spectrum, also travel at the speed of light. Light travels most rapidly in a vacuum, and moves slightly slower in materials like water or glass.
When light passes from one material to another material with a different density, is usually bends or changes course. Different colors of light bend by slightly different amounts. When blue light passes from air through a dense glass prism, for example, it bends slightly more than red light does. This is why a prism breaks white light up into a rainbow of different colors. Raindrops can become natural prisms, causing rainbows when sunlight passes through.
© 2018 UCAR with portions adapted from Windows to the Universe (© 2005 NESTA)
Electromagnetic waves are categorized according to their frequency f or, equivalently, according to their wavelength λ = c/f. Visible light has a wavelength range from ~400 nm to ~700 nm. Violet light has a wavelength of ~400 nm, and a frequency of ~7.5*1014 Hz. Red light has a wavelength of ~700 nm, and a frequency of ~4.3*1014 Hz.
Visible light makes up just a small part of the full electromagnetic spectrum. Electromagnetic waves with shorter wavelengths and higher frequencies include ultraviolet light, X-rays, and gamma rays. Electromagnetic waves with longer wavelengths and lower frequencies include infrared light, microwaves, and radio and television waves.
| gamma-rays | 1020 - 1024 | < 10-12 m |
| x-rays | 1017 - 1020 | 1 nm - 1 pm |
| ultraviolet | 1015 - 1017 | 400 nm - 1 nm |
| visible | 4 - 7.5*1014 | 750 nm - 400 nm |
| near-infrared | 1*1014 - 4*1014 | 2.5 μm - 750 nm |
| infrared | 1013 - 1014 | 25 μm - 2.5 μm |
| microwaves | 3*1011 - 1013 | 1 mm - 25 μm |
| radio waves | < 3*1011 | > 1 mm |
Problem:
Two microwave frequencies are authorized for use in microwave ovens, 900 and 2560 MHz. Calculate the wavelength of each.
Solution:
- Reasoning:
For all electromagnetic waves in free space λf = c. - Details of the calculation:
λ = c/f.
f = 900*106/s, λ = (1/3) m
f = 2560*106/s, λ = 11.7 cm.
Problem:
Distances in space are often quoted in units of light years, the distance light travels in one year.
(a) How many meters is a light year?
(b) How many meters is it to Andromeda, the nearest large galaxy, given that it is 2.54*106 light years away?
(c) The most distant galaxy yet discovered is 12*109 light years away. How far is this in meters?
Solution:
- Reasoning:
All electromagnetic waves in free have speed c. - Details of the calculation:
(a) 1 light year (ly) = distance light travels in one year
= (3*108 m/s)*(365*24*3600 s) = 9.46*1015 m.
(b) The distance to Andromeda is 2.54*106 ly * 9.46*1015 m/ly = 2.4*1022 m.
(c) The distance to this galaxy is 12*109 ly * 9.46*1015 m/ly = 1.14*1026 m.
Spectroscopy:
What can we learn by analyzing the EM spectrum emitted by a source?
The velocities of particles with thermal energy are changing almost all the time. The particles are accelerating. Accelerating charged particles produce electromagnetic radiation. The power radiated is proportional to the square of the acceleration. Higher rates of velocity change result in higher frequency (shorter wavelength) radiation. The observed intensity of thermal radiation emitted by as a function of wavelength can be described by the Planck Radiation Law (Physics 221).
The Planck Radiation Law gives the intensity of radiation as a function of wavelength for a fixed temperature. The Planck law gives a continuous distribution, which peaks at some wavelength. The peak shifts to shorter wavelengths for higher temperatures, and the area under the curve grows rapidly with increasing temperature. The diagram below shows the intensity distribution predicted by the Plank law in J/(m2s) for blackbodies at various temperature. By observing the continuous distribution of the thermal radiation emitted by an object, we can learn its temperature.
When light passes through or reflects or scatters of matter, it interacts with the atoms and molecules. Atoms and molecules have characteristic resonance frequencies. The preferentially interact with light waves of exactly those frequencies. When excited in collisions, atoms and molecules emit light with a set of characteristic frequencies. This results in a line spectrum. Only light with a discrete set of wavelengths is produced and the spectrum is not continuous, but consist of a set of emission lines. That set characterizes the atoms and molecules which produced it and can be used to identify those atoms and molecules and their environment.
When light with a continuous distribution of wavelengths passes through a low-density material, the atoms and molecules of the material absorb light waves with the same set of characteristic frequencies that appear in their emission spectrum. This produces an absorption spectrum, a nearly continuous spectrum with missing lines. The
absorption spectrum can also be used to identify those atoms and molecules and their environment.
Embedded Question 3
Please explore this simple simulations of various molecules interacting with electromagnetic radiation of different wavelength.
//phet.colorado.edu/en/simulations/molecules-and-light
Can you identify certain characteristics?
Discuss this with your fellow students in the discussion forum!