The speed of light

Light waves can propagate through air, sound, water, vacuum (image montage)Light waves can travel through space as well as many gases, liquids, and solids. The ability to propagate in a vacuum (space) makes light waves different from sound waves, which require matter (such as air) to propagate. Another big difference is speed. The speed of sound can be estimated by timing an echo with a stopwatch. Light is about a million times faster than sound and it takes sensitive experiments to measure the speed of light. Read the text aloud
Comparing properties of sound and lightIf you compare light and sound, you notice that light travels far faster than sound. The wavelength of visible light is the size of atoms whereas the wavelength of sound is the size of macroscopic objects. The frequency of light is trillions of hertz compared to audible sound, which ranges from 20 to 20,000 Hz. Read the text aloud
The frequency, wavelength, and speed of light are related by equation (22.1), which is similar to the relationship for other waves. The speed of light (or c) is the product of frequency and wavelength. Read the text aloud
(22.1) c=fλ
c  = speed of light (m/s) = 3.00×108 m/s in a vacuum
f  = frequency (Hz)
λ  = wavelength (m)
Speed of light
The speed of light is usually assumed to be in a vacuum, e.g., in outer space where there is no air or other gas. Just as the speed of sound changes depending on the material it is traveling through (such as air or water in the ocean), the speed of light differs when it travels through different materials. Light travels slightly slower when it passes through air and significantly slower in water. But the frequency of the light stays the same, even when it travels from one material to another. Read the text aloud
Calculate the frequency of visible light (in a vacuum) if the wavelength is 500 nm.
Asked: frequency f of the light
Given: wavelength of the light, λ = 500 nm; speed of light, c = 3×108 m/s
Relationships: c = f λ
Solution: First convert from nanometers to meters

500 nanometers = 5 × 10-7 m


Then, solve for frequency and substitute values. f= c λ = 3× 10 8  m/s 5× 10 7  m =6× 10 14 Hz
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What is the wavelength of an x-ray traveling with a frequency of 3×1018 Hz?
  1. 10−26 m
  2. 10−10 m
  3. 1010 m
  4. 1026 m
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