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- Physics 1C Lecture 24A Spectrum of EM Waves There are distinct forms of EM waves at different frequencies(and wavelengths Recall that the wave

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Physics 1C Lecture 24A

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Spectrum of EM Waves There are distinct forms of EM waves at different frequencies (and wavelengths). Recall that the wave speed is given by: v wave = c = λ f . Wavelengths for visible light range between 400nm (violet) and 700nm (red). There is no sharp division between one kind of EM wave and the next. For example, you can have an X-ray and a Gamma Ray with the exact same wavelength.
EM Spectrum Note the overlap between types of waves (such as UV and X-rays). All EM waves have the same speed in a vacuum, what distinguishes the types are their frequencies or wavelengths. Note that the visible section is a quite small portion of the spectrum. f n = v l = nv 4 L

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EM Spectrum Wavelengths of light can range from very long (radio, ~100km) to very short (gamma, ~1fm). Frequencies have an equally long range of possible values: (gamma, ~10 22 Hz) to (radio, ~10Hz). Visible light ranges from Red (700nm, 4x10 14 Hz) to Violet (400nm, 7x10 14 Hz) f n = v l = nv 4 L
EM Spectrum Radio waves have a long wavelength (~100m) and thus are good for use as a communication tool (TV, AM, FM). Microwaves are smaller (~1cm) and interfere easily with common things (μwave oven grates). Infrared waves are produced by hot objects. f n = v l = nv 4 L

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EM Spectrum Visible light (~500nm) is detected by the human eye. We are most sensitive to yellow-green (560nm). UV light (~100nm) that comes from the Sun is mostly absorbed by the Earth’s ozone layer. f n = v l = nv 4 L
EM Spectrum X-rays (~0.1nm) are associated with fast electrons hitting off of a metal target (medical applications). Gamma rays (~1fm) are emitted by radioactive nuclei. They can cause serious damage to living tissue as they penetrate deeply into most matter. f n = v l = nv 4 L

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Spherical Waves A spherical wave propagates radially outward from the source (for instance, your cell phone). The energy propagates equally in all directions. The intensity is: The average power is the same through any spherical surface centered on the source. Intensity will decrease as r increases.
Cell Phone Intensity Example A cell phone emits 0.60Watts of 1.9GHz radio waves. What are the amplitudes of the electric and magnetic fields at a distance of 10cm? Answer Assume the cell phone is a point source of electromagnetic waves (or r = 0).

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Cell Phone Intensity Answer The intensity of the radio waves at 10cm is: We want the maximum values (amplitudes) for the electric and magnetic fields.
Cell Phone Intensity Answer For magnetic field we can turn to:

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The amplitude of the oscillating electric field at your cell phone is 8 μ V/m when you are 10km from the broadcast antenna. What is the electric
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This note was uploaded on 01/30/2012 for the course PHYS 1C 1C taught by Professor Wethien during the Spring '11 term at UCSD.

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- Physics 1C Lecture 24A Spectrum of EM Waves There are distinct forms of EM waves at different frequencies(and wavelengths Recall that the wave

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