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Electromagnetic Waves | MCAT 2015 Electromagnetic Waves • Suppose we have an alternating current running through a wire • If we graphed the electric field generated by this current, it’d peak where the current is moving fastest, and it’d be 0 when the current stops to change direction • • Together, the two waves make up an electromagnetic wave Light travels via this type of wave E o c= B o c = speed of light E = electric field B = magnetic field The Electromagnetic Spectrum • Visible light makes up a small segment of the spectrum of electromagnetic waves o ~ 400-700 nm Electric Field of an AC Current over Time At maximum velocity, the current generates its maximum electric field. When the current halts, the electric field becomes 0. • • We know that an electric field generates a magnetic field o Thus, when the electric field is at a maximum, so is the magnetic field o And, when the electric field is 0, so is the magnetic field We could plot the magnetic field on the z-axis of the graph Electric and Magnetic Fields of AC Current over Time The electric and magnetic field peak and bottom out at the same points. The Electromagnetic Spectrum Energy and frequency increase to the right of visible light. Wavelength increases to the left. You can remember this if you know that the sun’s UV rays have so much energy that they burn your skin. Speed of Electromagnetic Waves • As with other waves, we can use v = λf • In a vacuum, light’s speed is a constant, c, 3x108 m/s • Like other waves, light changes velocity in different media o To help predict this, we assign an index of refraction, n, to media c o n= v o n = index of refraction c = speed of light in vacuum 1 © 2017 J Co Review, Inc., Accessed by Guest on 05-12-2017 Electromagnetic Waves | MCAT 2015 • v = speed of light in medium o Thus, the index of refraction is a ratio of light’s speed in a vacuum to light’s speed in the given medium In any medium, the medium’s atoms will absorb some of the light’s energy, thereby slowing it Light Being Slowed by Water Molecules • Since a vacuum is atom-free, light will travel quickest through a vacuum o Since c will always exceed v, n must always be greater than 1 o The greater n is, the slower light travels in that medium 2 © 2017 J Co Review, Inc., Accessed by Guest on 05-12-2017