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1 General Properties of Electromagnetic Radiation Lecture 1

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2 Electromagnetic radiation is looked at as sinusoidal waves which are composed of a combination of two fields. An electric field (which we will use, in this course, to explain absorption and emission of radiation by analytes) and a magnetic field at right angle to the electric field (which will be used to explain phenomena like nuclear magnetic resonance in the course of special topics in analytical chemistry offered to Chemistry students only).

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3 The classical wave model The classical wave model describes electromagnetic radiation as waves that have a wavelength, frequency, velocity, and amplitude. These properties of electromagnetic radiation can explain classical characteristics of electromagnetic radiation like reflection, refraction, diffraction, interference, etc. However, the wave model can not explain the phenomena of absorption and emission of radiation.

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4 We will only deal with the electric field of the electromagnetic radiation and will thus refer to an electromagnetic wave as an electric field having the shape of a sinusoidal wave. The arrows in the figure below represent few electric vectors while the yellow solid sinusoidal wave is the magnetic field associated with the electric field of the wave.

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5 Wave Properties of Electromagnetic Radiation

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6 Wave Parameters 1. Wavelength ( ) The wavelength of a wave is the distance between two consecutive maxima or two consecutive minima on the wave. It can also be defined as the distance between two equivalent points on two successive maxima or minima. This can be seen on the figure below:

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8 2. Amplitude (A) The amplitude of the wave is represented by the length of the electrical vector at a maximum or minimum in the wave. In the figure above, the amplitude is the length of any of the vertical arrows perpendicular to the direction of propagation of the wave.

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9 3. Frequency The frequency of the wave is directly proportional to the energy of the wave and is defined as the number of wavelengths passing a fixed point in space in one second. 4. Period (p) The period of the wave is the time in seconds required for one wavelength to pass a fixed point in space.

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10 5. Velocity (v) The velocity of a wave is defined as the multiplication of the frequency times the wavelength. This means: V = The velocity of light in vacuum is greater than its velocity in any other medium

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11 Since the frequency of the wave is a constant and is a property of the source, the decrease in velocity of electromagnetic radiation in media other than vacuum should thus be attributed to a decrease in the wavelength of radiation upon passage through that medium.

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13 6. Wavenumber ( ) The reciprocal of wavelength in centimeters is called the wavenumber. This is an important property especially in the study of infrared spectroscopy.

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