chem-201-chapter-18-20

chem-201-chapter-18-20 - 10/31/11 Light Light (cont.) We...

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10/31/11 1 Light We can use different terms to describe light: D Color D Wavelength D Frequency Light is composed of electromagnetic waves that travel through some medium. The properties of the medium determine how light travels through it. In a vacuum, light waves travel at a speed of 3.00 x 10 8 m/s or 186,000 miles/s. The speed of light in a vacuum is a constant that is tremendously important in nature and science—it is given the symbol, c. Light (con t.) Because light behaves like a wave, we can describe it in one of two ways—by its wavelength or by its frequency. λ = wavelength—distance between two adjacent wave crests. λ has units of distance—frequently nanometers (nm). ν = frequency—how many times the wave goes up and down in a period of time. n has units of inverse time (1/s Hz[hertz]). λ Light (con t.) If you know either the frequency or the wavelength, you can calculate the other quantity through the relationship: c = λ I ν c = speed of light (3.00 x 10 8 m/s) λ = wavelength (m) ν = frequency (s -1 ) A particle of light is called a photon. Light (con t.) The type of light (ultraviolet, visible, infrared, x-ray, etc.) is defined by either its frequency or wavelength: 10 3 m 10 2 Hz wavelength frequency gamma rays x-rays ultraviolet infrared microwave radio waves 10 -5 nm 10 -3 nm 1 nm 400 nm 700 nm 10 3 nm 10 6 nm 1 m 10 24 Hz 10 16 Hz 10 12 Hz 10 10 Hz 10 8 Hz 10 6 Hz 10 4 Hz 10 22 Hz 10 20 Hz 10 18 Hz visible light 7 x 10 14 Hz 4 x 10 14 Hz Light (con t.) The energy of light can be determined either from its wavelength or frequency: ν = λ = h E or c h E Planck s constant: h = 6.626 x 10 -34 J s Examples 4.3 x 10 13 Hz ( ν ) light (6980 nm = 6.98 μ m): E = (6.626 x 10 -34 J s)(4.3 x 10 13 s -1 ) = 2.85 x 10 -20 J = 17.2 kJ mol -1 670 nm ( λ ) diode laser: E = (6.626 x 10 -34 J s)(3.00 x 10 8 m s -1 ) = 2.97 x 10 -19 J (670 x 10 -9 m) = 179 kJ mol -1
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10/31/11 2 Examples (con t.) Violet light from a mercury lamp has a wavelength of 436 nm: E = (6.626 x 10 -34 J s)(3.00 x 10 8 m s -1 ) = 4.56 x 10 -19 J (436 x 10 -9 m) = 275 kJ mol -1 D Atoms and molecules absorb and emit light in the ultraviolet (UV), visible (vis), infrared (IR), and microwave ( μ wave) regions of the electromagnetic spectrum. D Absorption or emission of light in the UV and vis regions involves movement of electrons in the atom or molecule. D One reason UV light is so damaging is that the light has enough energy to break chemical bonds—biological and chemical systems D E ( λ = 300 nm) = 399 kJ mol D Average bond energy = 380 kJ mol -1 Spectroscopy P o P b dx light source sample cell detector The intensity of light entering the cell is P o . Some of the light is absorbed by the sample.
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chem-201-chapter-18-20 - 10/31/11 Light Light (cont.) We...

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