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Lecture 4sf

# Lecture 4sf - What is meant by and what is the difference...

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What is meant by and what is the difference between: Classical Mechanics Quantum Mechanics Classical Mechanics, sometimes called Newtonian Mechanics, is what is traditionally studied in physics. Its laws and equations describe inclined planes, gravitational acceleration, projectile motion, astronomical motions, harmonic oscillators, and a variety of other motions and interactions. Quantum Mechanics studies motion and interaction on the atomic level. In chemistry, this particularly means studying electrons in atoms: their locations and energies. Why is quantum mechanics necessary? Certain observations and experiments were not consistent with and could not be explained by classical mechanics. The experiments which led to the development of quantum mechanics are all related to the interaction of light and matter. To understand these interactions, we must have some background on the nature of waves. A wave is a disturbance of a medium. Dropping a pebble in water, pushing a string up and down--these disturbances propagate through the medium as a wave. We can characterize the wave by: λ = wavelength = distance between each wave unit ν = frequency = the number of waves (cycles) which pass a given point each second Units of λ : meter, mm, nm (length of each wave) Units of ν : sec -1 Hz (all per sec) The “cycles” or “waves” are understood when we say sec -1 A fundamental relationship for all waves is:

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λ x ν = speed of wave x = Light can be considered to be a wave phenomenon in which there does not have to be a medium. Light can travel through the vacuum of space. Light can be thought of as vibrating electric and magnetic fields, with the planes of these fields both being perpendicular to the velocity of the light. This is called electromagnetic radiation. When we are describing electromagnetic radiation, the speed of the radiation in a vacuum is always a constant, designated by the letter c (the speed of light). To three significant figures: c = 3.00 x 10 8 For electromagnetic radiation: λν = c Electromagnetic radiation encompasses radio waves, infrared, visible light, ultraviolet, x- rays and gamma-rays. Visible light is a relatively small portion of this spectrum, with wavelengths ranging from 400 nm to 700 nm. Only during this range can we use the radiation to see with. Radio | Infrared |Vis| UV | X-ray ---------> increasing frequency <---------- increasing wavelength Within the visible range, we perceive different wavelengths as different colors, ranging from red (700 nm) to violet (400 nm), and covering the colors of the rainbow: red, orange, yellow, green, blue, violet. |red orange yellow green blue violet | λ 700 nm 400 nm ν 4.3 x 10 14 s -1 7.5 x 10 14 s -1 ---------> increasing frequency <---------- increasing wavelength Calculate the frequency of light for a photon of violet light having wavelength 400 nm
λν = c ν = λ c ν =  9 - 8 10 400 10 3.00 x x = 7.5 x 10 14 sec -1 How does this frequency compare with radio waves?

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Lecture 4sf - What is meant by and what is the difference...

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