The origins of quantum mechanics
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص243-244
2025-11-19
38
The origins of quantum mechanics
The basic principles of classical mechanics are reviewed in Appendix2. In brief, they show that classical physics (1) predicts a precise trajectory for particles, with precisely specified locations and momenta at each instant, and (2) allows the translational, rotational, and vibrational modes of motion to be excited to any energy simply by controlling the forces that are applied. These conclusions agree with everyday experience. Everyday experience, however, does not extend to individual atoms, and careful experiments of the type described below have shown that classical mechanics fails when applied to the transfers of very small energies and to objects of very small mass. We shall also investigate the properties of light. In classical physics, light is described as electromagnetic radiation, which is understood in terms of the electromagnetic field, an oscillating electric and magnetic disturbance that spreads as a harmonic wave through empty space, the vacuum. Such waves are generated by the acceleration of electric charge, as in the oscillating motion of electrons in the antenna of a radio transmitter. The wave travels at a constant speed called the speed of light, c, which is about 3 × 108 m s−1. As its name suggests, an electromagnetic field has two com ponents, an electric field that acts on charged particles (whether stationary or moving) and a magnetic field that acts only on moving charged particles. The electromagnetic field is characterized by a wavelength, λ (lambda), the distance between the neighbouring peaks of the wave, and its frequency, ν (nu), the number of times per second at which its displacement at a fixed point returns to its original value (Fig. 8.1).
The frequency is measured in hertz, where 1 Hz = 1 s−1. The wavelength and frequency of an electromagnetic wave are related by
λν=c
Therefore, the shorter the wavelength, the higher the frequency. The characteristics of the wave are also reported by giving the wavenumber, # (nu tilde), of the radiation, where
Wavenumbers are normally reported in reciprocal centimetres (cm−1). [8.2] Figure 8.2 summarizes the electromagnetic spectrum, the description and classification of the electromagnetic field according to its frequency and wavelength. White light is a mixture of electromagnetic radiation with wavelengths ranging from about 380 nm to about 700 nm (1 nm = 10−9 m). Our eyes perceive different wavelengths of radiation in this range as different colours, so it can be said that white light is a mixture of light of all different colours. The wave model falls short of describing all the properties of radiation. So, just as our view of particles (and in particular small particles) needs to be adjusted, a new view of light also has to be developed.
Fig. 8.1 The wavelength, λ, of a wave is the peak-to-peak distance. (b) The wave is shown travelling to the right at a speed c. At a given location, the instantaneous amplitude of the wave changes through a complete cycle (the four dots show half a cycle). The frequency, ν, is the number of cycles per second that occur at a given point.
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