Q. 144.5( 2 Votes )
Let us list some of the factors, which could possibly influence the speed of wave propagation:
(i) Nature of the source.
(ii) Direction of propagation.
(iii) Motion of the source and/or observer.
(v) Intensity of the wave.
On which of these factors, if any, does
(a) The speed of light in vacuum,
(b) The speed of light in a medium (say, glass or water), depend?
(a) The speed of light in vacuum i.e. 3 × 108 m/s is universal constant and thus, it’s value stays the same in vacuum as there are no interacting particles, It is not effected by (i) the nature of the source, (ii) Direction of propagation (iii) Motion of the source or observer (iv) Wavelength and (v) Intensity of the wave.
Thus, the given factors will have no effect on the speed of light in vacuum.
(b) Since, the factors such as (i) the nature of the source (ii) the direction of propagation (iv) wavelength change the intrinsic property of the light but other factors given are external factors and does not affect the propagation property of light. Thus, The speed of light in a medium like glass or water is influenced only by (i) (ii), (iv).
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The optical properties of a medium are governed by the relative permittivity (εr) and relative permeability (μr ). The refractive index is defined as For ordinary material εr > 0 and μr > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with εr < 0 and μr < 0. Since then such ‘metamaterials’ have been produced in the laboratories and their optical properties studied. For such materials As light enters a medium of such refractive index the phases travel away from the direction of propagation.
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