91. 92. One beam of coherent light travels path P in arriving at point Q and another coherent beam (from same source) travels path P2 in arriving at the same point. If these two beams are to interfere destructively, the path difference P - P2 must be equal to ---- A) zero B) an odd number of half-wavelengths C) a whole number of wavelengths D) a whole number of half-wavelengths A student prepared a Young's double slit by drawing two parallel lines (with a separation less than a millimeter) on a smoked glass plate. [For smoking, the plate was held above the flame of a kerosene lamp]. Accidentally he used two different pins to draw the lines so that the widths of the slits (regions from where smoke was removed by the pin) were in the ratio 4:1. What will be the ratio of the intensity at the interference maximum to that at the interference minimum? A) 4:1 B) 2:1 C) 3:1 D) 9:1 93. In metals according to the classical theory the electrical conductivity is related to mass of electron 'm', charge 'e', its concentration 'n', and collision time 't' by the relation A) mnet B) me t/m C) ne t/m D) ne t/m 94. If the overlap of electronic wave functions centered on neighboring atoms, in a crystal, is small, the effective mass of the electron *m** A) will be very large as compared to mass of the free electron B) will be negative C) will be very small as compared to mass of free electron D) will be equal to the mass of free electron 95. The "Normal" and "Anomalous" Zeeman effect are observed when (Note: S' is the total spin angular momentum due to the coupling of individual spin angular momentum) A) S = 0 and S #0 respectively C) S 0 and S'=0 respectively B) S=0 and S = 0 respectively D) S' 0 and S' 0 respectively