Name the three different modes of propagation in a communication system. State briefly why do the electromagnetic waves with frequency range from a few MHz up to 30 MHz can reflect back to the earth. What happens when the frequency range exceeds this limit?

In Young’s double slit experiment, the two slits are separated by a distance of 1 �5 mm and the screen is placed 1 m away from the plane of the slits. A beam of light consisting of two wavelengths 650 nm and 520 nm is used to obtain interference fringes. Find

the distance of the third bright fringe for λ = 520 nm on the screen from the central maximum.

the least distance from the central maximum where the bright fringes due to both the wavelengths coincide.

(a) The figure shows the plot of binding energy (BE) per nucleon as a function of mass number A. The letters A, B, C, D and E represent the positions of typical nuclei on the curve. Point out, giving reasons, the two processes (in terms of A, B, C, D and E), one of which can occur due to nuclear fission and the other due to nuclear fusion.

(b) Identify the nature of the radioactive radiations emitted in each step of the decay process given below.

Define the terms ‘stopping potential’ and ‘threshold frequency’ in relation to photoelectric effect. How does one determine these physical quantities using Einstein’s equation?

Three circuits, each consisting of a switch ‘S’ and two capacitors, are initially charged, as shown in the figure. After the switch has been closed, in which circuit will the charge on the left-hand capacitor (i) increase, (ii) decrease and (iii) remain same? Give reasons.