State the principle,which helps us to determine the shape of the wavefront at a later time from its given shape at any time.Apply this principle to
Show that a spherical/plane wavefront continues to propagate forward as a sherical/plane wave front.

State the principle,which helps us to determine the shape of the wavefront at a later time from its given shape at any time.Apply this principle to Derive Snell's law of refraction by drwing the refracted wavefront corresponing to a plane wavefront incident on the boundary separating a rarer medium from a denser medium.

A plane wave front AB propagating from denser medium (1)into a rarer medium (2) is incident on the surface P_1P_2 separating the two media as shown in the figure. Using Huygen's principle, draw the secondary wavelets and obtain the refracted wave front in the diagram.

The shells of crabs found around a particular coastal location in Japan seem mostly to resemble the legendary face of a Samurai. Given below are two explanations of this observed fact. Which of these strikes you as a scientific explanation? - (a) A tragic sea accident several centuries ago drowned a young Samurai. As a tribute to his bravery, nature through its inscrutable ways immortalised his face by imprinting it on the crab shells in that area. (b)After the sea tragedy, fishermen in that area, in a gesture of honour to their dead hero, let free any crab shell caught by them which accidentally had a shape resembling the face of a Samurai. Consequently, the particular shape of the crab shell survived longer and therefore in course of time the shape was genetically propagated. This is an example of evolution by artificial selection.

The principle of 'parallax' is used in the determination of distance of nearby stars. The role of distant object O there is now taken by nearby distant stars. The baseline AB is the line joining the Earth's two locations six months apart in its orbit around the Sun. That is the baseline is about the disameter of the earth's orbit = 3 xx 10^11 m. However, even the nearest stars are so distant that with such a long baseline theyshow parallax only of the order 1 s of arc or so. A parsec is a convenient unit of length on the astronomical sxale. It is the distance of an object that will show a parallar of 1s of arc from opposite ends of a baseline equal to the distance from the Earth to the Sum How much is a parsec in terms of metre?

A coil in the shape of an equilateral triangle of side 0.02 m is suspended from a vertex such that it is hanging in a vertical plane between the pole pieces of a permanent magnet producing a horizontal magnetic field of 5 xx 10^(-2) T. Find the couple acting on the coil when a current of 0.1 A is passed through it and the magnetic field is parallel to its plane.

Fig. below shows planer loops of different shapes moving out or into a region of magnetic field, which is directed normal to the plane of the loop and away from the reader. Determine the direction of induced current. in each loop using Lenz's law.