Physics Questions and Answers

Answer: C

Because the refractive index of the different layers of the atmosphere keeps changing.

A star is effectively a point source, so its light passes through a narrow column of air. Turbulent cells of differing density bend the ray slightly and randomly, and the apparent position and brightness flicker. This is called atmospheric scintillation.

Answer: B

The pascal is the unit of pressure.

\[1\ \mathrm{Pa} = 1\ \mathrm{N\,m^{-2}}\] It is a small unit - atmospheric pressure is about \(10^{5}\) Pa - so the bar and the kilopascal are often used instead. Thrust is simply force, measured in newtons.

Answer: D

A mirage is called mrigatrishna, the deer's thirst, in Sanskrit.

The name comes from the deer running towards what looks like water on hot ground and never reaching it. The effect is caused by refraction through air layers of different density and total internal reflection.

Answer: B

The boat does not move.

The fan pushes air backwards and the air pushes the fan - and hence the boat - forwards; but the moving air then strikes the sail and pushes it backwards with an equal force. The two internal forces cancel, and with no external force the boat stays put. Removing the sail would let it move.

Answer: A

One thousand microns is \(10^{-3}\) m.

\[1000 \times 10^{-6}\ \mathrm{m} = 10^{-3}\ \mathrm{m}\] which is one millimetre. The micron, now called the micrometre, is \(10^{-6}\) m.

Answer: D

Light travels fastest in a vacuum.

Its speed there is \(3\times10^{8}\ \mathrm{m\,s^{-1}}\); in a medium it is \(c/n\), always less because \(n>1\). It slows to about \(2\times10^{8}\) in glass and \(2.25\times10^{8}\) in water, and that slowing is what produces refraction.

Answer: D

The work done is the same, but the rate of doing it is less when rolling.

Raising the stone through a height \(h\) takes \(mgh\) of work against gravity by either route. What changes is the force: on an incline of angle \(\theta\), only \(mg\sin\theta\) has to be overcome, applied over the longer distance \(h/\sin\theta\). The smaller force spread over more time is what makes it feel easier.

Answer: D

A rainbow is due to refraction and reflection of sunlight by water droplets.

Light entering a raindrop is refracted and dispersed into colours, reflected once off the inside of the far surface, then refracted again on leaving. It emerges at about 42° to the incoming ray, which fixes the radius of the bow.

Answer: D

Mica is used in the electrical industry.

It splits into thin transparent sheets, withstands high temperature, and is an excellent insulator with a high dielectric strength, so it is used in capacitors and as insulation in heating elements. India has among the world's largest reserves, in Jharkhand and Bihar.

Answer: B

Both acquire the same momentum.

By the impulse-momentum theorem \(Ft = \Delta p\), so equal forces acting for equal times give equal changes in momentum whatever the masses. Their velocities, accelerations and kinetic energies all differ.