Physics Questions and Answers

Answer: C

The relation is velocity = frequency × wavelength.

\[v = f\lambda\] In a given medium the speed is fixed, so frequency and wavelength vary inversely. When a wave passes into another medium the frequency stays the same - it is set by the source - and the wavelength changes with the speed.

Answer: A

A mirage is due to unequal heating of different parts of the atmosphere.

Hot ground warms the air just above it, making it optically rarer, so a gradient of refractive index forms. Light from the sky curves gradually downward through these layers and is finally totally internally reflected, so it appears to come from the ground.

Answer: C

Because they produce perfectly homogeneous solutions.

Ultrasound sets up rapid pressure oscillations that create and collapse microscopic bubbles - cavitation - and the resulting microjets mix the liquid down to a very fine scale, far more thoroughly than mechanical stirring.

Answer: C

Infrared rays are electromagnetic.

They are oscillating electric and magnetic fields and travel through vacuum at \(3\times10^{8}\ \mathrm{m\,s^{-1}}\). Sound and ultrasonic waves are mechanical and need a medium; cathode rays are a stream of electrons, which are particles.

Answer: C

Water is densest at 4 °C.

Below that temperature the hydrogen-bonded open structure of ice begins to form, and the molecules move slightly further apart, so cooling from 4 °C to 0 makes water less dense. This anomalous expansion is why ice floats and why a lake freezes from the surface down, leaving fish alive beneath.

Answer: B

Rectifiers convert alternating current to direct current.

A diode conducts in one direction only, so it passes half the AC cycle and blocks the other. A bridge of four diodes gives full-wave rectification, and a capacitor smooths the pulses into steady DC. The reverse conversion is done by an inverter.

Answer: B

The water level will decrease.

In the boat, each rock displaces its own weight of water; sunk on the bottom, it displaces only its own volume. Since the rocks are denser than water, the volume is the smaller of the two, so the total displacement falls and the level drops.

Answer: A

The spectrum of an oil flame is a continuous emission spectrum.

Incandescent solids and liquids - the glowing soot particles in a flame, a lamp filament, molten metal - emit at every wavelength, so the spectrum shows no gaps. A gas at low pressure gives a line spectrum instead, characteristic of its atoms.

Answer: C

Because the pressure of the skate melts the ice by lowering its melting point.

Water is unusual in expanding when it freezes, so pressure favours the denser liquid phase and the melting point falls. A thin film of water forms under the blade and acts as a lubricant, refreezing once the skate has passed.

Answer: C

The sound comes from air rushing in to fill the vacuum inside the bulb.

The bulb is either evacuated or filled with gas at low pressure. When the glass breaks, atmospheric air rushes in at high speed and the sudden compression produces the pop - an implosion rather than an explosion.