Physics Questions and Answers

Answer: A

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

Air close to hot ground becomes less dense and optically rarer, so a vertical gradient of refractive index is set up. Light from the sky bends gradually as it descends through the layers and is finally reflected totally, giving the appearance of water on the road.

Answer: D

A reactor is critical when it is ready to produce controlled energy.

Critical means the neutron multiplication factor \(k = 1\): exactly one neutron from each fission goes on to cause the next, so the chain reaction is self-sustaining at a steady rate. Below one it dies away; above one the power rises. The word carries no suggestion of danger.

Answer: B

The light year is a unit of distance.

It is how far light travels in a year, about \(9.46\times10^{15}\) m. Despite the word 'year' in the name it measures length, not time.

Answer: B

An object with constant speed might be accelerated.

Acceleration is the rate of change of velocity, which is a vector. A body moving in a circle at constant speed is changing direction continuously, so it is accelerating - towards the centre. Only constant velocity guarantees zero acceleration.

Answer: C

The image on the retina is real and inverted.

The eye's lens is convex and the object lies beyond its focal length, so the rays actually converge on the retina, forming a real image, and it is upside down and left-right reversed. The brain learns to interpret it the right way up.

Answer: A

The primary colours of light are red, green and blue.

They correspond to the three types of cone cell in the eye, and mixing them in the right proportions gives any colour, including white - which is how television and computer screens work. In pigments the primaries are different: cyan, magenta and yellow, which mix by subtraction.

Answer: A

Compressions and rarefactions characterise longitudinal waves.

The particles oscillate along the direction of travel, so regions of high and low density follow each other. Transverse waves have crests and troughs instead, with particles moving at right angles to the direction of travel.

Answer: A

The acceleration is constant in magnitude and directed radially inwards.

In uniform circular motion the speed does not change, so the magnitude \[a = \frac{v^2}{r}\] is fixed, but the direction always points at the centre and therefore turns with the body. It is constant in size and variable in direction - the reverse of what one might guess.

Answer: A

The P wave indicates atrial depolarisation.

It is the first deflection of the ECG and represents the spread of electrical activity across the atria, which then contract. The QRS complex is ventricular depolarisation and the T wave ventricular repolarisation; atrial repolarisation is hidden within the QRS.

Answer: B

The red ray is deviated least.

Deviation depends on refractive index, and \(n\) falls as wavelength rises - Cauchy's relation. Red has the longest visible wavelength, so glass bends it least; violet, at the short end, is bent most. That difference in deviation is dispersion, and is what spreads the light into a spectrum.