General Science

Atomic clocks use the frequency of atomic vibrations (typically cesium-133) to measure time with extreme precision - accurate to within one second in millions of years. They are far more precise than quartz clocks or astronomical methods.

Gauss is the CGS unit of magnetic induction (magnetic flux density). It is named after mathematician Carl Friedrich Gauss. Note: option B has a typo ('Gress' instead of 'Gauss') but the claimed answer is still correct.

Jet engines work on Newton's third law (action-reaction) which is based on conservation of linear momentum. The engine expels mass (high-velocity exhaust gases) backward, generating forward thrust. This is fundamentally about linear momentum change.

Fish plates are metal bars used to join the ends of two adjacent rails together in a railway track. They maintain continuity and alignment of the rail while allowing for expansion joints. They do not join coaches (couplers do that) or guide wheels (flanges do that).

Sodium vapor lamps produce nearly monochromatic yellow light (589 nm) with very high luminous efficacy - around 100-200 lumens per watt compared to 10-20 lumens per watt for incandescent lamps. This gives them much higher intensity of illumination for the same power consumption.

Rectifiers convert alternating current (A.C.) to direct current (D.C.) using diodes that allow current flow in only one direction.

Correct Answer: conducting the lightning safely to the ground

Hooke's law states that the extension of a spring is directly proportional to the applied force, which is the principle behind spring balances.

A periscope uses mirrors or prisms to reflect light, allowing submarine sailors to view objects on the surface while submerged.

Galvanometers measure direct current. AC would cause the needle to vibrate rapidly around zero, making reading impossible.

The piezoelectric effect generates an electric charge when mechanical stress is applied to certain crystals like quartz, providing precise timekeeping.

Safety wires fuse at low temperatures to break electrical circuits during dangerous current surges, preventing overheating and potential fires.

Biogas production requires anaerobic conditions where bacteria break down organic matter without oxygen, with optimal efficiency around 40°C.

A tubelight contains two filaments (for starting), fluorescent material coating (to convert UV to visible light), and mercury vapor (to produce UV when ionized).

Diesel engines use compression ignition - air is compressed to such high pressure and temperature that fuel auto-ignites without spark plugs.

Correct Answer: electro-magnetic waves of short wavelength

A bimetallic strip bends when heated because the two metals expand at different rates - it bends toward the metal with lower thermal expansion.

A vacuum (thermos) flask is designed to keep hot liquids hot and cold liquids cold. Essentially, we need to reduce the transfer of thermal energy by conduction, convection and radiation. The vacuum flask consists of a double-walled glass container, with a vacuum between the walls. Both walls are silvered on the vacuum side. The container is supported on foam plastic, which is a poor conductor of thermal energy. No thermal energy can enter or leave the flask by conduction or convection across the vacuum. The inner silvered surface reflects radiation from hot fluids back into the flask. The outer silvered surface reflects radiation in the external surroundings away from the flask. The foam plastic support and the plastic cap also minimise the thermal energy transmitted by conduction through the thin glass walls of the flask. Lastly, the plastic cap stops convection and evaporation.

Correct Answer: storage battery

A thermostat is a temperature-regulating device that maintains a constant temperature in systems like water baths or ovens by switching heating on/off.

A theodolite is a precision optical instrument for measuring angles in horizontal and vertical planes. It is primarily used by surveyors for tasks like land surveying, construction layout, and infrastructure development. While pilots, navigators, and cartographers use other tools, theodolite is specifically the surveyor's instrument for angular measurements.

Decibel (dB) is a unit that is used to measure intensity of sound level.

 Option 4 is correct.