General Science Previous Year Questions (PYQ) with Answers – Page 44

Starlight has to cross many layers of air before it reaches the eye, and these layers keep changing in temperature and density, so their refractive index keeps changing too.

Vitamin A, chemically retinol, is needed to make rhodopsin, the light-sensitive pigment in the rod cells of the retina that lets us see in dim light.

Lead has 82 protons in its nucleus, so its atomic number is 82. Its symbol is Pb, from the Latin plumbum, the root of the word "plumber", because Roman water pipes were made of lead.

Water is densest at about 4 degrees Celsius, and 4 plus 273 gives roughly 277 kelvin. Most liquids keep shrinking as they cool, but water behaves oddly: below 4 degrees Celsius it starts to expand again because its molecules begin forming the open, hexagonal pattern of ice.

Zinc has atomic number 30, meaning every zinc atom has 30 protons in its nucleus. It is the last element of the first transition series in period 4 of the modern periodic table, with the electronic configuration [Ar] 3d10 4s2, and it is placed in group 12.

Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.

Friction is a non-conservative force because the work it does depends on the path taken and the energy it removes is lost as heat.

A plane mirror always forms an upright image of the same size as the object, and the image cannot be caught on a screen.

In uniform circular motion the acceleration points towards the centre of the circle, at right angles to the velocity. In uniform circular motion the speed stays the same, but the direction of motion changes at every point, so the velocity keeps changing and the body is accelerating.

Slaked lime on the wall slowly absorbs carbon dioxide from the air and turns into a thin, shiny layer of calcium carbonate.

The more a colour is slowed by glass, the higher the refractive index for it and the more it bends.

Thrust is a force and impulse is force multiplied by time, so thrust divided by impulse has the unit 1/second (s⁻¹), which is the unit of frequency.

Sodium carbonate, called washing soda in its hydrated form Na₂CO₃·10H₂O, removes both temporary and permanent hardness of water. Hard water contains dissolved calcium and magnesium salts, which stop soap from lathering.

A soda-acid extinguisher holds a solution of sodium hydrogen carbonate (baking soda) and a small bottle of sulphuric acid; when the two mix, carbon dioxide is released.

Using kinetic energy = ½mv² with the mass in kilograms gives a speed of exactly 10 m/s. First convert the mass to SI units: 2000 g = 2 kg.

Cardiac muscle, found only in the wall of the heart, is made of cylindrical cells that branch and join with one another, and each cell usually has one central nucleus.

Using v² = u² − 2gh with final velocity zero at the top, u = √(2gh) = √(2 × 10 × 20) = √400 = 20 m/s. A ball thrown straight up slows down under gravity until its velocity becomes zero at the highest point.

Sound is a mechanical wave, so it needs a medium, and in air its particles vibrate along the direction of travel.

Planaria is a free-living flatworm, while the other three are parasites that live inside a host. Planaria belongs to the phylum Platyhelminthes and lives in freshwater ponds and streams, gliding under stones and feeding on small animals.

Faraday's law is a law of magnitude, not a direction rule. It states that the emf induced in a circuit is proportional to the rate at which the magnetic flux through it changes.