General Science Previous Year Questions (PYQ) with Answers – Page 15

A telescope's objective must gather as much light as possible from a distant object and form a large image, so it is both wide and of long focal length.

X-rays are electromagnetic waves made of uncharged photons, so electric and magnetic fields cannot bend them. Electric and magnetic fields exert force only on charged particles: a charge q feels a force qE in an electric field and qvB in a magnetic field.

Twenty crests and twenty troughs make twenty complete waves in 0.1 second, so the frequency is 200 Hz and the wavelength is 330 ÷ 200 = 1.65 m. One complete wave, or one cycle, contains one crest and one trough (for sound, one compression and one rarefaction).

X-rays have wavelengths from about 0.01 nm to 10 nm, so 1 nm lies well inside the X-ray band. In the electromagnetic spectrum, wavelength decreases from radio waves through microwaves, infrared, visible light and ultraviolet to X-rays and gamma rays.

One pascal is defined as a force of one newton acting on an area of one square metre. Pressure is force per unit area, and its SI unit, named after the French scientist Blaise Pascal, is the pascal (Pa): 1 Pa = 1 N/m².

When a conductor moves in a magnetic field and a current is induced in it, this rule gives the direction of that induced current.

Gravity is a conservative force, so the work it does never depends on the path. The work done by gravity on a body of mass m is mgh, where h is the vertical height between the start and the end.

One kelvin and one degree Celsius are the same size, so a rise of 30 K is also a rise of 30°C. The Kelvin and Celsius scales differ only in their starting point: T(K) = t(°C) + 273 (more exactly 273.15).

Force depends on how fast momentum changes, not on the momentum itself. Newton's second law states that the rate of change of momentum of a body is proportional to the net force on it and takes place in the direction of the force.

One parsec equals about 2,06,265 astronomical units, which rounds to 2 × 10⁵ A.U.

It gives the direction of the force on a current-carrying conductor placed in a magnetic field. Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular.

Power is the reciprocal of focal length in metres, and a concave lens has a negative focal length. The power of a lens is P = 1/f, with f in metres, and its unit is the dioptre (D).

Copper turns green because its surface slowly reacts with the air and gets covered by a layer of corrosion products, and that layer is green.

One mole of any gas occupies 22.4 litres at STP, so 5.6 litres is 5.6 divided by 22.4, that is one quarter of a mole.

Concentration tells how much solute is present in a given amount of solution, and the three standard percentage forms are mass by mass, mass by volume and volume by volume; volume by mass is not one of them.

The Tyndall effect is the scattering of a beam of light by particles suspended in a medium, and it needs particles large enough to scatter that light.

A haloalkane carries a halogen atom - fluorine, chlorine, bromine or iodine - bonded to a carbon chain, so no oxygen appears in its functional group at all.

C3H4 is propyne, and it fits the alkyne general formula CnH2n-2 when n equals 3. Alkynes are unsaturated hydrocarbons that carry at least one carbon-to-carbon triple bond, and the first member of the family is ethyne, C2H2, better known as acetylene and burnt with oxygen in welding torches.

A covalent bond is formed when two atoms share one or more pairs of electrons so that each completes its outermost shell.

A metal loses electrons and a non-metal gains them, so the transfer makes oppositely charged ions that attract each other, and that attraction is an ionic bond.