General Science MCQs – Page 11 (Q101-110)

The speed of light in a vacuum is approximately 3 x 10 to the 8 meters per second (about 299,792,458 m/s precisely), a universal constant denoted by the symbol c in physics equations, most famously in Einstein's mass-energy equivalence formula E = mc squared. This value forms the basis of the modern definition of the meter, since one meter is defined as the distance light travels in 1/299,792,458 of a second. Nothing with mass can reach or exceed this speed according to special relativity, though light slows slightly when passing through denser media like water or glass, which is what causes refraction. This figure appears frequently in the physics and general science sections of CSS, PMS, and NTS tests.

The speed of sound in air at room temperature (around 20 to 25 degrees Celsius) is approximately 343 meters per second, or roughly 1,235 kilometers per hour, a value that increases with temperature and varies depending on the medium. Sound travels faster through denser, more elastic media, moving about four times faster in water than in air and even faster still through solid steel, since molecules are packed closer together and transmit vibrations more efficiently. This is why the speed of sound is often mistakenly assumed to be constant, when in fact humidity, temperature, and altitude all cause it to vary. Aircraft speeds are often expressed as a Mach number, a ratio to the local speed of sound.

Sound cannot travel through a vacuum because it is a mechanical wave that requires a material medium, solid, liquid, or gas, whose particles vibrate and collide to transmit the wave's energy from one point to another. In the vacuum of outer space there are essentially no particles to carry these vibrations, which is why explosions in science-fiction films are dramatized with sound despite space actually being silent. This contrasts sharply with electromagnetic waves like light, radio, and X-rays, which do not need a medium and travel through the vacuum of space perfectly well, which is how sunlight reaches Earth across roughly 150 million kilometers of empty space.

Light travels vastly faster than sound, moving at about 300,000 kilometers per second in a vacuum compared to sound's roughly 343 meters per second in air, a difference of nearly a million times. This disparity is easily observed during a thunderstorm, where lightning is seen instantly while the accompanying thunder arrives several seconds later, allowing a rough distance estimate of about one kilometer for every three seconds of delay. The two also differ fundamentally in nature: light is a transverse electromagnetic wave that needs no medium, while sound is a longitudinal mechanical wave that requires matter to propagate, which is why sound cannot travel through the vacuum of space.

The law of reflection states that the angle of incidence, measured between the incoming ray and the normal, an imaginary line perpendicular to the reflecting surface, always equals the angle of reflection, measured between the outgoing ray and that same normal. This principle holds for both smooth, regular reflection off mirrors, called specular reflection, and the diffuse reflection off rough surfaces like paper, though diffuse reflection scatters light in many directions because the surface's microscopic irregularities create varying normals at each point. The law of reflection, together with the law of refraction known as Snell's law, underlies the design of mirrors, periscopes, telescopes, and countless other optical instruments.

Refraction occurs when light passes from one transparent medium into another of different optical density, causing it to change speed and consequently bend its path at the boundary, unless it strikes the surface exactly perpendicular. This bending happens because light travels at different speeds in different media, quantified by each material's refractive index, with denser media like glass or water slowing light down more than air does. Refraction explains why a straw appears bent when placed in a glass of water, why lenses can focus or spread light, and why a swimming pool appears shallower than it actually is. It is governed quantitatively by Snell's law.

A convex lens, also called a converging lens, is thicker at its center than at its edges and causes parallel light rays passing through it to bend inward and converge at a single point called the focal point. This converging property makes convex lenses useful in magnifying glasses, cameras, projectors, and the corrective lenses used for farsightedness, since they can focus light that would otherwise converge behind the retina back onto it. The distance from the lens to the focal point is called the focal length, and shorter focal lengths produce stronger convergence. The human eye's natural lens is also convex, helping focus light onto the retina.

A concave lens, also called a diverging lens, is thinner at its center than at its edges and causes parallel light rays passing through it to spread outward, or diverge, as if originating from a virtual focal point on the same side as the incoming light. Because it spreads light rather than focusing it, a concave lens always produces a smaller, upright, virtual image, making it useful for correcting myopia, or nearsightedness, where it diverges light slightly before it enters the eye so the eye's own lens can properly focus it on the retina. Concave lenses are also used in some telescope and binocular designs alongside convex lenses.

Convex mirrors are used as vehicle side mirrors because their outward-curving reflective surface diverges light rays, producing a smaller, upright virtual image that captures a much wider field of view than a flat mirror of the same size could show. This wider view helps drivers see more of the road and reduces blind spots, though it comes with the well-known trade-off of making objects appear farther away than they actually are, hence the common warning printed on such mirrors. Convex mirrors are also used in shops for security surveillance and at blind road corners, where a wide-angle view of oncoming traffic matters more than accurate distance perception.

Concave mirrors are used in torches and vehicle headlights because their inward-curving reflective surface can take light diverging from a source placed at their focal point and reflect it outward as a strong, nearly parallel beam, maximizing the light's throw distance and intensity. This is the reverse of how a concave mirror is used to focus incoming parallel rays, such as sunlight, to a single point, which is the principle behind solar cookers and reflecting telescopes. Concave mirrors are also used as shaving and makeup mirrors, where holding the face close to the mirror produces a magnified, upright image useful for close-up detail.