11. Frequently Asked Facts — Rapid Revision

The following one-line facts are drawn from patterns commonly seen in SSC (CGL, CHSL, MTS, GD) and RRB (NTPC, Group D) General Science sections. Reading through this list just before the exam is a useful last-minute revision exercise.

• The speed of light in vacuum is approximately 3 × 10⁸ m/s, the fastest speed possible in the universe.
• Among visible colours, violet has the shortest wavelength and highest frequency/energy; red has the longest wavelength and lowest frequency/energy.
• Violet light deviates (bends) the most, and red light deviates the least, when passed through a glass prism.
• The unit of power of a lens is the dioptre (D); P = 1/f with f in metres.
• A convex lens is also called a converging lens; a concave lens is also called a diverging lens.
• A straw (or pencil) appears bent at the point where it enters water because light rays from the submerged part of the straw bend (refract) as they pass from water (denser) into air (rarer) on their way to the observer's eye, so the brain perceives the submerged portion as displaced from its true position.
• A concave mirror is used in a torch, headlight, and search-light to produce a powerful parallel beam of light (source placed at the focus).
• A convex mirror is used as a rear-view mirror in vehicles because it gives a wider field of view (though objects appear smaller and closer than they really are).
• The refractive index of diamond is about 2.42, the highest among common natural transparent substances, giving it its critical angle of about 24° and its brilliant sparkle via total internal reflection.
• Optical fibres work on the principle of total internal reflection.
• Myopia (short-sightedness) is corrected using a concave lens; hypermetropia (long-sightedness) is corrected using a convex lens.
• The near point of a normal human eye is 25 cm, and the far point is at infinity.
• Presbyopia is corrected using bifocal lenses; it is caused by age-related weakening of the eye's accommodation ability.
• Cataract, a clouding of the eye lens, is treated by surgery, not by spectacles.
• The sky appears blue due to the scattering of sunlight by atmospheric molecules (Rayleigh scattering), which scatters shorter (blue) wavelengths more strongly than longer (red) wavelengths.
• Sunrise and sunset appear reddish-orange because sunlight travels a longer path through the atmosphere at these times, scattering away most of the blue light and leaving red/orange light to dominate.
• The Tyndall effect refers to the scattering of light by colloidal particles, making the path of a light beam visible.
• VIBGYOR is the standard mnemonic for the seven colours of the visible spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
• X-rays were discovered by Wilhelm Röntgen; gamma rays are emitted by radioactive nuclei and are the most penetrating and energetic form of EM radiation.
• Ultraviolet rays are mostly absorbed by the ozone layer; excessive UV exposure can cause skin cancer and cataracts.
• A rainbow forms due to refraction, dispersion, and total internal reflection of sunlight in water droplets suspended in the air.
• The image formed by a plane mirror is always virtual, erect, laterally inverted, and of the same size as the object.
• A simple microscope is a single convex lens of short focal length; a compound microscope uses two convex lenses (objective and eyepiece).
• An astronomical telescope forms an inverted final image; a terrestrial telescope forms an erect final image using an extra erecting lens or prism system.
• The SI unit of luminous intensity is the candela (cd); the SI unit of luminous flux is the lumen (lm); the SI unit of illuminance is the lux (lx).
• The photoelectric effect, explained by Einstein using the particle (photon) nature of light, earned him the Nobel Prize in Physics in 1921.
• Light shows both wave nature (interference, diffraction, polarization) and particle nature (photoelectric effect) — this is called wave-particle duality.
• Polarization proves that light is a transverse wave, since only transverse waves can be polarised.
• A periscope uses two plane mirrors (or prisms) each inclined at 45° to allow viewing over obstacles, as in submarines.

End of Notes — Volume 3: General Science, Light and Optics. Students are advised to revise the tables (electromagnetic spectrum, mirror/lens image formation, eye defects, and the formula reference) multiple times, as these form the backbone of both conceptual and numerical questions asked on this topic in SSC and RRB examinations.