Which of the following units is used for the measurement of Luminous Intensity?

The correct answer is A, Night blindness. 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. When the vitamin is short, the eye cannot adjust to darkness, and the person cannot see properly at night or in poor light; this is night blindness (nyctalopia). If the deficiency continues, the eye becomes dry and the cornea is damaged, a condition called xerophthalmia, which can end in total blindness. Vitamin A is fat-soluble and comes from carrots, papaya, mango, green leafy vegetables, milk, eggs and fish liver oil. B is wrong because scurvy, with bleeding gums, is caused by lack of vitamin C. C is wrong because marasmus is caused by a severe shortage of both calories and protein in young children. D is wrong because kwashiorkor is a protein deficiency disease marked by a swollen belly. Exam tip: vitamin A night blindness, C scurvy, D rickets, K bleeding; marasmus and kwashiorkor are protein-energy problems, not vitamin diseases.

The correct answer is A, B3. Pellagra is caused by a lack of vitamin B3, also called niacin or nicotinic acid. The body can also make niacin from the amino acid tryptophan, so the disease is common where people live mainly on maize, which is poor in both. Pellagra is remembered by the 'four Ds': dermatitis (rough, dark skin on parts exposed to the sun), diarrhoea, dementia and, if untreated, death. Niacin is found in meat, fish, groundnuts, whole grains and milk. B is wrong because vitamin B6 (pyridoxine) deficiency causes anaemia, skin problems and convulsions in infants. C is wrong because vitamin B2 (riboflavin) deficiency causes cracked lips and sores at the corners of the mouth, called ariboflavinosis. D is wrong because vitamin B12 (cobalamin) deficiency causes pernicious anaemia and nerve damage. Exam tip: B1 beriberi, B2 ariboflavinosis, B3 pellagra, B12 pernicious anaemia; pellagra = the four Ds.

The correct answer is C, N2O. Laughing gas is nitrous oxide, N2O, a colourless gas with a faintly sweet smell that makes people giggle and feel light-headed when they breathe it in. It was first prepared by Joseph Priestley in 1772, and Humphry Davy studied its effects and gave it the popular name. Dentists and surgeons use it mixed with oxygen as a mild anaesthetic and pain-killer, and it is also the propellant in whipped-cream cans. In the laboratory it is made by gently heating ammonium nitrate. It is a greenhouse gas too, and it damages the ozone layer. A is wrong because NO is nitric oxide, a colourless gas that turns brown in air. B is wrong because N2O3 is dinitrogen trioxide, a blue liquid at low temperature. D is wrong because NO2 is nitrogen dioxide, the reddish-brown, poisonous gas seen in vehicle smoke and acid rain. Exam tip: laughing gas = nitrous oxide = N2O, discovered by Priestley, used as a dental anaesthetic.

The correct answer is B, SF6. The name itself gives the formula: 'hexa' means six, so sulphur hexafluoride has one sulphur atom joined to six fluorine atoms, SF6. The molecule is octahedral in shape, with the six fluorine atoms arranged symmetrically around the sulphur. SF6 is a colourless, odourless, non-flammable and very unreactive gas, about five times heavier than air. Because it does not conduct electricity, it is used as an insulating gas inside high-voltage circuit breakers and switchgear. It is also one of the strongest greenhouse gases known, with a global warming potential thousands of times that of carbon dioxide, and it is listed under the Kyoto Protocol. A is wrong because S2F6 would need two sulphur atoms, which the name does not say. C is wrong because SF2 is sulphur difluoride, with only two fluorine atoms. D is wrong because S6F reverses the numbers and is not a real compound. Exam tip: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6; SF6 = one sulphur, six fluorine, used in electrical switchgear.

The correct answer is C, Pascal. Pressure is force acting per unit area, so its SI unit is the newton per square metre, which is given the special name pascal (Pa) after the French scientist Blaise Pascal, who stated the law of transmission of pressure in liquids used in hydraulic brakes and lifts. One pascal is a very small pressure, so larger units are common: 1 bar = 10⁵ Pa, 1 atmosphere = 101,325 Pa = 760 mm of mercury, and weather reports use the hectopascal (1 hPa = 100 Pa), while blood pressure is still read in mm Hg. Option A is wrong because the volt is the SI unit of electric potential difference (1 V = 1 J/C). Option B is wrong because the ampere is the SI base unit of electric current. Option D is wrong because the ohm is the SI unit of electrical resistance (1 Ω = 1 V/A). Exam tip: pressure = force ÷ area, N/m² = pascal; and the electrical trio: volt for voltage, ampere for current, ohm for resistance.