Match List–I with List–II and select the answer using the code given below the Lists : List–I (Device) A. Periscope B. Lightning conductors C. Thermos flask D. Levers List–II (Phenomenon) 1. Conduction and convection minimization 2. Moment of a force 3. Reflection by plane mirrors 4. Static electricity Code :

Correct — C, the code A-3, B-4, C-1, D-2. A periscope uses two plane mirrors set parallel at forty-five degrees to the line of sight, so the phenomenon is reflection by plane mirrors, item 3. A lightning conductor works by static electricity: the sharp point concentrates charge and encourages a discharge along a safe path to earth, so item 4. A thermos flask is built to stop heat leaving — a vacuum between double walls blocks conduction and convection, and the silvered surfaces cut radiation — so item 1. A lever turns on the moment of a force about a fulcrum, item 2. That gives 3, 4, 1, 2.

• (a)Code A B C D = 2 4 1 3 — Right about the lightning conductor and the thermos flask but exchanges the periscope and the lever, giving the periscope a moment of force and the lever a pair of plane mirrors.
• (b)Code A B C D = 3 1 4 2 — Right at the two ends but swaps the middle pair, assigning conduction and convection minimisation to the lightning conductor and static electricity to the thermos flask.
• (d)Code A B C D = 2 1 4 3 — Wrong at every position — it is option (a) with the middle pair also exchanged, so no single pairing survives.

Four everyday devices, each resting on a different branch of school physics. Optics gives the periscope, in which two parallel plane mirrors at forty-five degrees carry the line of sight around an obstruction, as in a submarine or a crowd. Electrostatics gives the lightning conductor, where charge accumulates at a sharp point and provides a low-resistance path to earth. Heat gives the thermos flask, whose evacuated double wall stops conduction and convection while its silvered surfaces reflect radiation. Mechanics gives the lever, where a small effort at a large distance from the fulcrum balances a large load close to it.

A match-list of this kind is best done by naming the branch of physics each device belongs to before looking at List-II at all. Once the periscope is labelled optics and the lever mechanics, only one code can be right, because the two items in List-II that belong to those branches are fixed. Note also the wording of item 1: a thermos flask minimises conduction and convection through the vacuum, and reduces radiation by silvering — the item names the two the vacuum handles.

• A periscope uses two plane mirrors inclined at forty-five degrees to the line of sight.
• A lightning conductor exploits the concentration of static charge at a sharp point to earth a discharge safely.
• A thermos flask stops conduction and convection with a vacuum between double walls and cuts radiation with silvered surfaces.
• A lever works on the principle of moments: effort times effort arm equals load times load arm.
• Levers are classified into three orders by the relative positions of fulcrum, effort and load.

• Assigning 'static electricity' to the thermos flask because both involve keeping something out.
• Reading item 1 as covering radiation as well; the vacuum stops conduction and convection, the silvering handles radiation.
• Choosing a code from one confident pairing when three of the four codes get at least one position right.

A device-to-principle match spread across four branches of physics, so it rewards classifying each device before touching the codes.

• practice — not a real PYQ

  The two plane mirrors in a simple periscope are placed

  • (a)perpendicular to each other
  • (b)parallel to each other at forty-five degrees to the line of sight
  • (c)at sixty degrees to each other
  • (d)facing away from each other

  Answer(b) parallel to each other at forty-five degrees to the line of sight — this carries the light around the obstruction without inverting the image.

• practice — not a real PYQ

  The vacuum between the double walls of a thermos flask mainly prevents heat transfer by

  • (a)conduction and convection
  • (b)radiation only
  • (c)radiation and conduction
  • (d)evaporation

  Answer(a) conduction and convection — both need a material medium, and the silvered walls deal separately with radiation.