IIT-JEE Exam

Diameter of a plano-convex lens is 6 cm and
thickness at the centre is 3 mm. If speed of light in
material of lens is 2 × 10^8 m/s, the focal length of
the lens is -
(1) 30 cm (2) 10 cm
(3) 15 cm (4) 20 cm

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IIT-JEE Other Question

A uniform cylinder of length L and mass M
having cross-sectional area A is suspended, with
its length vertical, from a fixed point by a
massless spring, such that it is half submerged in a
liquid of density σ at equilibrium position. The
extension x0 of the spring when it is in equilibrium
is - (Here k is spring constant) The supply voltage to a room is 120 V. The
resistance of the lead wires is 6Ω. A 60 W bulb is
already switched on. What is the decrease of
voltage across the bulb, when a 240 W heater is
switched on in parallel to the bulb ?
(1) 13.3 V (2) 10.04 V
(3) zero V (4) 2.9 V