MCQ-5

21.

Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F1. When the space between the two lenses is filled with glycerine (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F2. The ratio F1: F2 will be :

 (1)  2 : 1                           (2) 1 : 2

 (3) 2 : 3                           (4) 3 : 4

22. Increase in temperature of a gas filled in a container would lead to :

(1) Increase in its mass

(2) Increase in its kinetic energy

(3) Decrease in its pressure

(4) Decrease in intermolecular distance

23. An electron is accelerated through a potential difference of 10,000 V. Its de Broglie wavelength is, (nearly) : ( me=9×10^-31 kg)

(1) 12.2 ×10^-13  m              (2) 12.2 ×10^-12 m

(3) 12.2 ×10^-14  m              (4) 12.2 nm

24. A copper rod of 88 cm and an aluminium rod of unknown length have their increase in length independent of increase in

temperature. The length of aluminium rod is 

(1)  6.8 cm                           (2)  113.9 cm

(3)  88 cm                            (4)  68 cm

25)  Pick the wrong answer in the context with rainbow.

(1) When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed

(2) The order of colours is reversed in the secondary rainbow

(3) An observer can see a rainbow when his front is towards the sun

(4) Rainbow is a combined effect of dispersion refraction and reflection of sunlight

EXPLANATION :

21)

 Answer : (2)

22)

Answer : (2)

Increase in temperature would lead to the increase in kinetic energy of gas (assuming far as to be ideal) as U=F/2nRT

23)

Answer : (2)

24)

Answer : (4)

25)

Answer : (3)

Rainbow can't be observed when observer

faces towards sun.