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) : (   m e =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 o