Magnetism and Matter — Physics STD 12 Science — Question

10 gm of radioactive material of half-life 15 year is kept in store for 20 years. The disintegrated material is(a) 12.5 g(b) 10.5 g(c) 6.03 g(d) 4.03 g

A lens of power + 2 diopters is placed in contact with a lens of power – 1 diopoter.  The combination will behave like(a) A divergent lens of focal length 50 cm(b) A convergent lens of focal length 50 cm(c) A convergent lens of focal length 100 cm(d) A divergent lens of focal length 100 cm

Two rays A and B being reflected by a mirror and going as A' and B'. The mirror:

Electrons are emitted by a hot filament and are accelerated by an electric field, as shown in the figure. The two stops at the left ensure that the electron beam has a uniform cross-section.

1. The speed of the electrons is more at B than at A.
2. The electric current is from left to right.
3. The magnitude of the current is larger at B than at A.
4. The current density is more at B than at A.

Assertion  :   Leclanche cell is used, when constant supply of electric current is not required.  
Reason     : The e.m.f. of a Leclanche cell falls, if it is used continuously.(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.(c) If assertion is true but reason is false.(d) If the assertion and reason both are false.

When NPN transistor is used as an amplifier(a) Electrons move from base to collector (b) Holes move from emitter to base(c) Electrons move from collector to base(d) Holes move from base to emitter

A fisherman is standing on the bank of a lake. To a fish underwater viewing obliquely the man looks:

The electric potential of free space is :

In which frequency range, space waves are normally propagated(a) HF(b) VHF(c) UHF(d) SHF