In the phenomenon of electric discharge through gases at low pressure, the coloured glow in the tube appears as a result of

A 5 watt source emits monochromatic light of wavelength 5000 $$\mathop A\limits^ \circ $$. When placed 0.5 m away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the number of photoelectrons liberated will be reduced by a factor of

A beam of electron passes undeflected through mutually perpendicular electric and magnetic fields. If the electric field is switched off, and the same magnetic field is maintained, the electrons move

Monochromatic light of frequency 6.0 $$ \times $$ 10¹⁴ Hz is produced by a laser. The power emitted is 2 $$ \times $$ 10^$$-$$3 W. The number of photons emitted, on the average, by the source per second is