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

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

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

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