A 5 W source emits monochromatic light of wavelength 5000 A. 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

Intensity of light is inversely proportional to square of distance.That  is,  I∝1r2  or  I2I1=r12r22Given,    r1=0.5m,      r2=1.0 mTherefore  I2I1=0.5212=14Now, since number of photoelectrons emitted per second is directly proportional to intensity, so number of electrons emitted would decrease by factor of 4.