mistakes

Question 1

The experiment is repeated using a photocell in which a different metal Y is illuminated
with light of the same wavelength as in part (a). There is no photoemission when the
metal surface is uncharged

why?

Answer 1

work function of Y is greater than that of X
photons (in each experiment) have the same energy (as in a)
OR energy given to an electron is same

Photon energy is insufficient to liberate an electron from Y

function (of Y) is the minimum energy needed by an electron to escape
OR energy to liberate an electron in greater for Y than for X when a (conduction) electron in the metal absorbs a photon, it gains all the energy of the photon

Question 2

failure of wave theory

Answer 2

wave theory predicts that incident light (of any frequency)
would cause photoelectric emission (from any metal)

wave theory could not explain why light below a certain frequency (or below a threshold frequency) could not cause
photoelectric emission 

this (threshold) frequency is characteristic of the metal (or depends on the metal) 

wave theory could not explain the instantaneous
emission of photoelectrons

Question 3

failure of wave theory

Answer 3

wave theory predicts that incident light (of any frequency)
would cause photoelectric emission (from any metal)

wave theory could not explain why light below a certain frequency (or below a threshold frequency) could not cause
photoelectric emission 

this (threshold) frequency is characteristic of the metal (or depends on the metal) 

wave theory could not explain the instantaneous
emission of photoelectrons

Question 4

2 postulates - mean?

Answer 4

physical laws have same form in all intertial forms of reference:
inertial frame is stationary or moving at constant velocity 
which Newton’s law(s) of motion obeyed in inertial reference frames
OR
Same laws of physics are obeyed in all inertial frames of reference.

speed of light in FREE space is invarient/independent of
motion of source and motion of observer