A sodium surface is illuminated with light of wavelenght 300nm. The work function of the metal is 2.4eV. 1. Determine the K. E
2. The cutt-off wavelenght of sodium​

1) 1.67eV

2) 505nm

Explanation:

The maximum kinetic energy of photoelectrons ,

 KEmax=λhc−W=(0.3×10−6)(1.6×10−19)(6.62×10−34)(3×108)eV−2.46eV=1.67eV

If λ0 is the cut-off wavelength, W=λ0hc

or λ0=Whc=2.46×1.6×10−19(6.62×10−34)(3×108)=5.05×10−7m=505×10−9=505nm

We know that the work function is the minimum photon energy for taking place of photoelectric effect. 

it is called kinetic energy

3. dark matter

empty space could also be a simple vacuum