(1) A threshold frequency exists: below it no electrons are emitted regardless of intensity. Photon model: one photon delivers all its energy to one electron; if $hf < \Phi$, no single photon can free an electron. (2) Emission is instantaneous even in dim light: energy arrives in concentrated packets rather than accumulating gradually over the surface. (Also acceptable: $E_{max}$ depends on frequency, not intensity.)

$E_k = eV = 150\ \text{eV} = 2.4\times10^{-17}\ \text{J}$. Momentum: $p = \sqrt{2mE_k} = \sqrt{2(9.11\times10^{-31})(2.4\times10^{-17})} \approx 6.6\times10^{-24}\ \text{kg m s}^{-1}$. Then $\lambda = h/p \approx 1.0\times10^{-10}\ \text{m}$ — atomic spacing, which is why crystals diffract electrons.

The photon transfers some of its energy and momentum to the recoiling electron in an elastic collision. Less energy means, via $E = hc/\lambda$, a longer wavelength. The shift $\Delta\lambda = \frac{h}{m_ec}(1-\cos\theta)$ is maximal at $\theta = 180°$ (backscattering), where it equals twice the Compton wavelength.