Einstein and Photoelectric Effect

Question 1

What did Einstein propose about EM waves in relation to Planck’s work?

Answer 1

Einstein proposed that EM radiation is made of discrete packets called photons, which have particle-like interactions and carry energy E=hf.

Question 2

How does photon theory explain the threshold frequency in the photoelectric effect?

Answer 2

An electron can absorb only one photon. If hf≥ϕ (work function), a photoelectron is emitted. The threshold frequency is when hf=ϕ, so below that, no emission occurs.

Question 3

Why is photoelectron emission immediate in Einstein’s model?

Answer 3

Because the photon’s entire energy is transferred instantly to a single electron during interaction.

Question 4

How does Einstein’s model explain the effect of increasing light intensity?

Answer 4

Higher intensity means more photons per second, so more photoelectrons are emitted, but each has the same energy if the frequency stays constant.

Question 5

Why do photoelectrons have a range of kinetic energies in Einstein’s model?

Answer 5

• Some electrons start deeper in the metal and lose energy in collisions.

Electrons may do work against surface charges, reducing their energy.
Thus, not all electrons reach the surface with maximum kinetic energy.

Question 6

What happens when a positive potential is applied to the metal surface?

Answer 6

Photoelectrons must do more work against electrostatic attraction, reducing their kinetic energy. As the potential increases, fewer electrons have enough energy to escape.

Question 7

What is the stopping potential, and what does it measure?

Answer 7

The stopping potential Vs is the potential needed to stop even the most energetic photoelectrons. It allows us to calculate maximum kinetic energy: Ek(max)=eVs.

Question 8

What is Einstein’s photoelectric equation?

Answer 8

hf=ϕ+Ek(max) or hf=ϕ+eVs.

Question 9

What does a graph of stopping potential Vs vs frequency f look like?

Answer 9

A straight line with:

Gradient = h/e

Y-intercept = -ϕ/e

X-intercept = threshold frequency f0.

Question 10

How did the stopping potential vs frequency graph support Einstein’s photon model?

Answer 10

The linear relationship confirmed E=hf, validating that light behaves as quantised photons, not just waves.