Question

Which one of the four graphs showing lines P, Q, R and S between maximum kinetic energy (E) and intensity of incident radiation (I) is correct?

• A. P
• B. Q
• C. R
• D. S

Hint:

Maximum kinetic energy depends on frequency, not on intensity, so the graph against intensity is horizontal.

Answer 1

The Correct Option is A

Step 1: Fix the frequency:
Take light of a fixed frequency above the threshold. Each photon has energy $h\nu$, and an electron leaves with at most $h\nu - \phi$.

Step 2: Double the intensity:
Doubling the intensity doubles the number of photons, not the energy of each photon. So the number of emitted electrons doubles, but the maximum kinetic energy of each one is unchanged.

Step 3: Conclusion:
A graph of $E_{max}$ against I is therefore a line parallel to the I axis. Lines Q, R and S slope upward, which would imply a link between intensity and energy that the photoelectric effect does not have. Only P is horizontal.

Final Answer:
The answer is P. \[ \boxed{\text{P}} \]