VCE · Topic explainer

Reading the gradient of a photoelectric-effect graph

For a graph of maximum electron kinetic energy in joules against light frequency in hertz, the gradient is Planck’s constant. The horizontal intercept is the threshold frequency. Check the axes first because a stopping-voltage graph or energy measured in electronvolts changes the gradient’s units and numerical value.

Worked example

Suppose a straight line rises from 1.0 × 10⁻¹⁹ J at 6.0 × 10¹⁴ Hz to 3.0 × 10⁻¹⁹ J at 9.0 × 10¹⁴ Hz. The rise is 2.0 × 10⁻¹⁹ J over 3.0 × 10¹⁴ Hz.

The gradient is about 6.7 × 10⁻³⁴ J s. If your graph plots the magnitude of stopping voltage instead, its gradient is h/e. Multiply that voltage gradient by the elementary charge to recover h in joule-seconds.

Check the energy and frequency axes. Gradient = change in energy ÷ frequency. For joules against hertz, slope = h.

Official exam practice

2025 VCE Physics, Section B, Question 16

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