Why does increasing light intensity below the threshold frequency produce NO photoelectrons?

You can work it out this way: think about what a single photon's energy depends on.

Sodium has a work function φ = 2.28 eV. What is the minimum threshold frequency?

You can work it out this way: use f_threshold = φ/h, convert φ to joules first.

Light of f = 5 × 10¹⁴ Hz shines on copper (φ = 4.5 eV). Are photoelectrons emitted?

You can work it out this way: calculate photon energy E = hf and compare with φ.

Pro 🔒~25 min

Photoelectric Effect

Discover how light frequency — not intensity — ejects electrons

How it works

Einstein's 1905 explanation of the photoelectric effect treats light as discrete photons each carrying energy E = hf. An electron can only escape the metal surface if a single photon supplies energy exceeding the work function φ. Intensity controls how many photons arrive per second but does not increase individual photon energy, so higher intensity below threshold still produces zero photoelectrons.

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Step-by-step

Increase light frequency until the electron counter starts registering — that is the threshold frequency.
Note that raising intensity below threshold never produces electrons.
Switch to Pro mode to select different metals and measure the stopping voltage that exactly halts the fastest electrons.

Key formulas

$E = hf \quad (h = 6.626 \times 10^{-34}\,\text{J}\cdot\text{s})$ Photon Energy
$KE_{max} = hf - \varphi$ Einstein Photoelectric Equation
$eV_{stop} = KE_{max}$ Stopping Voltage relation
$f_{threshold} = \frac{\varphi}{h}$ Threshold Frequency

Frequently asked questions

Why does increasing light intensity below the threshold frequency produce NO photoelectrons?: You can work it out this way: think about what a single photon's energy depends on.
Sodium has a work function φ = 2.28 eV. What is the minimum threshold frequency?: You can work it out this way: use f_threshold = φ/h, convert φ to joules first.
Light of f = 5 × 10¹⁴ Hz shines on copper (φ = 4.5 eV). Are photoelectrons emitted?: You can work it out this way: calculate photon energy E = hf and compare with φ.