Photons, Photoelectric Effect & Wave–Particle Models — Practice · Grade 12 Science

Chapter: Modern Physics

What this practice covers

By the end of Photons, Photoelectric Effect & Wave–Particle Models you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: work function, threshold frequency, electronvolt.

Problem 1

Photon 5 eV, work function 2 eV. Find Kmax.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

3 eV

5−2=3.

Problem 2

Why does brighter below-threshold light not eject electrons in the basic model?
Answer and explanation

Each photon lacks the required energy; adding more such photons does not change the single-photon energy.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

Using hc=1240 eV·nm, find photon energy at 620 nm.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

2 eV

1240/620=2.

Problem 4

Which observation supports a wave model and which supports a photon model?
Answer and explanation

Interference supports wave superposition; threshold-dependent photoemission supports quantized energy exchange.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

With work function 2.4 eV and photons of 2.0 eV, is emission predicted by the single-photon model? Show the reasoning, not just the conclusion.
Hints

Below threshold, increased intensity alone does not supply more energy to each photon.

Answer and explanation

No. Each photon is below threshold; do not report negative kinetic energy as an emitted-electron result.

Problem 6

For fixed above-threshold frequency, what can increased light intensity change in the ideal photoelectric model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

It can increase emission rate/current, while maximum electron kinetic energy is set by photon energy minus work function.

Problem 7

Maximum kinetic energy and stopping potential need an electron accountMaximum kinetic energy and stopping potential need an electron accountQuantitySupplied valueCalculationPhoton energy5.0 eVGiven frequency via hfWork function3.0 eVEscape thresholdKmax2.0 eV5.0−3.0Stopping-potential magnitude2.0 VeVs=KmaxMaximum energy is not every electron’s energy.
Original photoelectric energy/voltage account for the selected threshold model.
Interpret the supplied visual. What is 2.0 eV in joules?
Answer and explanation

3.204×10⁻¹⁹ J per electron, rounded appropriately for the supplied data.

Photoelectrons can emerge with a range of kinetic energies; the ideal relation specifies the maximum. A retarding stopping potential magnitude Vs is sufficient to stop even those fastest electrons when eVs=Kmax in the stated setup. Converting eV to joules uses 1 eV=1.602×10⁻¹⁹ J. Keep electron charge magnitude separate from the signed field/voltage convention when describing the apparatus, and do not confuse a 2 eV energy with a 2 V potential.

3.204×10⁻¹⁹ J per electron, rounded appropriately for the supplied data.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

A material has work function 2.2 eV. Photons carry 3.7 eV. Find Kmax in eV and joules, and stopping-potential magnitude.
Answer and explanation

Kmax=1.5 eV=2.403×10⁻¹⁹ J; stopping-potential magnitude=1.5 V. At fixed photon energy, increasing intensity changes available electron number rather than this maximum.

Kmax=1.5 eV=2.403×10⁻¹⁹ J; stopping-potential magnitude=1.5 V.

At fixed photon energy, increasing intensity changes available electron number rather than this maximum.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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