A7LAM CSCA ACADEMYCSCA Academy / Physics / Modern physics
CSCA Physics · Lesson 15 of 16Photoelectric effect and atoms
Effet photoélectrique et atome · الأثر الكهروضوئي والذرة
▶ Watch the video lesson (A7LAM students)
Light also behaves as particles called photons. Their energy explains the photoelectric effect and the exact colours that atoms emit.
Key ideas
- Photon energy E = hf, with h = 6.63 × 10⁻³⁴ J·s.
- Electrons leave a metal only if hf > W₀ (work function); brighter light gives more electrons, not faster ones.
- Einstein: Eₖ(max) = hf − W₀.
- Atoms have fixed energy levels; a jump down emits a photon with hf = Eₘ − Eₙ.
Formulas to know
- E = hf = hc/λ
- Eₖ = hf − W₀
- hf = Eₘ − Eₙ
Worked example
Photons of 5.0 eV hit a metal with W₀ = 2.3 eV. Maximum kinetic energy of the electrons?
- Law: Eₖ = hf − W₀
- Replace: Eₖ = 5.0 − 2.3
- Compute: Eₖ = 2.7 eV
Answer: 2.7 eV
Practice questions
1. Brighter light of the same colour on a metal gives:
more electrons, same maximum energy ✓ · faster electrons · no change · fewer electrons
Intensity changes the number of photons, not their energy.
2. Hydrogen: E₃ = −1.51 eV, E₂ = −3.40 eV. Photon energy for 3 → 2?
1.89 eV ✓ · 4.91 eV · 3.40 eV · 1.51 eV
−1.51 − (−3.40) = 1.89 eV.
3. Which has the most energetic photons?
Red light · Green light · Ultraviolet ✓ · Infrared
Highest frequency.