Photoelectric effect: Class 12 practice
A photon has energy E = hf = hc/λ. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Units: eV and J
The idea behind photoelectric effect
A photon has energy E = hf = hc/λ. Einstein's equation: K_max = hf − φ, where φ is the work function. No electrons are emitted below the threshold frequency f₀ = φ/h. Stopping potential V₀ = K_max/e.
Step-by-step method
- Find photon energy in eV (E = 1240/λ(nm)).
- K_max = E − φ.
- If K_max < 0, no emission; V₀ = K_max in volts.
Worked example 1 (Basic)
Find the energy of a photon of wavelength 250 nm in eV.
- E = 1240/250
- = 4.96 eV
- Answer: 4.96 eV
Worked example 2 (Standard)
Light of 350 nm falls on a metal with work function 2.1 eV. Find the maximum kinetic energy of the electrons.
- E = 3.54 eV
- K = 3.54 − 2.1 = 1.44 eV
- Answer: 1.44 eV
Worked example 3 (Challenge)
Find the threshold wavelength for a metal of work function 2.75 eV.
- λ₀ = 1240/2.75
- = 450.91 nm
- Answer: 450.91 nm
Common mistakes and misconceptions
- Thinking brighter light raises K_max
- Mixing J and eV
Practice questions
- (Basic) Find the energy of a photon of wavelength 400 nm in eV.
- (Basic) Find the energy of a photon of wavelength 300 nm in eV.
- (Standard) Light of 300 nm falls on a metal with work function 4.5 eV. Find the maximum kinetic energy of the electrons.
- (Standard) Light of 450 nm falls on a metal with work function 4.5 eV. Find the maximum kinetic energy of the electrons.
- (Challenge) Find the threshold wavelength for a metal of work function 4.5 eV.
- (Challenge) Find the threshold wavelength for a metal of work function 2.3 eV.
Answer key with full working
- Q1: E = 1240/400 → = 3.1 eV → Answer: 3.1 eV
- Q2: E = 1240/300 → = 4.13 eV → Answer: 4.13 eV
- Q3: E = 4.13 eV → E < φ → no electrons emitted → Answer: No emission
- Q4: E = 2.76 eV → E < φ → no electrons emitted → Answer: No emission
- Q5: λ₀ = 1240/4.5 → = 275.56 nm → Answer: 275.56 nm
- Q6: λ₀ = 1240/2.3 → = 539.13 nm → Answer: 539.13 nm
Syllabus note
This skill is listed under "Dual Nature of Radiation and Matter" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is photoelectric effect in?
"Dual Nature of Radiation and Matter", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.