Photoelectric Effect
Dual Nature of Matter
80
JEE Qs
6%
Hard
60
min
Master the graphs relating photoelectric current, stopping potential, intensity, and frequency; they are frequently tested and reveal a deep understanding of the concepts.
🧮 Key Formulas
✅ Key Points for JEE
- 1The photoelectric effect is an instantaneous process, occurring without any time lag, provided the incident photon energy exceeds the work function.
- 2The maximum kinetic energy (K_max) of emitted photoelectrons depends linearly on the frequency (f) of incident light, not its intensity.
- 3The photoelectric current is directly proportional to the intensity of incident light, provided its frequency is above the threshold frequency.
- 4Below a certain threshold frequency (f_0), characteristic of the material, no photoemission occurs, regardless of the intensity of light.
- 5The work function (φ) is the minimum energy required to eject an electron from the metal surface.
⚠️ Common Mistakes
- ✕Confusing the roles of intensity and frequency: Students often mistakenly believe that higher intensity increases K_max, or that higher frequency increases photoelectric current.
- ✕Ignoring the threshold condition: Applying photoelectric equation without checking if incident frequency is greater than threshold frequency, leading to negative K_max.
- ✕Units conversion errors: Frequently making mistakes while converting energy units (Joules to eV or vice-versa) or wavelength units (nm, Å to m).
📝 Practice Questions
See allQ42.A light source of wavelength λ illuminates a metal surface and electrons are ejected with maximum kinetic energy of 2 eV . If the same surface is illuminated by a light source of wavelength λ , then the maximum 2 kinetic energy of ejected electrons will be (The work function of metal is 1 eV ) (1) 3 eV (2) 2 eV (3) 6 eV (4) 5 eV
Q45.In photoelectric effect an em-wave is incident on a metal surface and electrons are ejected from the surface. If the work function of the metal is 2.14 eV and stopping potential is 2 V , what is the wavelength of the em- wave ? (Given hc = 1242eVnm where h is the Planck's constant and c is the speed of light in vaccum.) 2025 (23 Jan Shift 2) JEE Main Previous Year Paper (1) 300 nm (2) 400 nm (3) 600 nm (4) 200 nm
Q30.An electron in the ground state of the hydrogen atom has the orbital radius of 5.3 × 10−11 m while that for the electron in third excited state is 8.48 × 10−10 m . The ratio of the de Broglie wavelengths of electron in the excited state to that in the ground state is (1) 3 (2) 16 (3) 9 (4) 4
Q40.The work functions of cesium (Cs) and lithium (Li) metals are 1.9 eV and 2.5 eV , respectively. If we incident a light of wavelength 550 nm on these two metal surfaces, then photo-electric effect is possible for the case of (1) Both Cs and Li (2) Neither Cs nor Li (3) Cs only (4) Li only
Q34.A sub-atomic particle of mass 10−30 kg is moving with a velocity 2.21 × 106 m/s. Under the matter wave consideration, the particle will behave closely like (h = 6.63 × 10−34 J. s) (1) Visible radiation (2) Gamma rays (3) Infra-red radiation (4) X-rays
Q32.An electron of mass ' m ' with an initial velocity →v = v0^i (v0 > 0) enters an electric field E = −Eo^k . If the initial de Broglie wavelength is λ0 , the value after time t would be (1) λ0 (2) e2E20t2 m2v2o √1+ e2E02t2m2v02 λo√1 + (3) λ0 (4) λ0 √1−e2Eo2t2m2v2o
NCERT Chapters
- Class 12 Physics Ch 11: Dual Nature of Radiation and Matter