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Electron, Photon, Photo-electric Effect and X-Rays — JEE physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceJEE physicsElectron, Photon, Photo-electric Effect and X-Rays1 Mark
MCQ
Threshold frequency for a metal is $10^{15} \mathrm{~Hz}$. Light of $\lambda=4000 \mathring A$ falls on its surface. Which of the following statements is correct
  • ✓
    No photoelectric emission takes place
  • B
    Photo-electrons come out with zero speed
  • C
    Photo-electrons come out with $10 \mathrm{~m} / \mathrm{sec}$ speed
  • D
    Photo-electrons come out with $10 \mathrm{~m} / \mathrm{sec}$ speed
✓

Answer

Correct option: A.
No photoelectric emission takes place
Frequency of light of wavelength $(\lambda=4000 \mathring A)$ is $v=\frac{c}{\lambda}=\frac{3 \times 10^8}{4000 \times 10^{-10}}=0.75 \times 10^{15}$ which is less than the given threshold frequency.
Hence no photoelectric emisssion takes place.

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