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A monochromatic light is incident on a metallic plate having work function φ . An electron, emitted normally to the plate from a point A with maximum kinetic energy, enters a constant magnetic field, perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point B. The distance between A and B is: (Given: The magnitude of charge of an electron is e and mass is m, h is Planck's constant and c is velocity of light. Take the magnetic field exists throughout the path of electron)

1. eB / hc m ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ − λ
2. eB / hc m 8 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ − λ
3. 2 eB / hc m ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ − λ
4. eB / hc m 2 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ − λ

Correct answer: eB / hc m 8 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ − λ

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