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The threshold wavelength for lithium metal is 6250 A. For photoemission, the wavelength of the incident light must be ______.

Asked in GUJCET 2014 · Threshold frequency and wavelength

• (1) more than 6250 A
• (2) exactly equal to 6250 A
• (3) equal to or more than 6250 A
• (4) equal to or less than 6250 A

Step-by-step solution

Why the other options are wrong

• (1) A longer wavelength means a lower photon energy (hc)/λ, below the work function, so no electron is emitted.
• (2) Exactly 6250 A is only the limiting case. Every shorter wavelength also works, since its photons carry more energy.
• (3) Wavelengths longer than 6250 A carry too little energy per photon. Emission needs the wavelength at or below the threshold, not above it.

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