The threshold wavelength for photoelectric emission from a material is 5500 A∘. Photoelectrons will be emitted, when this material is illuminated with monochromatic radiation from aA. 75 W infra-red lampB. 10 W infra-red lampC. 75 W ultra-violet lampD. 10 W ultra-violet lampChoose the correct answer from the options given below :B and C onlyA and D onlyC onlyC and D only
Question
The threshold wavelength for photoelectric emission from a material is 5500 A∘. Photoelectrons will be emitted, when this material is illuminated with monochromatic radiation from aA. 75 W infra-red lampB. 10 W infra-red lampC. 75 W ultra-violet lampD. 10 W ultra-violet lampChoose the correct answer from the options given below :B and C onlyA and D onlyC onlyC and D only
Solution
The photoelectric effect depends on the frequency (or wavelength) of the incident light, not its intensity. Therefore, the power of the lamp (75 W or 10 W) is irrelevant.
The threshold wavelength for photoelectric emission from the material is given as 5500 Å. The frequency (ν) corresponding to this wavelength (λ) can be calculated using the speed of light (c) equation: c = λν.
The frequency of infrared light is less than the frequency of visible light, and the frequency of ultraviolet light is higher than the frequency of visible light. Since the threshold wavelength is in the visible light range, only ultraviolet light will have a high enough frequency to cause photoelectric emission.
Therefore, the correct answer is "C and D only".
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