The question as it appears in the 11/13 edition of Specs & Techs from GlobalSpec:
Electrons are ejected from the surface of a metal plate exposed to light with wavelength . A uniform, retarding electric field E is imposed on the plate, and it is observed that the ejected electrons do not travel more than a distance d from the surface of the plate. What is the threshold wavelength for the plate material?
(Update: Nov 20, 8:49 AM EST) And the Answer is...
The threshold wavelength is defined as the minimum wavelength capable of ejecting electron form the metal plate. This wavelength, , is a material property.
When light of wavelength impinges on the surface of the plate the ejected electrons will acquire a kinetic energy equal to
(1)
When the electrons move in the direction of the retarding electric field and reach the maximum distance d, all this energy is converted into potential energy given by
(2) 
Equating equations (1) and (2), we get
(3) 
Solving for we get

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