When radiation of wavelength `2000 Å` is incident on a nickel plate, the plate gets positively charged. But when wavelength of incident radiation is raised to `3440 Å`, the plate remains neutral. Explain.

Explanation

Photoelectric effect takes place for incident radiation of wavelength `2000 Å` and due to emission of photoelectrons, the plate becomes positively charged. No photoelectric -effect takes place when radiation of wavelength for `3400 Å` is incident on the plate, as threshold wavelength for photoelectric effect is more than `2000 Å` but less than `3400 Å`