What is the property of ZINC SULPHIDE which make it glow when cathode rays strike ?
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Pure ZnS doesn't fluoresce. The stuff used as a phosphor has a little cadmium and silver added to it. These impurities are stuck in the latice and the electrons associated with them have a different set of permitted energy levels from the Zn atoms.
The electrons from the cathode rays (or UV light) will promote electrons to a higher energy level associated with one of these impurity atoms. When the electrons fall back down they give out light.
The electrons from the cathode rays (or UV light) will promote electrons to a higher energy level associated with one of these impurity atoms. When the electrons fall back down they give out light.
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