Explain why a blackened platinum strip when placed at the radius of curvature of turns red hot, only when the cathode taken has concave shape.

1. The factors responsible for the strip to turn red hot are :

(a) The electrons when travel from a concave cathode they are almost in a straight line for a reasonable distance.
(b) As radius of curvature is close enough,so the object kept on radius of curvature of the concave cathode is impinged by a maximum rays of electron rich high energy radiation.

2. Thus,when the blackened platinum strip kept at the radius of curvature is subject to a high intensity of radiation that not only excite the electrons on the surface of the metal but also increase the energy of underlying electrons.

3. This makes the strip of platinum red hot when placed on the radius of curvature of a concave cathode.

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