answer is c
When a certain metal was irradiated with light of frequency 3.2 x 1016 Hz. Photoelectrons emitted had twice the kinetic energy as did photoelectrons emitted when the same metal was irradiated with light of frequency 2.0 x 1016Hz. Hence, threshold frequency is:
a 0.8 x 1015Hz
b 8.0 x 1015Hz
c 0.8 x 1014Hz
d 6.4 x 1016Hz

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