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प्रश्न
Define the term "cut off frequency" in photoelectric emission. The threshod frequency of a metal is f. When the light of frequency 2f is incident on the metal plate, the maximum velocity of photo-electrons is v1. When the frequency of the incident radiation is increased to 5f, the maximum velocity of phto-electrons is v2. Find the ratio v1 : v2.
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उत्तर
That particular frequency of incident radiation for which the minimum negative potential V0 given to a plate for photelectric current becomes zero is called the cut-off frequency.
Given that threshold frequency of metal is f. For light of frequency 2f, using Einstein's equation for photoelectric effect, we can write
\[h\left( 2f - f \right) = \frac{1}{2}m {v_1}^2\] .......... (1)
Similarly, for light having frequency 5f, we have
\[\frac{f}{4f} = \frac{{v_1}^2}{{v_2}^2} \Rightarrow \frac{v_1}{v_2} = \sqrt{\frac{1}{4}}\]
\[ \Rightarrow \frac{v_1}{v_2} = \frac{1}{2}\]
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