• # question_answer Ragini was worried about her grand mother who stays all alone in her residence in a crowded area of Varanasi. To ensure safety of her grand mother, she installed a device known as Burglar's alarm under the guidance of her Physics teacher. While doing the experiment, she recorded the following observations. When radiations of frequency ${{10}^{15}}$ Hz are incident on two photosensitive surfaces A and B, in Burglar's alarm, For surface A No photoemission took place. For surface B Photoemission took place but photoelectrons have zero velocity. Read the above passage and answer the following questions: (i) What are the values displayed by Ragini? (ii) Explain the above observations on the basis of Einstein's photoelectric equation.

Answer:

(i) Ragini is very intelligent, aware of technology and caring. (ii) Einstein's photoelectric equation states that $K{{E}_{max}}=h(v-{{v}_{0}})$                     ...(i) where,   v = frequency of incident light ${{v}_{0}}$= threshold frequency. For surface A No photoemission takes place from surface A, it means $v<{{v}_{0}},$ i.e. incident frequency is less than threshold frequency, because $K{{E}_{\max }}$ can never be negative. For surface B $K{{E}_{max}}=0$ $\Rightarrow h(v-{{v}_{0}})=0\Rightarrow v={{v}_{0}},$ i.e. frequency of incident light is equal to the threshold frequency of the photosensitive material.

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