• # 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.

(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.