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.