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NEET Sample Paper NEET Sample Test Paper-13

  • question_answer
    A 5 W source emits monochromatic light of wavelength\[5000\text{ }\overset{\text{o}}{\mathop{\text{A}}}\,\]. When placed 0.5 m away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m the number of photoelectrons liberated will be reduced by a factor of

    A)  4                                

    B)  8

    C)  16                               

    D)  2

    Correct Answer: A

    Solution :

     Intensity of light is inversely proportional to square of distance. That is, \[I\propto \frac{1}{{{r}^{2}}}\]or \[\frac{{{I}_{2}}}{{{I}_{1}}}=\frac{{{({{r}_{1}})}^{2}}}{{{({{r}_{2}})}^{2}}}\] Given, \[{{r}_{1}}=0.5\,m,\,{{r}_{2}}=1.0\,m\] Therefore, \[\frac{{{I}_{2}}}{{{I}_{1}}}=\frac{{{(0.5)}^{2}}}{{{(1)}^{2}}}=\frac{1}{4}\] Now, since number of photoelectrons emitted per second is directly proportional to intensity, so number of electrons emitted would decrease by factor of 4.


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