<p>Phtoelectric emission is observed form a surface for frequency&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>v</mi><mn>1</mn></msub><mo>&#160;</mo><mi>a</mi><mi>n</mi><mi>d</mi><mo>&#160;</mo><msub><mi>v</mi><mn>2</mn></msub></math>&nbsp;of the incident radiation&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><msub><mi>v</mi><mn>1</mn></msub><mo>&#160;</mo><mo>&#62;</mo><mo>&#160;</mo><msub><mi>v</mi><mn>2</mn></msub></mrow></mfenced></math>. If the maximum kinetic energies of the photoelectrons in the two cases are in the ratio 1 : k, then the threshold frequency&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>v</mi><mn>0</mn></msub></math>&nbsp;is given by:</p>