Monochromatic radiation of wavelength `"640.2 nm "(1nm = 10^(-9) m)` from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage.

Explanation

Use `e V_(0) = h v -phi_(0)` for both sources. From the data on the first source, `phi_(0) = 1.40eV`. Use this value to obtain for the second source `v_(0) = 1.50 V.`