Work Function Comparison

NCERT establishes that a metal emits photoelectrons only when the incident photon energy exceeds its work function. Violet light of wavelength around 400 nm carries a photon energy of roughly $\frac{1242}{400}\approx 3.1$ eV. This comfortably exceeds caesium's 2.1 eV work function, so caesium emits electrons, but it falls well short of platinum's 5.6 eV barrier, so platinum does not respond. This is exactly why low-work-function metals like caesium are used in practical photocells. The option favouring only platinum is wrong because a higher work function makes emission harder, not easier. The option claiming both emit equally is wrong because emission depends critically on whether photon energy surpasses each metal's specific work function. The option claiming neither emits is wrong because caesium's low barrier is clearly overcome by the 3.1 eV violet photons. As stated in NCERT Class 12, Chapter 11, alkali metals with small work functions are photosensitive even to visible light. This selectivity is the operating principle behind practical photocells and light meters, where a low-work-function coating is chosen so that ordinary visible light is enough to trigger emission. It also underlies why photographic and light-sensing devices historically favoured alkali-metal photocathodes over refractory metals. A consistency check confirms that with 3.1 eV photons, only the metal whose work function lies below this value, namely caesium, will emit.