Which type of light can cause electrons to be emitted from a photosensitive surface?

• A. Red light
• B. Ultraviolet light
• C. Blue light
• D. Infrared light

Answer: (B)

Light causes electrons to be emitted only when each photon delivers at least the energy needed to overcome the surface’s work function. This is the photoelectric effect: a photon with energy E = hf must exceed φ, the work function, for an electron to be released. Ultraviolet light has photons with higher energy, so it readily surpasses that barrier for most photosensitive surfaces, leading to emission. Red and infrared photons carry less energy and generally don’t make it over the barrier, while blue light has more energy than red but may still be below the threshold for some materials; ultraviolet is the type that most reliably causes emission. The kinetic energy of emitted electrons is hf − φ.