If you want to design a phototube that ignores all visible light, what should its cathode's work function be?
A) Below $1.77 \mathrm{eV}$
B) Any value between $1.77 \mathrm{eV}$ and $3.1 \mathrm{eV}$
C) Some specific value between $1.77 \mathrm{eV}$ and $3.1 \mathrm{eV}$
D) Above $3.1 \mathrm{eV}$
E) Some other value or range (specify)

Question

Quizplus · Accepted Answer

The visible light spectrum ranges from about 400 nm (violet) to 700 nm (red), corresponding to photon energies from approximately 3.1 eV to 1.77 eV, respectively. To ignore all visible light, the cathode's work function needs to be above the highest energy of visible light, which is above 3.1 eV. This ensures that photons from the visible spectrum do not have enough energy to eject electrons from the cathode.

If You Want to Design a Phototube That Ignores All 1.77eV1.77 \mathrm{eV}1.77eV

If You Want to Design a Phototube That Ignores All $1.77 \mathrm{eV}$