Which practical technology depends primarily on the photoelectric effect?

The diagram shows electric field lines around two point charges. What is the sign of each charge?

In the circuit shown, a 12 V battery drives current through two resistors in series: R₁ = 4 Ω and R₂ = 8 Ω. What is...

The ray diagram shows an object placed farther than 2f from a converging (convex) lens. What is TRUE of the image?

The diagram shows energy levels of an atom. An electron transitions from level n = 3 to level n = 1. What happens?