A teacher writes four claims on the board about wavefronts and Huygens' construction. Which of the claims are correct?

• I. A point source in a uniform medium produces spherical wavefronts.
• II. Rays are perpendicular to the wavefronts, along the direction of energy flow.
• III. A thin convex lens turns a plane wavefront into a spherical wavefront converging to its focus.
• IV. The energy carried by a light wave depends on its speed, not on its amplitude.