After twenty minutes under a lamp, which surface will feel warmer?

• A. The glass will reflect all of the light after twenty minutes.
• B. The stone will reflect all of the light after twenty minutes.
• C. The glass will feel warmer than the stone after twenty minutes.
• D. The stone will feel warmer than the glass after twenty minutes.

Answer: (D)

The main concept is that surfaces heat up mainly by absorbing light. When a lamp shines on a surface, the energy that is absorbed is converted into heat, warming that spot. Glass tends to reflect or transmit much of the light rather than absorb it, so less energy becomes heat at the surface. Stone, especially when it's dark, absorbs more of the incoming light and converts more of that energy into heat in the surface layer. So after twenty minutes, the stone will feel warmer than the glass because it has taken in and turned more of the lamp’s energy into heat. The other ideas imagine the surface reflecting all the light, which isn’t realistic; in practice, no surface reflects all light and different materials absorb different amounts.