Question

dentists use a soft, clay - like material to make a filling for a cavity. the filling stays soft under the light of the dentist’s lamp, but it hardens when the dentist shines an ultraviolet flashlight on it. why does light from the ultraviolet flashlight harden the filling when light from the lamp does not?
options:
because ultraviolet light carries energy, and light from the lamp does not.
because the filling material can take in energy from ultraviolet light, but not from the light from the lamp.
because ultraviolet light pulls energy out of the filling material, and light from the lamp does not.
because ultraviolet light carries hardening molecules, but light from the lamp does not.

Explanation:

1. Analyze Option 1: All light carries energy, so the claim that lamp light has no energy is wrong. Eliminate this.
2. Analyze Option 2: The filling material (a photopolymer) absorbs energy from UV light to initiate polymerization (hardening), while it doesn't absorb energy from the lamp's light (likely visible light, which doesn't trigger the reaction). This matches the process.
3. Analyze Option 3: UV light provides energy to the filling (polymerization needs energy input), not pull energy out. Eliminate this.
4. Analyze Option 4: Light doesn't carry "hardening molecules"—it provides energy for a chemical reaction. Eliminate this.

Answer:

B. Because the filling material can take in energy from ultraviolet light, but not from the light from the lamp.