An aluminum foil is placed across a beam of radiation from a radioactive source. Which of the following types of radiation will be stopped by it?

Analyze the question: A thin aluminum foil is placed in the path of radiation. We need to find which radiations it stops.

Concept: Penetrating power of radiation

| Radiation | Nature | Penetrating power | Stopped by | |---|---|---|---| | Alpha (α) | Helium nucleus (2 protons + 2 neutrons) | Lowest | Paper, a few cm of air, thin foil | | Beta (β) | Fast electron | Medium | A few mm of aluminum | | Gamma (γ) | High-energy EM wave | Highest | Thick lead or several cm of concrete |

Step-by-step reasoning:

1. Alpha particles are heavy and strongly ionizing, so they lose energy quickly. Even a sheet of paper stops them, so thin aluminum foil certainly does.
2. Beta particles are lighter and more penetrating. Thin aluminum foil is typically enough to stop or largely absorb them, which is why aluminum sheets a few mm thick are the standard absorber for beta in laboratory work.
3. Gamma rays are very penetrating. Aluminum foil barely reduces them, and only thick lead or concrete reduces them substantially.

So the foil stops alpha and beta, but gamma passes through.

Checking the options:

• A. alpha and beta: correct
• B. alpha and gamma: wrong, gamma is not stopped
• C. beta and gamma: wrong, gamma is not stopped
• D. gamma only: wrong, gamma is the one that gets through

Answer: A

Exam tip: Remember the order of penetration as "α < β < γ": Paper → Aluminum → Lead. Whatever stops the more penetrating one also stops those before it. Since aluminum is the second barrier, it stops both alpha and beta.