Timing the Invisible: Half Life Basics Physics Quiz

9th Grade

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| Questions: 20 | Updated: Mar 9, 2026

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1. Half-life is the time it takes for:

All atoms to decay

Half the radioactive nuclei in a sample to decay

The sample to double in mass

Radiation to become visible

Concept: definition of half-life. Half-life measures how quickly decay happens. It is the time for the number of undecayed radioactive nuclei to drop to half its starting value.

Explanation

Concept: definition of half-life. Half-life measures how quickly decay happens. It is the time for the number of undecayed radioactive nuclei to drop to half its starting value.

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About This Quiz

Timing The Invisible: Half Life Basics Physics Quiz - Quiz

This assessment explores the fundamentals of half-life, a key concept in nuclear physics. It evaluates understanding of radioactive decay, the mathematical calculations involved, and applications in real-world scenarios. Mastering these concepts is essential for students and professionals in science and engineering fields, enhancing their comprehension of nuclear reactions and thei... see moreimplications. see less

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2. Half-life is a property of the isotope.

True

False

Concept: isotope-specific decay. Each isotope has its own characteristic half-life. This value is determined by nuclear structure and does not depend on how the isotope is used.

Explanation

Concept: isotope-specific decay. Each isotope has its own characteristic half-life. This value is determined by nuclear structure and does not depend on how the isotope is used.

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3. If you start with 200 radioactive nuclei, after one half-life you have about:

150

100

Concept: halving rule. Half remain after one half-life. So 200 becomes about 100 because the sample is reduced by a factor of 2.

Explanation

Concept: halving rule. Half remain after one half-life. So 200 becomes about 100 because the sample is reduced by a factor of 2.

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4. After two half-lives, the fraction remaining is ______.

Concept: repeated halving. (1/2)^2 = 1/4. Each half-life multiplies the remaining amount by 1/2, so two half-lives gives (1/2) × (1/2).

Explanation

Concept: repeated halving. (1/2)^2 = 1/4. Each half-life multiplies the remaining amount by 1/2, so two half-lives gives (1/2) × (1/2).

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5. A sample starts with 80 g of a radioisotope. After one half-life, it has:

60 g

40 g

20 g

10 g

Concept: half-life applies to amount of radioisotope. Mass halves after each half-life (for the radioactive part). If all 80 g is the radioisotope, after one half-life about 40 g remains undecayed.

Explanation

Concept: half-life applies to amount of radioisotope. Mass halves after each half-life (for the radioactive part). If all 80 g is the radioisotope, after one half-life about 40 g remains undecayed.

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6. Half-life depends on how much of the substance you have.

True

False

Concept: half-life independence from sample size. Half-life does not depend on sample size. A bigger sample has more nuclei, but each nucleus still has the same decay probability per unit time.

Explanation

Concept: half-life independence from sample size. Half-life does not depend on sample size. A bigger sample has more nuclei, but each nucleus still has the same decay probability per unit time.

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7. If the half-life is 5 days, after 10 days the amount remaining is:

1/2

1/4

1/8

1/16

Concept: counting half-lives. 10 days = 2 half-lives → (1/2)^2 = 1/4. Two halvings reduce the amount to one quarter of the original.

Explanation

Concept: counting half-lives. 10 days = 2 half-lives → (1/2)^2 = 1/4. Two halvings reduce the amount to one quarter of the original.

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8. Radioactive decay is random for a single nucleus.

True

False

Concept: randomness at the single-nucleus level. You can’t predict exactly when one nucleus will decay. You can only talk about probabilities, which is why we use half-life for large numbers of nuclei.

Explanation

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9. Radioactive decay for a large sample is predictable because:

Every atom decays at the same time

Statistics averages out random behavior

It is controlled by temperature

It is controlled by pressure

Concept: statistics in large samples. Large numbers give stable averages. Even though individual decays are random, the overall fraction that decays in a given time becomes predictable.

Explanation

Concept: statistics in large samples. Large numbers give stable averages. Even though individual decays are random, the overall fraction that decays in a given time becomes predictable.

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10. If you start with 1000 nuclei, after 3 half-lives you have about:

500

250

125

100

Concept: repeated halving sequence. 1000 → 500 → 250 → 125. Three half-lives means three successive halvings, so the result is one eighth of the original.

Explanation

Concept: repeated halving sequence. 1000 → 500 → 250 → 125. Three half-lives means three successive halvings, so the result is one eighth of the original.

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11. After 3 half-lives, the fraction remaining is ______.

Concept: fraction remaining after n half-lives. (1/2)^3 = 1/8. Each half-life multiplies the remaining fraction by 1/2, so after three you get 1/8.

Explanation

Concept: fraction remaining after n half-lives. (1/2)^3 = 1/8. Each half-life multiplies the remaining fraction by 1/2, so after three you get 1/8.

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12. Half-life is the same as 'time for all atoms to decay.'

True

False

Concept: exponential decay never hits exact zero. Decay is exponential and never reaches exactly zero. After each half-life, some nuclei still remain, so the amount keeps decreasing but does not abruptly finish.

Explanation

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13. Which isotope would decay the fastest?

Half-life 1 year

Half-life 10 years

Half-life 1 day

Half-life 100 years

Concept: interpreting half-life length. Shorter half-life means faster decay. A 1-day half-life means the sample halves every day, which is much quicker than years.

Explanation

Concept: interpreting half-life length. Shorter half-life means faster decay. A 1-day half-life means the sample halves every day, which is much quicker than years.

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14. If half-life is 2 hours, the time for the sample to drop to 1/4 is:

2 hours

4 hours

6 hours

8 hours

Concept: two half-lives gives one quarter. 1/4 requires 2 half-lives → 2×2=4 hours. Halving twice takes two half-life intervals.

Explanation

Concept: two half-lives gives one quarter. 1/4 requires 2 half-lives → 2×2=4 hours. Halving twice takes two half-life intervals.

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15. Which statements about half-life are true?

It is a property of the isotope

After one half-life, half remains

It depends on sample size

Amount decreases exponentially

Concept: core half-life facts. a, b, d are true. Half-life is isotope-specific, the amount halves each interval, and repeated halving produces an exponential decrease.

Explanation

Concept: core half-life facts. a, b, d are true. Half-life is isotope-specific, the amount halves each interval, and repeated halving produces an exponential decrease.

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16. After many half-lives, there may still be a tiny amount left.

True

False

Concept: exponential decay tail. Exponential decay approaches zero gradually. The remaining amount can become extremely small, but it may not be exactly zero.

Explanation

Concept: exponential decay tail. Exponential decay approaches zero gradually. The remaining amount can become extremely small, but it may not be exactly zero.

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17. A material with a long half-life is:

Very unstable

More stable (decays slowly)

Not radioactive

Guaranteed to be safe

Concept: stability vs half-life. Long half-life means slow decay, not 'non-radioactive.' It can still be radioactive; it just changes more slowly over time.

Explanation

Concept: stability vs half-life. Long half-life means slow decay, not 'non-radioactive.' It can still be radioactive; it just changes more slowly over time.

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18. If you start with 64 units, after 2 half-lives you have:

Concept: applying two halvings. 64 → 32 → 16. Two half-lives means divide by 2 twice, giving one quarter of the starting amount.

Explanation

Concept: applying two halvings. 64 → 32 → 16. Two half-lives means divide by 2 twice, giving one quarter of the starting amount.

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19. The half-life stays the same over time for a given isotope.

True

False

Concept: half-life constancy. It’s constant for that isotope. The decay probability per unit time stays the same, so the halving time does not change.

Explanation

Concept: half-life constancy. It’s constant for that isotope. The decay probability per unit time stays the same, so the halving time does not change.

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