What is the half-life of a radioactive substance?

Last Updated: 25 Jun 2026

What is the half-life of a radioactive substance? Here, the relevant answer is “Time for half the atoms to decay.” A sound solution relies on the governing concept rather than a memorised option letter, because option order may change in another paper. While reading a question, first note whether it asks for a definition, SI unit, formula, direction, cause, effect, or practical use.

ATime for complete decay

BTime for half the atoms to decay

CTime for double the activity

DTime constant of decay

Correct Answer: Time for half the atoms to decayYour Answer: B

Explanation

— the scientifically appropriate response is Time for half the atoms to decay. Option-wise review: Time for half the atoms to decay is correct under the condition given in the stem. Time for complete decay is incorrect because it describes a different quantity or phenomenon and does not meet the stated condition. Time for double the activity is also incorrect; although it may be a valid physics term, its definition or application is not the one asked here. Time constant of decay is incorrect because it fails the governing relation or characteristic relevant to this case. Therefore, option B, Time for half the atoms to decay, is factually and logically consistent with the question. Half-life is the time taken for half of the radioactive nuclei in a sample to decay.

Important Notes

Read the complete stem before looking at the options. Identify whether the demand is a definition, unit, formula, direction, cause, effect, image characteristic, or device principle. In this record, the confirmed answer is Time for half the atoms to decay.
Learn each concept as a five-part card: definition; common symbol; SI unit; principal equation; and one real-life application. This works for mechanics, heat, optics, sound, electricity, magnetism, and modern physics.
Use dimensional consistency as a quick check for formulas. Both sides of a valid physical equation must have the same dimensions. Units can eliminate distractors quickly, although a dimensionally correct option must still satisfy the actual law.

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FAQ

SSC CGL Physics FAQs

Common questions and clear answers for this topic.

How many physics questions usually appear in the SSC CGL exam?

Physics typically makes up a small portion of the General Science questions, usually focused on concepts rather than numerical problems.

Which physics topics are most important for SSC CGL?

Basic mechanics, heat, sound and light, and electricity and magnetism are the areas that come up most consistently across previous papers.

Do I need to solve numerical problems for SSC CGL physics?

Rarely. Most questions are conceptual and test everyday understanding rather than calculation skills.

Is NCERT physics enough for SSC CGL preparation?

NCERT textbooks from classes 9 and 10 build a strong enough conceptual base for most of what SSC CGL asks.

How can I make physics concepts easier to remember for SSC CGL?

Connecting concepts to everyday observations, like why a thermos keeps tea warm, makes them far easier to recall than memorizing definitions.

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