Define half life of a radioactive material.

Explanation

**Step-by-Step Solution:**

1. **Understanding Radioactive Decay**: Radioactive materials consist of unstable atoms that decay over time, transforming into different elements or isotopes while emitting radiation.

2. **Defining Half-Life**: The half-life of a radioactive material is defined as the time required for half of the original quantity of radioactive atoms to decay. This means that if you start with a certain number of radioactive atoms, after one half-life, only half of those atoms will remain undecayed.

3. **Expressing the Concept Mathematically**: If \( N_0 \) is the initial quantity of the radioactive substance, after one half-life \( t_{1/2} \), the quantity remaining \( N \) can be expressed as:
\[
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