To determine which statements about magnets are correct, let's analyze each statement one by one.
**Step 1: Analyze Statement I**
- Statement I: "A bar magnet becomes weak if broken along its length."
- When a bar magnet is broken along its length, each piece retains its magnetic properties, but the overall strength of the magnet can decrease. Therefore, this statement is **correct**.
**Step 2: Analyze Statement II**
- Statement II: "If a bar magnet is broken into two pieces, the total number of north poles becomes 4."
- When a bar magnet is broken, each piece becomes a new magnet with its own north and south pole. Thus, if a bar magnet is broken into two pieces, there will be a total of 2 north poles (one on each piece), not 4. Therefore, this statement is **false**.
**Step 3: Analyze Statement III**
- Statement III: "Magnetism is induced in an iron nail placed near a magnet."
- When an iron nail is brought close to a magnet, the magnetic field of the magnet induces magnetism in the iron nail. This means the nail will temporarily behave like a magnet. Therefore, this statement is **correct**.
**Step 4: Analyze Statement IV**
- Statement IV: "Magnetic effect cannot pass through non-magnetic substances."
- Magnetic fields can penetrate non-magnetic materials, but they do not induce magnetism in them. For instance, a magnet can attract iron but will not attract plastic. Therefore, this statement is **correct**.
**Conclusion:**
- The correct statements are I, III, and IV. Thus, the correct answer is that statements I, III, and IV are correct.
