Why a thin oil film spread over water appears colourful in light?

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Setup**: When a thin film of oil is spread over water, it creates a layer that has different optical properties than the water beneath it. The oil film is very thin, which is crucial for the phenomenon we are about to discuss.

2. **Incident Light**: When light strikes the surface of the oil film, some of it is reflected off the top surface of the oil, while some penetrates the oil and is reflected off the oil-water interface below.

3. **Reflection of Light Rays**: The two reflected rays (one from the top surface of the oil and the other from the oil-water interface) will travel different paths. The ray that reflects off the top surface travels a shorter distance compared to the ray that reflects off the bottom surface.

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