A person has normal vision, but he cannot distinguish between red-green colours. Why ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Normal Vision**: A person with normal vision can see clearly at various distances and can perceive straight lines without any distortion. This means that their eye structure is functioning properly in terms of focusing light.

2. **Identifying the Problem**: The question states that the person cannot distinguish between red and green colors. This indicates a specific issue related to color perception rather than a general vision problem.

3. **Role of Cone Cells**: The human retina contains specialized cells known as cone cells, which are responsible for detecting colors. There are three types of cone cells, each sensitive to different wavelengths of light corresponding to red, green, and blue colors.

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