What does phenomenon of red drop refer to?

Explanation

### Step-by-Step Solution:

1. **Definition of Red Drop**: The phenomenon of red drop refers to a specific observation in photosynthesis where the rate of photosynthesis decreases significantly when light wavelengths exceed a certain point, particularly above the red light region.

2. **Historical Context**: This phenomenon was discovered by researchers Emerson and Lewis in 1943. They conducted experiments to study the effects of different wavelengths of light on the rate of photosynthesis.

3. **Wavelength Reference**: In their experiments, they identified that the red light region, specifically around 680 nm, is crucial for photosynthesis. Below this wavelength, the rate of photosynthesis is relatively high.

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