I. The colour of light depends on its frequency II. When white light is dispersed by a triangular prism…

To solve this question, let’s analyze each statement based on the principles of optics and the physics of light.

1. Analyzing the Statements

Statement I: The colour of light depends on its frequency.

• Concept: The frequency of a light wave is an intrinsic property that remains constant even when the light passes from one medium to another. The colour we perceive is directly determined by this frequency.
• Verdict: This statement is correct.

Statement II: When white light is dispersed by a triangular prism, yellow is deviated more than green.

• Concept: According to Cauchy’s equation and the refractive index of glass, light with a shorter wavelength (higher frequency) deviates more than light with a longer wavelength (lower frequency).
• Spectrum Order: The order of colours in a spectrum (from least deviated to most deviated) is ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
• Comparison: Since Green has a shorter wavelength than Yellow, Green is deviated more than Yellow. Therefore, saying Yellow is deviated more than Green is incorrect.
• Verdict: This statement is incorrect.

Statement III: Rainbows are formed when rains fall heavily.

• Concept: Rainbows are formed by the refraction, internal reflection, and dispersion of sunlight in water droplets. While rain is necessary, "heavy rain" is not the requirement; in fact, rainbows are best seen when the sun is shining while it is drizzling or when there is a light mist/rain. Heavy rain often implies thick cloud cover, which blocks the sunlight needed to create the rainbow.
• Verdict: This statement is incorrect.

2. Conclusion

Since only Statement I is correct, the correct option is B.

3. Exam Tip

Remember the acronym ROYGBIV for the visible spectrum. Always remember that Red is at the top (least deviation, longest wavelength) and Violet is at the bottom (most deviation, shortest wavelength). If you are ever asked about deviation, just remember: the closer the colour is to Violet, the more it bends!