Case Study:

After a heavy afternoon rainfall in Shillong, a group of Class 10 students noticed a bright rainbow appearing across the sky at around 5:15 PM. Interestingly, the colour sequence was always the same - red on the outer side and violet on the inner side. Their science teacher asked them an important question: If sunlight appears colourless, where do these colours come from, and why do they always appear in a fixed order?

"If you've ever wondered about the deeper physics behind this, you can explore more about the magic of light dispersion, which turns white light into a masterpiece."

The students observed that the rainbow became brighter when the Sun was low in the sky and faded when clouds covered the sunlight. They also learned that millions of tiny water droplets suspended in the air act like small prisms. When sunlight enters a raindrop, it first undergoes refraction. During this process, white light splits into its seven constituent colours because different colours bend by different amounts. This phenomenon is called dispersion of light.

Inside the raindrop, the light then undergoes internal reflection from the back surface of the droplet. Finally, when the light comes out of the droplet, it refracts again before reaching the observer’s eyes.

This isn't just limited to raindrops;  the role of refraction in everyday optical phenomena explains everything from bent spoons in water to twinkling stars.

Since red light deviates the least and violet light deviates the most, the colours spread out in a fixed sequence every time.

The students concluded that a rainbow is not just a colourful pattern in the sky but the result of refraction, dispersion, and internal reflection occurring together inside countless raindrops.

CASE-BASED QUESTIONS

 MCQ 

Q1. Why does sunlight split into different colours inside a raindrop?

A. Due to absorption of light
B. Due to scattering of light
C. Due to dispersion during refraction
D. Due to reflection from clouds

Q2. Which colour bends the most while forming a rainbow?

A. Red
B. Yellow
C. Green
D. Violet

 Assertion - Reason 

Q3. Assertion (A): The colours in a rainbow always appear in the same order.
Reason (R): Different colours of light deviate by different amounts when passing through water droplets.

A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, but R is not the correct explanation of A.
C. A is true, but R is false.
D. A is false, but R is true.

 Application - Base 

Q4. A student says that if all colours deviated equally inside raindrops, a rainbow would still form normally. Do you agree? Give a reason.

Q5. Why is a rainbow usually visible only when the Sun is behind the observer?

 Data/Logic - Based 

Q6. A science group recorded the deviation of two colours while passing through water droplets:

Based on this data, identify the incorrect observation and explain why.

ANSWER KEY WITH EXPLANATION

A1. C. Due to dispersion during refraction
Explanation:   During refraction inside water droplets, white light splits into different colours due to dispersion.

A2. D. Violet
Explanation:   Violet light has the maximum deviation because it bends more than other colours.

A3. A. Both A and R are true, and R is the correct explanation of A.
Explanation:   The fixed order of colours occurs because each colour deviates differently during refraction.

A4. No. A rainbow forms because different colours deviate by different amounts during dispersion. If all colours deviated equally, white light would not separate into different colours, and no rainbow would be visible.
Explanation:   Equal deviation would prevent colour separation, so dispersion would not occur.

A5. The sunlight must enter raindrops in front of the observer and reflect toward their eyes. If the Sun is in front, the reflected light will move away from the observer, preventing rainbow formation.

Since our eyes are the final 'lens' that captures this light, understanding how the human eye works helps connect the physics of the sky to our own biology.

A6. The data is incorrect because violet light deviates more than red light. In a rainbow formation, red bends the least while violet bends the most due to greater refraction.

To see how light behaves differently during the day and evening, check out this guide on the scattering of light and why the sky changes color.

HOTS EXTENSION QUESTIONS

Q1. Suppose Earth had no atmosphere containing water droplets after rainfall. Predict how this would affect natural rainbow formation.

Q2. A scientist creates artificial raindrops using liquids of different refractive indices. Predict how the rainbow pattern may change.

Physics also helps us fix how we see these wonders; learn about common eye defects and how spectacles help correct our vision.

Still curious? Head over to our community to ask your own questions or challenge yourself with more interactive science quizzes.

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