Draw a ray diagram to show the formation of a rainbow and mark the point where (i) dispersion, (ii) internal reflection occurs.
Draw a ray diagram to show the formation of a rainbow and mark the point where (i) dispersion, (ii) internal reflection occurs.
A rainbow is a natural spectrum that appears in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets suspended in the atmosphere. The water droplets act like small prisms. Sunlight entering a raindrop first undergoes refraction and dispersion at the surface of the droplet (splitting into its seven constituent colours), then undergoes internal reflection at the opposite inner surface, and finally undergoes refraction again as it comes out of the raindrop. The red light bends the least and violet the most, so the seven colours reach the observer's eye as a band. A rainbow is always formed in a direction opposite to the Sun.
Marking Scheme
- 11 mark: Correct ray diagram showing the path of sunlight through a water droplet (incident ray, refracted/dispersed ray inside, internally reflected ray, and emergent coloured rays).
- 21/2 mark: Clearly marking point of dispersion.
- 31/2 mark: Clearly marking point of internal reflection.
Hint
Think of the raindrop as a tiny prism combined with a mirror. Light enters, splits, bounces inside, and then exits spread out into colours.
Quick Oral Answer
Inside a raindrop, sunlight is first refracted and dispersed at the upper surface, then totally internally reflected at the inner back surface, and finally refracted again as it leaves the droplet. Dispersion occurs at the point of first refraction and internal reflection occurs at the inner surface of the droplet.
Analysis & Explanation
The NCERT text explicitly states: 'The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer's eye.' The diagram above captures the three key optical events in a single water droplet: dispersion at first refraction, total internal reflection at the back, and a second refraction while exiting. The drawing is essential for full marks in a 2-mark question — both a clear diagram and the labelling of points (i) and (ii) are needed.
Common Mistakes
- 1Showing only refraction and forgetting internal reflection inside the droplet.
- 2Drawing emergent rays that are not spread into VIBGYOR colours.
- 3Forgetting to mark the two specific points asked in the question.
- 4Drawing a prism instead of a spherical water droplet.
Interesting Facts
A rainbow is always a circle (full circle is visible from an aeroplane) but from the ground we see only a semicircular arc.
The angle between the Sun, the raindrop and the observer is about 42 for the red band and 40 for the violet band — this is why the colours form an arc.
A secondary, fainter rainbow can sometimes be seen due to two internal reflections inside each raindrop; its colour sequence is reversed.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2012?
This question carries 2 marks in the CBSE Class 10 Science 2012 examination.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Formation of a rainbow" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2012 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.