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Dispersion is one of the most drawn diagrams in Class 10 science and one of the most often drawn wrong. The colours come out in the wrong order, or the rays bend the wrong way at the second face. This post explains why the colours separate, puts numbers on it with real glass data, and ends with a checklist for the diagram.
Where does this sit in the syllabus?
The CBSE Class X Science curriculum for 2026-27 lists "Refraction of light through a prism, dispersion of light, scattering of light, applications in daily life (excluding colour of the sun at sunrise and sunset)" and includes tracing rays through a glass prism as practical 13. The text is sections 10.3 and 10.4 of the NCERT Class 10 chapter, The Human Eye and the Colourful World (2026-27 reprint). Refraction through a prism returns in Class 12 and in the JEE (Main) 2026 syllabus. Check your current syllabus.
What happens to one ray in a prism?
NCERT's Activity 10.1 traces a single ray. At the first face, light goes from air into glass and bends towards the normal. At the second face, it goes from glass into air and bends away from the normal. Because the two faces are not parallel, the ray does not come out parallel to where it started, as it would from a glass slab. The angle between the incident ray and the emergent ray is the angle of deviation.
Why does white light split into colours?
When NCERT's Activity 10.2 sends a narrow beam of sunlight through a prism, a band of colours appears on the screen: violet, indigo, blue, green, yellow, orange, red, remembered as VIBGYOR. The band is the spectrum, and the splitting is dispersion.
NCERT's reason: different colours bend through different angles, red least and violet most. The deeper reason is that glass has a slightly different refractive index for each colour. OpenStax College Physics 2e explains that the refractive index rises steadily as wavelength falls, and its Table 25.2 gives crown glass 1.512 for red (660 nm) and 1.530 for violet (410 nm).
Worked example: how far apart do red and violet travel?
Use crown glass, n(red) = 1.512 and n(violet) = 1.530, c = 3.00 × 10⁸ m/s, and an angle of incidence of 50° at the first face.
Speed in glass, from n = c/v:
Red: v = 3.00 × 10⁸ / 1.512 = 1.984 × 10⁸ m/s.
Violet: v = 3.00 × 10⁸ / 1.530 = 1.961 × 10⁸ m/s.
Violet is slower in glass by about 2.3 × 10⁶ m/s.
Angle of refraction, from n = sin i / sin r, so sin r = sin 50° / n, with sin 50° = 0.766:
Red: sin r = 0.766 / 1.512 = 0.5066, so r ≈ 30.4°.
Violet: sin r = 0.766 / 1.530 = 0.5007, so r ≈ 30.0°.
After the first face the two colours are only about 0.4° apart. The second face bends them again, and the gap grows. Carrying the same calculation through the second face of a 60° prism (a Class 12 step) gives a total deviation of about 38.2° for red and 39.8° for violet. A difference of 1.6° is small, which is why the screen needs to be some distance away before the spectrum spreads out clearly.
What did Newton's second prism show?
NCERT describes Isaac Newton trying to split a single colour again with a second prism and getting no new colours. When he placed a second identical prism upside down, the colours recombined into white light. That suggested sunlight is a mixture of colours, not something the glass adds. Newton's own account, his 1672 letter on his new theory about light and colours in the Philosophical Transactions, begins by recalling that in 1666 he procured a triangular glass prism to test the well-known phenomena of colours.
A rainbow uses the same idea. NCERT explains that raindrops act like small prisms: they refract and disperse sunlight, reflect it inside, and refract it again on the way out. Our post on scattering of light and the rainbow works through that geometry.
Which mistakes cost marks?
Violet at the top. Red deviates least, so it is nearest the prism's apex side; violet is lowest, nearest the base.
Bending away at the first face. Air to glass bends towards the normal.
Drawing all colours leaving the first face together. They already separate inside the glass, slightly.
Saying the prism creates colours. Newton's recombination shows it separates colours already present.
Using a glass slab argument. A slab's parallel faces give no net deviation, and the colours come out parallel, so you see no spectrum.
A practice routine
Draw a prism with one ray, marking i, r, e and the angle of deviation, from memory.
Draw it again with white light, showing only red and violet, then shade the rest.
Redo the worked example with i = 40°.
Revise refractive index and Snell's law in our post on light, reflection and refraction for Class 10.
Some ideas are easier to see than read. The Fermi Simulation Lab has interactive simulations for ideas like this, so you can change the angle and watch where each colour goes.
Before your next test, draw the dispersion diagram twice from memory and check the order of colours.
