NCERT Class 10 Science Chapter 10 Activities — The Human Eye and the Colourful World

NCERT Class 10 Science — The Human Eye and the Colourful World, Activities 10.1 and 10.2 on pages 165 and 166.

Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.

This chapter has only two activities — fewer than any other in the book — and they are consecutive, on facing pages, both using a prism. Everything else in the chapter, the eye and its defects, atmospheric refraction, scattering and the blue sky, is taught from diagrams and explanation rather than from apparatus.

That makes the pair unusually easy to plan for: one prism, one sunny window, and both are done in a single lesson. They also belong together, because the second only makes sense once the first has been done.

• Activity 10.1 traces a single ray through a prism and finds it deviated — bent away from its original direction, not merely displaced as it was by a glass slab in Chapter 9.
• Activity 10.2 sends white light in instead of a single narrow ray, and the deviation turns out to be different for each colour.

Key insight. The whole of dispersion follows from one fact: the amount a ray is deviated depends on its colour. Activity 10.1 establishes that a prism deviates; Activity 10.2 shows that it deviates violet more than red. White light is not changed by the prism — it is sorted by it, and Newton’s second prism recombining the colours is the proof of that.

Activity 10.1

Fix a sheet of white paper to a drawing board. Stand a glass prism on its triangular base and trace its outline. Draw a straight line PE meeting one refracting surface AB at an angle. Fix two pins on PE, look through the opposite face AC and fix two more pins so that all four appear in line. Remove the prism and pins, join the points, and mark the angle of incidence, the angle of refraction and the angle of emergence.

What you should observe. The ray comes out travelling in a different direction from the one it went in with — not merely shifted sideways.

Why a prism deviates where a slab did not. Compare this directly with Chapter 9’s Activity 9.10. A glass slab has parallel faces, so the second refraction undoes the first and the emergent ray runs parallel to the incident one, merely displaced. A prism’s faces are inclined to each other, so the second refraction adds to the first instead of cancelling it. The chapter puts it exactly this way: “the peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray”.

What goes wrong. Two things, both fatal to the measurement. Pins that are not vertical make the sighted line wrong, exactly as in Activity 9.10. And moving the prism after tracing its outline — it must sit back precisely inside the traced triangle, or the angles measured afterwards belong to a set-up that no longer exists. Trace the outline, and do not lift the prism until the pins are placed.

Activity 10.2

Make a small hole or narrow slit in a thick sheet of cardboard and let sunlight fall on it, giving a narrow beam of white light. Let the beam fall on one face of a glass prism and turn the prism slowly until the light coming out appears on a nearby screen.

What you should observe. Not a white patch, but a band of colours in a fixed order.

Why the colours separate. Because the deviation measured in Activity 10.1 is not the same for every colour. In the chapter’s words, “different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most.” Each colour therefore leaves along a slightly different path, and by the time the light reaches the screen those paths have spread far enough apart to be seen as separate bands.

Why the order is always the same. It is the order of how much each colour is deviated — violet most, at one end, red least, at the other. VIBGYOR is not a list to memorise so much as a ranking, and a student who knows that violet bends most can reconstruct it.

Newton’s second prism, which is the real proof. Newton was the first to get the spectrum this way, and he tried splitting the colours further with a second prism — and could not. He then placed a second identical prism inverted relative to the first, and the colours recombined into white light. That experiment settles what would otherwise be an open question: the prism is not adding colour to the light, it is separating colour that was already there.

What goes wrong. Three things, and all of them produce a washed-out band rather than no band at all, which makes them hard to diagnose:

• A slit that is too wide. Each colour’s image of the slit overlaps its neighbours’ and the colours blur into near-white. The slit needs to be genuinely narrow.
• A screen too close. The colours leave the prism along paths that differ by very little; they need distance to separate. “Nearby” in the instructions means a metre or so, not a few centimetres.
• Too much stray light. The spectrum is faint. It is far more convincing in a darkened room with one bright beam than at a sunny bench.

And the standing caution from Chapter 9 applies here too: the beam comes from the Sun, so never look back along it or at the Sun through the prism.

Common mistakes

• Treating the prism as a thick glass slab. The slab displaces; the prism deviates. The difference is that the prism’s faces are inclined, and it is the whole reason a spectrum is possible at all.
• Saying the prism “adds” colour to white light. It separates colours already present. Newton’s inverted second prism, which recombines them into white, is the evidence — and it is worth describing even if the second prism is not available.
• Getting the deviation order backwards. Red bends least, violet most. Everything about the order of the spectrum follows from that one sentence.
• Reporting a pale band as a failed experiment. It is usually a slit that is too wide, a screen too close, or a room too bright. The fix is one of those three, not a different prism.
• Confusing the angle of deviation with the angle of the prism. The angle of the prism is the angle between its two refracting faces — a property of the glass. The angle of deviation is between the incident and emergent rays, and it changes with the angle of incidence.

Practise next

• The end-of-chapter Exercises for this chapter, where the dispersion of Activity 10.2 returns as questions on the rainbow and on why the sky is blue, and the prism of Activity 10.1 as questions on the angle of deviation.
• The in-text questions of this chapter, which ask what causes dispersion and why the sun appears reddish at sunrise — both resting on the colour-dependent bending shown here.
• The activities of Chapter 9, especially Activity 9.10, whose glass slab is the comparison that makes the prism’s behaviour meaningful.
• Chapter 11, Electricity, the next chapter, which leaves optics behind entirely.