UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q1(i). Colour of a red flower in green light

Question

A flower is red in colour in daylight. What will be its colour in green light at night? Justify your answer.

It will look black (very dark). The petal reflects only red; green light is absorbed and no red light is present to be reflected.

The colour of an opaque object is the colour of the light it reflects. The pigment absorbs the rest.
In daylight (all wavelengths) the petal pigments absorb the blue-green region, about 490–550 nm. The remaining light, mainly red, is reflected. Red is the complementary colour of the green absorbed.
Green light at night contains only the green wavelengths. These are exactly what the pigment absorbs.
No red light falls on the petal, so none can be reflected. Almost no light reaches the eye from the petal, and it appears black.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

The colour of an opaque object is the colour of the light it reflects. The pigment absorbs the rest.
In daylight (all wavelengths) the petal pigments absorb the blue-green region, about 490–550 nm. The remaining light, mainly red, is reflected. Red is the complementary colour of the green absorbed.
Green light at night contains only the green wavelengths. These are exactly what the pigment absorbs.
No red light falls on the petal, so none can be reflected. Almost no light reaches the eye from the petal, and it appears black.

A white flower or a green leaf would look green under the same lamp.

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