Explain why some materials are transparent to visible light but opaque to ultraviolet radiation.

Eduqas GCSE Physics — 6.2 Interactions of electromagnetic radiation with matter · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Different materials interact differently with different parts of the electromagnetic spectrum. Glass, for example, allows visible light to pass through but blocks most ultraviolet radiation.

Model answer (4 marks)

Transparent materials transmit visible light because the photons pass through without being absorbed. The photons of ultraviolet radiation have a higher frequency (and therefore more energy) than visible light photons. The higher‑energy UV photons are absorbed by the atoms or electrons in the material. Because UV is absorbed and does not pass through, the material appears opaque to UV.

Examiner tips

Use the word ‘transmit’ for visible light and ‘absorb’ for UV; link energy to frequency (E=hf).
Show the energy hierarchy: UV > visible, explaining why only UV is absorbed.
Mention that absorption prevents transmission, giving opacity.

Common mistakes

Confusing ‘transparent’ with ‘opaque’ for visible light; stating that UV is transmitted instead of absorbed.
Failing to mention the higher energy/frequency of UV photons.
Using vague terms like ‘block’ without explaining absorption.

Mark scheme (4 marks)

Transparent materials transmit visible light — photons pass through without being absorbed
The photons of ultraviolet radiation have higher frequency / more energy than visible light photons
The higher-energy UV photons are absorbed by the atoms / electrons in the material
Because UV is absorbed and does not pass through, the material appears opaque to UV

Key terms in this question

transparent · opaque · ultraviolet radiation

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