Current Affairs · · GS3 · Science & Technology

Why glass is transparent: light passes because electrons cannot absorb it

Ordinary glass is transparent because the energy of visible light is too small to excite its electrons, so the light passes through without being absorbed. Glass also has no internal grain boundaries to scatter light. Higher-energy ultraviolet light can be absorbed, which is why glass blocks some UV.

Event date:

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    WHY IN NEWS?

    • A Hindu science explainer answers a common question: why is glass transparent?
    • The answer lies in how light interacts with the electrons and structure of glass.
    • The same physics explains why metals are shiny, why some materials block UV, and how solar cells work.
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    THE SHORT ANSWER

    • Light is made of photons, each carrying a fixed amount of energy.
    • An electron can absorb a photon only if the photon's energy matches a jump to a higher energy level.
    • In glass, this jump, called the band gap, is much larger than the energy of visible light.
    • So visible photons cannot be absorbed. They pass straight through.
    • Ultraviolet photons carry more energy, and some are absorbed. Glass therefore blocks part of the UV range.

    Think of it as a step too high to climb: visible light cannot reach it, so it walks past.

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    KEY CONCEPTS

    1. What is a photon?

    It is a packet of light energy. Blue and ultraviolet light carry more energy per photon than red light.

    1. What is a band gap?

    It is the energy an electron needs to move from a filled level to an empty one. Glass has a wide band gap; metals have none, which is why metals absorb and reflect light.

    1. What is scattering?

    It is when light bounces off in different directions. Fine powders, frosted surfaces and materials made of many tiny crystals scatter light and look opaque or white.

    1. Why does structure matter?

    Ordinary glass is amorphous, with no grain boundaries inside. A single, flawless crystal such as quartz is also transparent. Materials made of many small crystals scatter light at the boundaries between them.

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    EVERYDAY LINKS

    • Sunburn through windows: Ordinary glass blocks most UV-B, but some UV-A passes through.
    • Frosted glass: A rough surface scatters light, so it lets light in but blurs images.
    • Coloured glass: Added metal ions absorb certain colours of visible light.
    • Solar cells: Silicon has a narrow band gap, so it absorbs visible light and turns it into electricity, the opposite of glass.
    • Optical fibres: Very pure glass carries light over long distances with little loss.
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Sources

Syllabus

PaperSubjectSub-topic
GS3Science & TechnologyScience and Technology, everyday science, materials
PrelimsScience & TechnologyElectromagnetic spectrum, photons, band gap, scattering

Topics

IndustryInformation and Communication TechnologyPhysics

Practice questions

  1. Ordinary glass is transparent to visible light mainly because:

    1. It has free electrons that reflect light
    2. Visible photons lack enough energy to excite its electrons across the band gap
    3. It absorbs all visible light
    4. It has a crystalline structure
    Show answer

    Answer: B. With no absorption and little scattering, light passes through.

    Difficulty: medium · statement

  2. Which type of radiation is partly absorbed by ordinary window glass?

    1. Radio waves
    2. Visible light
    3. Ultraviolet
    4. Microwaves
    Show answer

    Answer: C. UV photons carry more energy, enough to be absorbed.

    Difficulty: medium · statement

  3. Frosted glass appears translucent rather than transparent because:

    1. It absorbs all light
    2. Its rough surface scatters light
    3. It has a smaller band gap
    4. It contains metals
    Show answer

    Answer: B. Scattered light still passes but images blur.

    Difficulty: medium · statement