Transparency of glass

March 18, 2017 06:24 pm | Updated March 19, 2017 12:07 am IST

Why is glass transparent to visible light but opaque to UV light?

Bhavani, Bengaluru

When light is incident on a material, it can be either reflected, absorbed or transmitted, or a combination of all the above phenomena might occur. As far as opacity (or transparency) of a material is concerned, we need to think in terms of absorption and transmission. Based on the material’s composition and property, a specific wavelength or a range of wavelengths might have enough energy to transfer an electron from the ground state to the excited state and hence that wavelength gets absorbed. The material thus appears as opaque to that wavelength. On the other hand, if the material transmits most of the incident light, it appears as transparent with respect to the incident light. In the case of glass which mostly consists of silica and aluminates, the energy associated with of UV light (~7eV) is enough to excite electrons from the ground state (valence band) to excited state (conduction band) and consequently this light gets absorbed. Beyond the range of UV light (wavelength >400 nm), the energy of visible and infrared light are not enough to excite the electrons and most of the incident light gets transmitted. Thus glass appears transparent to visible and infrared light.

Chandan Kumar Mishra. JNCASR, Bengaluru

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