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The refractive index of a material depends on the wavelength of the light incident upon it which is why dispersion happens. When we say that glass has refractive index 1.5 which wavelength do we have in mind?

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    $\begingroup$ If a high school or introductory college textbook says this, it typically means it's approximately $1.5$ throughout the visible region, where it doesn't vary much. Of course, a real reference work will specify the wavelength. $\endgroup$ – knzhou Feb 12 at 11:24
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In this Wikipedia page it says:

Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of 589 nanometers.

Therefore it's in the middle of the visible light band. In the list provided in that page you can see that glass (it calls it fused silica) at 20 Celsius degrees has a refractive index of around $1.5$ at that wavelength.

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You can look at wikipedia : https://en.wikipedia.org/wiki/List_of_refractive_indices

"Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of 589 nanometers."

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