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Author Topic: Why does light change colour when going through coloured glass?  (Read 10200 times)

Offline Super Hans

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I have some blue glass in the door, when the sunlight shines through, the light that has passed through the blue glass now looks blue on the floor...why?


 

Offline evan_au

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Blue glass is very effective at absorbing red and green light, allowing mostly blue light to pass through.

When this blue light reflects off the floor, there is mostly blue light present, so this patch of floor looks blue.
 

Offline David Cooper

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The important thing to understand is that there is less light hitting the floor with the blue glass in the way than without it, so it isn't a matter of the light changing colour, but of it losing some of its colours. The result of this colour loss is that the light ends up looking more colourful to our eyes than white light. White light is actually the most colourful kind of light because it contains many colours, but we just see it as a boring white. We only see light as being more attractively coloured once it's lost some of its colours. So, we're dealing with removal of colour, and if you send light through deep blue glass and then through deep red glass, hardly any light will get through both filters.
 

Offline Super Hans

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If the glass is blue shouldn't blue light be reflected and not pass through?

I'm new to this so sorry for my ignorance and thanks for your answers.
 

Offline chiralSPO

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There are different ways that things can be colorful.

Colored glass is a filter that are largely transparent, but absorb some frequencies of light (some colors). White light goes in and, as it passes through, the glass absorbs some of the light at specific frequencies. Thicker glass will be darker because the light has to travel farther through it. Light that reflects off of the glass may be blue, or potentially yellow (it depends how the glass gets it's blue color) The primary color scheme for filters (like printer ink) is magenta (white minus green), cyan (white minus red) and yellow (white minus blue).

You can also have pigments that change the color of reflected light, but it's the same basic idea.

Colors can also come from emitted light, without any need for another source of light. Neon lights, fireworks, LEDs all are colorful based on the spectrum of light that is emitted. This also contributes to DayGlo colors (they absorb light and fluoresce) Primary color scheme is red, green and blue (like in a computer screen, and in our eyes [really primary colors just have to do with the construction of our eyes--we have three types of cones that pick up in red, green and blue, so there are going to be three primary colors]).
 

Offline chris

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Hans, you raise a very good point, which is that we are told that things look a certain colour because they are reflecting light of that colour from their surface while absorbing other colours (wavelengths) of light. The result is a surface that appears to be coloured.

With glass, however, it's usually transparent. The addition of certain chemicals to the glass gives it the ability to absorb certain wavelengths (colours) leaving others to pass through.

This means that if you put your eye behind such a piece of glass, the only lay rays reaching you will be those with wavelengths that the glass does not absorb. And because the light is coming through the glass, it makes the glass itself look that colour, partly because some of the light scatters (refracts) inside and on the surface of glass, so there are light rays issuing from the glass at different angles, which is why you can see that the glass surface is there.

The result is a pane that looks coloured but also transmits light of the same colour.

I hope that makes it slightly easier to see.

Chris
 

Offline PmbPhy

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Quote from: Super Hans
If the glass is blue shouldn't blue light be reflected and not pass through?
No. As evan explained, the glass is blue because blue light is passing through it. I.e. when white light, such as the light from the sun or a lamp, passes through the glass then your eyes are seeing blue light. If it let through red and green light as well then it'd be letting all the light through and you'd have normal clear glass. The glass is blue because the non-blue light is taken out of it by the glass absorbing non-blue light.

Make sense?
 

Offline Super Hans

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Yeah so certain chemicals are added to the glass that absorb all wavelengths of light except blue, which goes through...so the blue light from the sun is the only wavelength not absorbed so wherever it hits ends up looking blue, and we can see that the actual glass is blue because some of the light refracts and reflects off the surface.

Yep that's clear now, thanks for taking the time to explain, everybody - much appreciated.

« Last Edit: 15/09/2014 00:30:26 by Super Hans »
 

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