Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Fozzie on 09/11/2009 16:09:57

Title: How do all the photons that leave the Sun combine to make white ?
Post by: Fozzie on 09/11/2009 16:09:57

The photons which leave the surface of the sun have a range of energy levels which can be split up by a prism forming the familiar rainbow of colors, blue for high energy photons and red for low energy etc. How is it then that the combination of all these colors is just right so that it produces white? It seems rather a coincidence! (OK, I know the sun is officially classed as yellow, but it looks white to me!). Surely if the energy levels of just one group of those photons varied, the sun would change color?

Title: How do all the photons that leave the Sun combine to make white ?
Post by: Mr. Scientist on 09/11/2009 16:23:00

White? White what.. light?

Title: How do all the photons that leave the Sun combine to make white ?
Post by: Madidus_Scientia on 09/11/2009 16:29:40

Actually, the sun is white. It looks yellow during sunset or sunrise due to atmospheric scattering, but if you look at it at midday you will notice it's white.

Title: How do all the photons that leave the Sun combine to make white ?
Post by: syhprum on 09/11/2009 17:18:44

Our eyesight has evolved with the assumption that the light from the sun is white, if it was a dull red or a high temp blue we would still think of it as white.

Title: How do all the photons that leave the Sun combine to make white ?
Post by: RD on 09/11/2009 19:01:19

All colours are not equally represented in sunlight: the sun is currently yellow-green star ...

Quote

Most of the sun's radiation comes to us in the yellow-green part of the visible spectrum.

http://www.nasa.gov/worldbook/sun_worldbook.html

Title: How do all the photons that leave the Sun combine to make white ?
Post by: lightarrow on 09/11/2009 19:55:01

Quote from: Fozzie on 09/11/2009 16:09:57

The photons which leave the surface of the sun have a range of energy levels which can be split up by a prism forming the familiar rainbow of colors, blue for high energy photons and red for low energy etc. How is it then that the combination of all these colors is just right so that it produces white? It seems rather a coincidence! (OK, I know the sun is officially classed as yellow, but it looks white to me!). Surely if the energy levels of just one group of those photons varied, the sun would change color?

White is not the colour of a single wavelenght, so it has not a physical meaning, but only perceptive. If you try to define the word "white" you begin to understand something...

Title: How do all the photons that leave the Sun combine to make white ?
Post by: Soul Surfer on 11/11/2009 09:07:25

We evolved under sunlight and colours are essentially differences between the illumination under sunlight.  White is an arbitrary measure of even reflection of all wavelengths and in eyesight varies according to the light source. For example compared with sunlight ordinary tungsten filament light is a reddish orange. If you took colour pictures with old fashioned film cameras you had to compensate for this with filters.  Modern digital colour sensors automatically compensate for this because they are programmed to adjust the colours to match what the human eye would see.

Title: How do all the photons that leave the Sun combine to make white ?
Post by: lightarrow on 11/11/2009 20:18:11

It's not so simple, Ian. We perceive as white even what is not necessarily sun light. Do you know the phenomenon of coloured shadows? The colour white is not only in our retinas, but in our minds too.

Title: How do all the photons that leave the Sun combine to make white ?
Post by: Soul Surfer on 12/11/2009 18:32:15

I agree but From the level of the question I thought that a simple answer would be adequate. 

I am well aware its is possible for us to see a full range of (rather attenuated) colours from objects illuminated with only a relatively narrow (but non zero) spectrum of yellowish light. That is with no red or blue light presence at all.  This is because our perception of colour depends on the difference in reflectivity of objects with our three types of colour sensors in our eyes