Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does 'white light' from the Sun look yellow?

  1. Jul 13, 2014 #1
    Light from the sun can be dispersed into its spectrum of colours. But when we look around us, and at the sun, it appears yellow. Then shouldn't it only be emitting the yellow part of the spectrum? We also use 'white light' sources when diffracting light through double slits, but then I couldn't really find in my books that I own what the source of that 'white light' is.
     
  2. jcsd
  3. Jul 13, 2014 #2

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

  4. Jul 13, 2014 #3

    Drakkith

    User Avatar

    Staff: Mentor

    The sun is not yellow. It is white. You just never look directly at the sun unless it is low in the sky and the shorter wavelength light is being filtered out by the atmosphere, turning it yellow/orange.
     
  5. Jul 13, 2014 #4
    But then if the blue light is scattered, then how do we still see the blue end of the spectrum when we disperse light close to the surface of the Earth? Also, if it's more intense in the yellow spectrum, then isn't it equally intense in the blues and reds spectrum by the look of dispersion of light images. As in, it's all the same frequency, the same energy, thus the same power...
     
  6. Jul 13, 2014 #5
    Actually, isn't it that, the colour that we see, isn't actually the real true colour?

    Like a red rose is not actually red. In reality we can only see with a small spectrum of light. So we only see the rose that colour because of that. If we could see in more spectrums of light we'd see more colours (infrared/ultraviolet etc). It probably wouldn't look as beautiful. Certain things can break up the full spectrum of light as well such as the atmosphere and stratosphere even possibly even earths magnetic poles. But many will say from the earths perspective that the sun is yellow or orange but if you was in space looking at the sun it would just appear as a white or a sort of slightly off-white until you got too close to it then it will get very dark again.

    Deep.
     
  7. Jul 13, 2014 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Wavelength is physics.
    Color is perception - part of psychology.

    It is known that our color perception saturates, so the color of the sun depends on how far away from it you are: it's not an intrinsic property of the object. So this is really more of a psychological question.
     
  8. Jul 13, 2014 #7

    Drakkith

    User Avatar

    Staff: Mentor

    Note that dispersing and scattering are not quite the same thing. The atmosphere isn't acting like a prism that turns white light into a rainbow of colors, it is acting more like a filter. When the sun is close to the horizon, the amount of air the light has to travel through to get to your eye is MUCH greater than when the sun is high in the sky, so a much larger percentage of the blue end of the spectrum is filtered out. This means that the sun appears white when it is high in the sky, and gradually becomes more and more reddish as it approaches the horizon.

    No, the intensity is different in the different colors. Also, the human eye isn't very good at measuring light intensity between different colors. A blue light that looks the same intensity as a yellow light is in reality MUCH brighter thanks to our eyes low efficiency in the blue end of the spectrum.

    I don't really know what you mean by saying it's all the same frequency, power, etc. Each color is a different frequency, and the sun shines at different intensities in different frequencies.

    Color is a subjective property, so we can't really say that a rose is "really" red in color. My dad is red-green colorblind. A rose to him would look similar in color to a blade of grass. Every person differs in their ability to discriminate colors, so there is no "true" color of anything. The only way to quantify the light reflecting from the rose would be to measure the intensity in every wavelength using a spectrometer.
     
  9. Jul 13, 2014 #8

    UltrafastPED

    User Avatar
    Science Advisor
    Gold Member

    The dispersion of the blue gives the sky its blue color ... which means that the blue is coming to us from all directions!

    So if you do like Isaac Newton, and make a hole in your window shutter to let in just a beam of sunlight for your prism, the blue will come from (a) the portion of the original sunlight that was not scattered, plus (b) the contribution from that portion of the sky.

    So the amplitude of the blue light will be less than if you were to take the same spectrum on the surface of the moon.

    Here is a brief description of "Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres"; there are many factors! http://rredc.nrel.gov/solar/pubs/spectral/model/section2.html
     
  10. Jul 13, 2014 #9
    So, it is white light then. That solves it. Thank you.

    The spectrum does not have common frequency but common speed. My bad. Sorry.

    =D So that's why we get a complete spectrum. But then, what source of white light do they use in double slit diffraction experiments in a closed room? It isn't explicitly stated in the textbooks what they use as source for diffraction of white light to form those spectum-ly bands. Though lasers are mentioned for otherwise diffraction experiments with red lights. And sometimes filament lamps for yellow. And thank you for the reference. =)
     
  11. Jul 14, 2014 #10
    I suspect another reason that the sun looks yellow can be explained by color appearance models, one of whose functions is to predict how the perceived color of an object depends on its immediate surround. Specifically, an object will be biased towards the complementary hue of its surround, and yellow (sun) is the complement of blue (sky).
     
  12. Jul 14, 2014 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    There have been three letters missing in many of the above posts. This letters are "ish".
    The sky looks blueISH and the Sun look yellowISH. Both of those colours (and most of the other colours we see in everyday life) are highly desaturated as they are a mix of all visible wavelengths. It's only when you do Newton's experiment (or buy a laser or LED, these days) that you see anything like spectral colours. When you get nearly equal amounts of light over the visible spectrum, you will call that 'white', if they are equal enough. The Sun 'looks' yellowish largely because you are viewing it next to a 'blueish' sky.

    There as many 'whites' around as you have had hot dinners. Look at the options on your Screen Controls and you will see a range of so called colour temperatures for your screen 'white'.

    Our eyes are constantly trying to eliminate the variations in perceived colour, due to the illumination. If you want to appreciate just how well it does, look at what a digital camera makes of a scene with mixed illuminants (e.g. light from a window and a tungsten lamp). The colours it 'sees' will vary wildly over the picture, whilst your eye can iron out all those variations and you manage to ignore them when you are actually there.
     
  13. Jul 14, 2014 #12

    UltrafastPED

    User Avatar
    Science Advisor
    Gold Member

    They mean sunlight, let in through a hole into a darkened room ... just like Newton. Sunlight has the spatial coherence required for diffraction due to the distance of the sun.

    In modern experiments they use lasers or gas discharge tubes.

    For more on white light sources see http://www.rp-photonics.com/white_light_sources.html
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why does 'white light' from the Sun look yellow?
  1. Light from the sun . (Replies: 8)

  2. Why is our sun yellow? (Replies: 66)

Loading...