If yellow light is produced by mixing red and green how is sodium vapour light monochromatic and yellow?

Answer 1

A good question. We perceive color because each of the cones in our eyes is receptive to only a specific range of frequencies, and so light of a given frequency will only stimulate some of these cells and not others. Any combination of light that stimulates the same cells by the same amount will be perceived as the same, even if spectroscopically it is quite different. Yellow light, such as that emitted by sodium vapor, falls well within the frequency range of both the "red" cones and the "green" cones, and so stimulates both, causing us to perceive yellow. However, if we were to provide both red and green light simultaneously, the red cones would be stimulated by red light and the green cones by green light, and we would also perceive yellow. Because we have but three types of cones, we can, both in principle and in practice, provide the perception of (almost) any color of light by varying luminosities of just the three primary colors, which is why we say that other colors are combinations of these three primary colors. However, monochromatic light of a frequency in between the frequencies of the primary colors will also appear to be a secondary color, even though it is obviously not a mixture of primary colors. Thus the grade school simplification of other colors as being literally composed of the three primary colors is, I'm afraid, quite misleading.

Answer 2

The process is through atomic emission. The filaments of the lamp sputter fast moving electrons, which hit the sodium atoms causing the valence electrons of the sodium atoms to excite to higher energy levels; and the electrons thus excited relax by emiting the characteristic monochromatic bright yellow light(589nm).
The yellow light produced by the red and green is composed yellow light meaning you have two wavelengths red and green. The monochramatic yellow light from sodium electrons is only yellow.

Answer 3

It depends what you mean by colour.

It is true that you could assign a colour to any single wavelength in the visible spectrum, or say that it lay in a colour range in the specturm. And in this case the twin lines of the sodium spectrum lie somewhere in what would be called the orange part of the spectrum. They are indeed, strictly monchromatic.

But your eyes are not sensitive to frequency. You cannot look at a picture and pick out just the bits that reflect light at just 500 nm. (By contract, your ears can pick out frequencies - you can pick out the violin in an orchestra for instance). Nor would a bit of an image consist of just one wavelength of light - a green leaf, for instance, simply reflect more green light than it does blue or red, but it certainly reflect some blue and some red.

Your eyes have just three colour sensors, so if you split the visual spectrum into three bits then you can combine them at the right intensity to give the eye the impression that it is looking at a colour anywhere along the visible spectrum. This is because you can arrange for each of the three types of receptor to get just the right amount of stimulation to see that colour.

The three colour ranges you split white light into for this purpose are - as you point out - red, green and blue. The right combinations of these can stimulate the eye to see any colour. Look closely at a TV set or computer screen and you can see the red, green and blue dots that make up.

Primary paint colours are different because you are not adding intensities of light in this case - you are subtracting. The primary reflective paint colours are red, blue and yellow, though these are not the colours that are used in printing. Printing inks are transparent, the light is reflected from the white of the page and the inks subrtract colours from them. So their colours are the inverse or red, green and blue - ie cyan, magenta and yellow.

Answer 4

Just to complement the answers already given, You have combinations of 3 celules in the eyes one detects blue, other green and the last one red. If you lock the right grafics you will see that red covers a lot of region, blue a smaller one and green also, the conbination on the exitation of this waves is what gives you the impression of color, when you detect sodium vapor light you are exiting both the green sensor and the red one, the color your brain processes is the thing that is called yellow.

Answer 5

Firstly you cannot "mix" light. Light is a wave with a range of wavelengths. When you mix red and green paints the substance formed reflects the light in the yellow region of wavelength , hence appears yellow.Please do not be under the impression that every time you need to generate a wave in the yellow region you need waves of red and green wavelengths.
Strictly speaking "red","green","yellow" are names given to ranges of wavelengths and not any specific wavelength.
You may have heard of interference and diffraction , these are mechanisms where 2 waves interact("mix").
Hope you are satisfied.

Answer 6

your question is best answered by understanding that "color perception" is accomplished by the brain and is not a physical property of the light itself. ( e.g. various optical illusions will prove this ).

secondly, light is NOT mixed like paints to produce colors. what happens when red and green light are projected on the retina is that they are PERCIEVED as yellow. ( yellow light is NOT made of red and green ).

yellow light itself is percieved as yellow ( no duh ! ).

you may want to read up on the theory of color perception and optics ( physics ) to gain a better understanding of these processes.

hope your question was answered !

Answer 7

Primary colors cannot be made by mixing any other colors. They are blue, yellow, and red.

Answer 8

noooooo yellow light can't be made by mixing red & green light
bcoz yellow is primary color
green is not primary color it's sec. color obtd. by mixing yellow & blue light

Answer 9

I don’t want to repeat the answers already given by Epidavros and Pascal. The have explained well.