I have got a serious doubt.
I have read, "yellow light from a low pressure sodium vapour is monochromatic"
How can it be monochromatic when yellow light is a combination of red and green primary colours?
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The key here is that there are a basically infinite number of different mixings of photons of different wavelengths that will be perceived as yellow by our eyes or a camera. This is because our eyes are not spectrometers, and use a relatively crude three-color system (red, green, and blue-sensitive cone cells) to identify colors. (Incidentally, this is why color TVs are feasible to make - they use essentially the same crude setup as our eyes, with red, green, and blue-emitting pixels.) But there is definitely a physical difference between 550nm monochromatic yellow light and the mix of red and green light that will appear to be the same color from your eyes. The monochromatic light will not disperse in a prism, while the mix of red and green will disperse into its red and green components.
You are right that yellow is a secondary colour in light - where RGB is primary. But you are slightly confusing the difference between how RGB works to create colours in a screen compared to sodium vapour.
You can tell whether an apparently yellow source is made up of a combination of red and green, or is just yellow by using a prism: different wavelengths of light have different refractive indexes for the same material, which is why a prism splits up colours. Sodium happens to produce (almost) only light in a single frequency/wavelength, but a different yellow source that looks to be the same colour could well be a combination of red and green wavelengths.
Color is not something that happens in the physical world. Color happens in your brain. In the physical world, every light source has a spectrum. The topic of how your brain and eyes reduce the spectrum of some light source to what we call "color" is quite deep.
The short answer is, there are many different spectra that your brain and eyes reduce to the color that we call "yellow." The light emitted by a low-pressure sodium lamp is one such spectrum that contains all of its energy in two very narrow, and very closely spaced emission lines. But, you can experience the same "yellow" color by looking at a light source (e.g., a color computer screen) that emits a totally different spectrum containing lines that you would call "red" and "green" if you saw them individually.
you can use a simple spectroscope to see the spectra of different light sources.
The notion of primary colors comes about from the way we perceive color (how our eyes and brain work) and the techniques we can use to represent color (printing, painting, television, computer graphics).
Light is electromagnetic energy. Just as acoustic energy (sound) has wavelength, so does electromagnetic energy. When electromagnetic energy has a wavelength in a certain range (typically 100km down to about 1m), we call it radio; when it has a wavelength in the range of 400nm to 700nm, we can perceive it with our eyes and we call it light. The longest visible wavelengths we perceive as red, the shortest we perceive as blue, and intermediates we perceive as green; wavelengths between what we perceive as red and what we perceive as green we perceive as yellow.
Our eyes have three kinds of color-sensitive receptors which will each respond to a portion of the electromagnetic spectrum; one responds most strongly to wavelengths around "red", another to wavelengths around "blue", and another to wavelengths around "green". Light having a wavelength between "red" and "green" will produce a response in both the "red" and "green" receptors, so we can trick our eyes into perceiving yellow by using a combination of red and green wavelengths. Thus what we perceive as yellow could be either wavelengths between "red" and "green" ("true yellow") or a combination of "red" and "green" wavelengths. What this also means is that the light falling on any given spot on the retina could include an almost limitless combination of wavelengths, each having some intensity, but the photoreceptors will produce an aggregate response as if it had only three components - red, green, and blue, and that an identical response could be obtained from different combinations of wavelengths.
For what it's worth: computers and TVs use a "primary color" system consisting of red, green, and blue; yellow is produced as an additive result of red and green. Printing uses a different system of primary colors consisting of yellow, cyan, and magenta inks; red is produced as an additive result of magenta and yellow (adding the dyes but effectively subtracting from the reflected light). Thus "primary color" isn't a fixed thing at all, but rather a human invention built upon a biological invention.