White, is it a colour or absence of colours? Our chemistry sir and we had an argument today at the lab, he says that white actually is not a colour, it is the abscence of colour, but we say that it is a colour and we gave the following point to substanciate our point that white is a colour:
When we see an object in red colour, it actually reflects red colour and absorbs all the other colours, in this point of view, a white object reflects all colours which fall on it, so it is a colour.
We do not know who is correct, I am posting this question in hope that I will get the correct answer.
 A: The explanation you give is correct. A white body reflects all wavelengths. We call it white when all colors (all wavelengths) are reflected from an object and hit our eye. Black is the opposite. 
I would say that white is all colors, as you do. But maybe he sees it from the perspective that since all is reflected and nothing is absorbed, there is "no light" left. I mean, it depends on what he means by "no light". It could be a matter of definition of the words, so maybe you actually agree on what happens but call it differently.
Nevertheless, your explanation is correct.
A: To add to Steeven's answer:
Any wavelength of input light will be perceived as lighter or darker in color depending on the intensity of the light.  At high intensities, we only 'perceive' whiteness.  I'm not sure whether the brain just ignores the overloaded cones (color-sensitive retinal elements) in favor of the rods (non-color elements), or whether a saturated cone-signal is interpreted directly as "white."  
Further, it is the case that very low-intensity signals don't even stimulate the cones, so everything is interpreted as white/grey via the rods' signals.  
A: One thing is certain: White is not a spectral color.
Beyond that, people argue. People who ascribe to additive light theory (e.g., the manufacturers of your computer screen) are apt to say that white is a color but black is not. People who ascribe to subtractive light theory (e.g., artists) are apt to say that black is a color but white is not.
To complicate matters, there are a number of Jedi mind tricks ("optical illusions") that our eyes and and mind play on us. One such illusion has recently popped up on the internet: Is this dress white and gold or blue and black?

There's a lot, a whole lot, of image processing that goes occurs between photons hitting receptors in our retinas and our mind saying "that's white and gold!" versus "that's blue and black!".
To complicate matters even more, our eyes only have three different kinds of color receptors. People who design lights take advantage of this. White LEDs are a prime example. The spectrum of a white LED is very, very different from the (almost) black body spectrum of the now-banned white incandescent bulbs. Iur eyes don't know the difference because we only have three kinds of receptors.
To complicate matters even more than that, our mind tries to compensate for varying lighting conditions. My living room has cool white lighting. The adjacent hallway, soft white lighting. The hallway walls look yellowish when viewed from the living room. When I go to the hallway, the walls look white.
To complicate matters yet more, our eyes and mind adjust what we think we see based on everything else we see. Suppose you have two objects, one you would definitely  say is "gray" and the other you would definitely say is "white". Which of squares A and B below is brighter?

The answer is that they're exactly the same. Our mind plays Jedi mind tricks on itself.

Bottom line, from a physics point of view:


*

*White is not a spectral color.

*White is an optical illusion.

A: we define color based on which range of wavelength it reflect. as u said a red object is red because it can only reflect red , so it is not any other color. maybe we could call black colorless, and i think our teacher just used a conventional form of speech (dont u think?)
ask our teacher what he would call the colour of water?
A: Colors have been defined by the International Commission on Illumination. They have defined the CIE XYZ color space where white is a color defined by the point x = y = z = 1/3. 
