Distinction between the terms white and colourless in Physics

[Prathamesh]

Is there any difference in the term 'White' and 'Colourless' in Optics? Is a light ray colourless or white. In many books colourless and white means one and the same.

But white is the colour of milk while water is colourless.

[Sisyphus]

Because light travels as a wave, all light has a frequency. Of the light that the eye can see, we interpret this frequency as color. "Red" is light with the lowest frequency that we can see, followed by orange, yellow, green, blue, and finally violet with the highest frequency.

 

Therefore, light cannot be colorless, because that would mean it didn't have a frequency. I would describe an object as colorless if it was transparent.

 

There is also no such thing as "white light," really. What we perceive as white is just a mixture of different colors. If red, green, and blue light are all hitting your eyes at once from the same direction, you perceive it as white. A white object like milk is perceived as such because it reflects all the colors of visible light in approximately equal amounts.

[Prathamesh]

Because light travels as a wave, all light has a frequency. Of the light that the eye can see, we interpret this frequency as color. "Red" is light with the lowest frequency that we can see, followed by orange, yellow, green, blue, and finally violet with the highest frequency.

 

Therefore, light cannot be colorless, because that would mean it didn't have a frequency. I would describe an object as colorless if it was transparent.

 

There is also no such thing as "white light," really. What we perceive as white is just a mixture of different colors. If red, green, and blue light are all hitting your eyes at once from the same direction, you perceive it as white. A white object like milk is perceived as such because it reflects all the colors of visible light in approximately equal amounts.

 

But practically speaking light is colourless. (We dont percieve light untill it is incident upon something, correct?) I am still confused. Consider the following:

1. When a beam of light is passed through a prism it disperses into its constituent colours. When the constituent colours are again combined by a prism we again get a white light (which is practically colourless)

2. When we paint the constituent colours of light on a disc in equal proportions and spin it we percieve the disc as white.

 

Please distinguish between the above two phenomena.

[Sisyphus]

Light is the only thing you perceive. You see it when it enters your eye and hits your eye. It can get there directly from the source (the sun, light bulbs), or reflected off something else (most things you see).

 

Those two phenomenon are the same. In both cases, you're getting a large number of photons ("particles of light") of different colors all hitting the same places in your eye at once. Your eye has three different kinds of color sensors, each most sensitive to a different range of colors. You distinguish colors based on the proportion of light that each detects. When they're roughly equal, you see that as white.

 

EDIT: I just realized what you probably meant with the colorless until incident upon something comment: different objects can appear to be different colors under the same light source, e.g. the sun. The reason for this is not that light is colorless until it hits something. It is because different objects absorb and reflect different colors of light.

 

So, for example, sunlight is made up of all the colors of visible light, in various quantities. An object that appears red in sunlight would be one that reflects red light, but absorbs the other colors. Thus the only portion of the sunlight that hits your eyes when you look at it is the red light. However, under a light source that doesn't include any red light, it would not look red, because there is no red light for it to reflect.

 

And again, objects that reflect a wide range of colors in roughly equal quantities will appear on the gray scale, from bright white if it reflects all light, to perfectly black if it absorbs all light.

[Prathamesh]

Light is the only thing you perceive. You see it when it enters your eye and hits your eye. It can get there directly from the source (the sun, light bulbs), or reflected off something else (most things you see).

 

Those two phenomenon are the same. In both cases, you're getting a large number of photons ("particles of light") of different colors all hitting the same places in your eye at once. Your eye has three different kinds of color sensors, each most sensitive to a different range of colors. You distinguish colors based on the proportion of light that each detects. When they're roughly equal, you see that as white.

 

EDIT: I just realized what you probably meant with the colorless until incident upon something comment: different objects can appear to be different colors under the same light source, e.g. the sun. The reason for this is not that light is colorless until it hits something. It is because different objects absorb and reflect different colors of light.

 

So, for example, sunlight is made up of all the colors of visible light, in various quantities. An object that appears red in sunlight would be one that reflects red light, but absorbs the other colors. Thus the only portion of the sunlight that hits your eyes when you look at it is the red light. However, under a light source that doesn't include any red light, it would not look red, because there is no red light for it to reflect.

 

And again, objects that reflect a wide range of colors in roughly equal quantities will appear on the gray scale, from bright white if it reflects all light, to perfectly black if it absorbs all light.

 

 

Thank You very much...I got it now