the colour of objects
It is logical to say we need light in order to see an object. But does it still sound logical if we say in order to see the colour of an object the light falling on it must contain all the necessary wavelengths. After all isn’t a red object always red? Well, not necessarily. If you cut your finger in the garden in the afternoon, you will see red blood, whether it’s sunny or cloudy. Cut your finger whilst scuba diving a few metres underwater and your same blood will appear anything from green to near black depending on how deep you are. Whilst seeing green blood can be very disconcerting, physics can prevent you from going into panic. Upon entering water, light wavelengths will either reflect light back, penetrate right through it or become absorbed. Which action takes place depends on the wavelength and the depth of the water. At relatively shallow depths the longer red wavelengths are absorbed by the water. Objects too will also reflect, absorb or allow light wavelengths to pass through. So in order for the red blood to be perceived as red it must reflect back the red wavelength and absorb all others. So without the red wavelength underwater the colour we perceive is green-black.
So it is important to recognise that the colour of an object is not solely an intrinsic quality of the object, but a combination of its colour characteristic and the wavelength of the light that falls on it. When a white light falls on an object, the object will absorb all colour wavelengths except for the colour wavelength of its own appearance, which it will reflect back. If an object’s surface reflects back all colours it appears white and if it absorbs all colours it appears black.
The blue vase pictured is illuminated by the white light of the sun which as we now know is a combination of all the colour wavelengths of the visible spectrum. Red, green and blue in varying combinations will create all of the other spectrum colours. The red and green wavelengths are absorbed by the blue vase, but the blue wavelength is reflected back enabling the eye to perceive the object as blue in colour.
Similarly, the multi coloured feathers are each reflecting back their own colour wavelength only and absorbing all others.