This is the second part of our discussion about dispersion and color. And we’ll learn about:
1- Color of objects in white light.
2- Light filters.
3- Appearance of colored objects in colored light.
4- Primary and secondary colors.
5- Mixture of colored lights.
6- Mixing colored pigments.
and 7- Infrared and ultraviolet light.
So let’s begin our course!.
Mixing colored lights explained
1- Color of objects in white light
When white light falls on any particular body, then either all the colors in white light may be reflected from the body, when it appears white, or only some of them may be reflected while the others are absorbed.
In the latter case the body appears colored. The energy of light absorbed is generally converted into internal energy (for more details about internal energy, read work, energy and power part 2 ) so that the body becomes slightly warmer.
The color which the body presents to the eye is the the color of light which it reflects. Thus the leaves of plants appear green, since they reflect green light and absorb the other colors.
White paper reflects all the colors of the spectrum while black paper absorbs all of them. Blackness is thus due to the absence of light of any color.
2- Interesting results. Light filters
Interesting results are obtained when light filters in the form of sheets of gelatin colored with various dyes are placed, in turn, in front of the slit you saw in the previous article “I think you did”.
By this means, light transmitted by the filter can be analyzed into into its component colors. It is observed that certain colors, depending on the color of the filter, are now absent from the spectrum.
The missing colors are those of light which has been absorbed by the filter while the remaining colors have been transmitted.
An unusual results with yellow filter
Now one would expect red gelatin to transmit only red light, green gelatin only green light and so on. Indeed, this generally proves to be so when tested by experiment. But an unusual results is obtained with yellow gelatin. The spectrum of the light passing through most types of yellow gelatin is found to consist of red and green as well as yellow.
What is even more striking is that this particular yellow light looks just the same to the eye as that which comes from a filter passing only pure yellow.
To distinguish between the two, the former kind of yellow is called compound yellow light.
Experiment shows that the yellow petals of flowers and most yellow paints are examples of compound yellow.
3- Appearance of colored objects in colored light
A convenient source of light of various colors may be obtained by placing an ordinary electric lamp in a box with an opening which may be covered with gelatin sheets of different colors (I did this experiment in the classroom and my students were very happy!).
By means of such a lamp the appearance of different colored objects in different colors of light may be examined in a dark room.
Then it is found that red bodies look red in red light while green and blue ones look black, since they have absorbed the red light.
In like manner a red poppy appears black in green light.
On the other hand, the compound yellow petals of a daffodil appear black only in blue or violet light. The daffodil appears yellow only in yellow or white light. In red light it looks red and in green light, green.
4- Primary and secondary colors
Although yellow may be produced by mixing red and green lights, it is not found possible to produce either red, green or blue by mixing two other colors. For this reason, red, green and blue are called primary colors.
Yellow is called a secondary color. The other two secondary colors are
cyan: made by mixing green light and blue light
magenta: by mixing red light and blue light.
5- Mixture of colored lights
Before proceeding further it must be pointed out that mixing colored paints is an entirely different thing from mixing colored lights. ( a brief discussion about paints in the next headline).
The effect of mixing colored lights may be investigated by using three projectors fitted with slides of various colored gelatin sheets, and arranged so as to produce overlapping images on a white screen.
In this way it may be shown that a mixture of the three primary colors, red, green and blue, gives a white patch on the screen.
However, a successful result is obtained only by using the right kind of red, green and blue gelatin for producing the color and by having each light of the correct intensity “ for more details about wave intensity you can read: Electromagnetic waves illustration: origin and sources”.
This can only be done by experimenting with different types of gelatin and by having lamps of the appropriate brightness.
By using two projectors only the following facts may also be verified:
Red + Green = Yellow
Red + Blue = Magenta
Blue + Green = Cyan
These results, together with the knowledge that a mixture of the three primaries gives white, lead us to expert that:
Red + Cyan = White
Green + Magenta = White
Blue + Yellow = White
A further experiment with two projectors using appropriate gelatins shows that these interferences are correct.
Two colors such as those described above which give white light when added together are called complementary colors.
6- Mixing colored pigments. Comparison between color Subtraction and addition
One of the first thing that a student of painting learns is that green paint can be made by mixing yellow and blue paint.
This would not be possible if the paints available were pure yellow and pure blue.
The success of this method of making green paint depends on the fact that the pigments in common use are impure colors.
Yellow paint is a compound yellow so that, when illuminated by white light, it reflects red, yellow and green light and absorbs the blue.
Similarly, blue paint is not a pure color: it reflects blue and green and absorbs red and yellow.
When the two paints are mixed, then between them they absorb red, yellow and blue. The only color they both reflect is green.
Consequently, the mixture looks green.
This process is called color mixing by subtraction to distinguish it from the effect of mixing colored lights by reflection from a white surface which is called color mixing by addition.
7- Infrared and ultraviolet light
If a spectrum from an electric arc lamp or from the sun is produced on a screen it may be shown that invisible radiant energy is incident on the screen just beyond each of the extreme ends of the visible spectrum “as shown in the image”.
Just beyond the red end of the spectrum is a region occupied by infrared radiation. This is invisible, but its presence may be demonstrated by placing a thermopile where the rays can fall on it.
A galvanometer connected to the thermopile will give a deflection. Details of the thermopile are given in this article Radiant heat: useful application of radiation (the link is above at the word “radiant”.
Ultraviolet light can be detected in several ways.
If a piece if photographic paper is placed on the screen it will become darkened to a variable extent where the spectrum falls on it, but maximum darkening is caused by ultraviolet light just beyond the violet end.
Ultraviolet light also causes certain substances to fluoresce, i.e., glow with visible light. Paper lightly smeared with Vaseline behaves similarly.
Laundry washing powders usually contain small quantities of a substance with fluoresces with a bluish white light under the action of ultraviolet light in sunlight.
This enhances the whiteness of linen and helps to combat the natural darkening of the material with age.