Light and Color

What colors can be produced when light mixes?

Three lights are turned on.  These lights are red, green and blue in color.  If the lights shine on a ‘white’ board, and overlap, what colors will you see?  How are these colors formed?

The bulbs of three ‘colored’ lights.

The three lights shine on a ‘white’ board, a hand blocks some of the light

What is seen in areas where none of the colored light shines?

Can you draw the path the light takes as it travels from the light-bulbs, until it is seen?  Where must the light end-up if you are to see it?  What happens to the light as it travels this path?  Read on to understand.....

What path does light take from its source to the point where you see it?

When light leaves the blue bulb, what direction does it travel in?  Is it surprising that it travels in ALL directions, except those blocked by the hood that covers part of the bulb?  As the light travels and then strikes a surface, like the front wall, what happens to it after it strikes the surface?  If Fred sees light coming from the white-board at the front of the room, what path did the light take so that Fred could to see it?  What must happen to the light as it strikes the white-board for Fred to see the light?  What would you call this process? (click here)  Do you think the one path shown for the blue light striking the white board makes sense?  Remember that each blue arrow on the diagram can be called a ray of blue light.  The ray describes the path that light travels.

Can Gwen see blue light coming from the same area on the board as Fred?  How many blue light rays leave the board from point X? (click here)  This type of reflection is called diffuse reflection.  Any process that causes light rays to leave a surface and move away in all directions can also be referred to as scattering.

Now that you understand how light spreads out from a bulb, lets think about what happens to light from each of the three colored bulbs.  When all the lights are turned on, which colors of light make it to the front whiteboard?  Which parts of the board does each color of light make it to?  What colors of light does Fred see coming from the front board?  Read on to explore these questions...

Red+Green

Red+Blue

Red+Blue+Green

Blue+Green

Overlapping Colors

Resulting Color

In the photos above the three ‘colored’ lights shine on a ‘white’ board.  Study this photo carefully.  The area where the red light shines partially overlaps the areas where the green and blue light shine.  What color is produced where only the red and green light overlaps?  What color is produced where only the red and blue light overlap?  Can you complete the chart below for what color is produced when these primary colors of light overlap?  Click here to see the answers.  Remember that red, blue and green are the primary colors of light.

What happens if all three lights are turned on?  What will the shadows on the board look like then?  Check the picture below to see!  This time a basketball is blocking the light.

What happens to the colors of light you see when an object blocks some of the light from getting to the front board?

A basketball held in front of the three colored lights.  Notice you see six colors and a black region with no light.  The colors are Blue, Red, Green, Cyan, Magenta, Yellow.  The RED REGION is where the basketball is blocking the GREEN and BLUE lights.  The MAGENTA REGION is where the basketball is blocking the GREEN light.  Can you list what light is blocked from the other regions?

Cyan

Magenta

Green

Red

Yellow

Blue

no Light

Yellow

Suppose the green light is turned off and the red and blue lights are left on.  Also suppose that there are no other lights in the room.  Then a large P.E. ball is placed in front of the lights as shown above.  What will Fred see on the board?  Does red light make it to all the parts of the front board?  Does blue light strike everywhere on the board?

Trace the paths of the light rays above to see where red light strikes the board and where blue light strike the board.  Try to identify the area where only red light strikes the board.  Which area is this?  Try to identify what colors Fred would see on the board in areas 1 to 5.  Click here to see if you are correct.  Click here to see the video of this demonstration.  Which areas on the front-board above could be called shadows?

copyright Marcus Milling  2009

Here is a demo with a cubic box blocking light and only red and green lights are turned on.  When the white room lights are turned off the pattern below is seen on the front board.

Note that you see two shadows produced by two lights and one object blocking light.  Notice that the RED and GREEN light in the picture above are set at the same height and so the shadows they produce are also at the same height on the board.  The region where the green light is blocked (the shadow of green) is red in color.  The region where the red light is blocked (the shadow of red) is green in color.

What would the shadows look like if only the red and blue lights were on.  Try to predict and then click here to see the picture.

What would the shadows look like if only the blue and green lights are on.  Try to predict and then click here to see the picture.