Shadows are everywhere.
Puppet show effectively make use of shadows. Mostly they are black. Sometimes we can make these shadows colorful as well.
Shadow is the region where light doesn't reach. Mostly there are black. But sometimes you have seen colored shadows. Lets understand more about colored shadows with this activity.
Let us take this A4 sheet and place it vertically with the help of a corrugated sheet stand.
We will mount this pencil here.
When the flashlight of the phone is switched on, we get a shadow of the pencil . It is black in color.
I will replace this white light from the phone with this colored LED light. I made it from 1 watt LED bulb, 9Volt battery and PVC pipe as a container.
How the shadow will look like when I switch on this red LED light ? It is still black in color.
Now let us bring in another light with green color. This time I see two shadows. There is no black shadow at all.
Let me bring in the third color which is blue. Can you guess what will happen ?
This time there are three colors but they are all of different colors.
What is the secret behind this color change ?
Let me switch off one light at a time and let you think about it. Can you make some observations ?
With only two lights red and blue, I get two shadows but of different color.
Let us turn off one light and observe the shadow.
With only two lights red and green , i get two shadows again of different color.
How about moving the pencil around. Will it change the color of the shadow ?
Not really.
How about moving the light sources ? Let us move the sources apart from each other.
Let us move the red light source away. What do you observe ?
As the red light source is moved away, other colors become darker.
The coloured shadows are possible due to one important property of light.
It travels in a straight line.
With the help of a ruler let us make this pencil and red light in a straight line.
When red light is switched on, we get a black shadow.
For our analysis, we will consider just one ray from source, travelling along the tip of the pencil.
This ray is blocked by a pencil. It can not travel further to the screen. As a result, the shadow is black.
Same is true in the case of blue color as well as green color.
In case of two light sources, for the green region or shadow on the screen, pencil blocks the ray from red light. Green rays are free to travel in that region. As a result we see the green shadow.
For the red shadow, green rays are blocked by the pencil. Rays from red light are free to travel in that region. As a result, we see a red shadow.
For the same reason, we see blue and red shadows as well.
Let us turn on the third light source - Green.
For the yellow shadow, rays from blue light are blocked by pencil. Rays from red and green light are free to travel to the screen. As a combination of red and green color results in yellow color, we see yellow shadow.
Let us replace pencil with slightly wider card sheet.
We can move it around and explore different formations.
Rays from blue and red sources can reach here . We see magenta for this region.
Rays from green and red sources can reach here . We see yellow for this region.
In this region, only green light can reach while in this region only blue light can reach.
In the middle, only Blue and green light can reach. We see a cyan color.
Taking the strip nearer to the screen reveals another color - Black.
This is the region where no light reaches and we see a black color.
Strip with a square opening in the middle behaves differently.
This opening enables light rays from all sources to reach the screen at different locations on the screen.
As a result, we see three white squares on the screen. A combination of three primary colors Red Green and Blue.
Try with different shapes and patterns for the opening.
If we block all the light except one circle in the center like this, we get another familiar pattern.
Additive color mixing is the kind of mixing you get if you overlap primary colors red, blue and green in a dark room.
If you don't have colored light sources, you can make use of flashlights of mobile phones and transparent color paper. Use corrugated sheets to construct a simple stand for these phones .
Do try this at home.
Thank You.
Credits : Exploratorium - Coloured shadows lecture