Shadows are not always black!

Shadows are absolutely
fascinating to play around with.

In this Exploratorium demonstration
you can see that a black shadow is only a subset of shadows that can be
formed on the screen.


If you have multiple sources of light with different colors, then you can additively combine colors to get shadows of various shades.


In this case where you have multiple sources of light, only when the object blocks off light from all three colored light sources do you get a black shadow.

Since we do not deal with multiple colored sources of light on a daily basis, go ahead, give this simple experiment a try. It’s totally worth it!

Have a good one!

** Other FYP explorations on Shadows :

Can a single point of light always illuminate an entire room ?

On disappearing shadows of Birds and airplanes



Can you crush a soda can with lightning ?

Here you can see the lightning striking the empire state building three times.

As you might know skyscrapers like these have a lightning rod that will carry the lightning bolt’s electrical charge through
the path of least resistance along the cable into the ground, reducing
the risk of fire or heat damage from the strike.


But you might know from high school or college physics that when you have two current carrying wires parallel to each other, then they experience an attractive force towards each other.


                                               Source Video                    

Lightning carries 30000 amps

An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA).

So if you were to pass such high currents through a rod, then surely they must experience a substantial magnetic force towards each other and get crushed right ?


Absolutely, Physics always works ! Have a look at this metal tube:


This is what would happen when electric lightning is passed through a metal tube. Using magnetic forces to compress electrical filaments is known as ‘Pinching’.

Can Crusher

One cool application for this would be for forming metal cans into interesting shapes.


As soon as the spark gap fires the capacitor discharges an enormous amount
of current through the coil (tens of thousands of amperes).  This
discharge creates a magnetic field around the coil. 

As the flux lines
pass through the cross section of the can, current is induced and flows
around the can.

 This induced current creates its own magnetic field
which opposes the magnetic field from the coil.  Between the two magnetic
fields there is now a force pushing inward on the can and outward on the
coil.  Once the force is strong enough the can is crushed.


And with large enough voltage, one can blow the can in opposite directions too!

From generating X-rays to forming of metal into shapes, Pinches have a lot of interesting applications. The references section of the wikipedia is surely a fun ride; check it out and have a good one!