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Reflected Energy

A flashlight shining on a mirror will reflect almost 100% of the energy
The same flashlight shining on a sheet of glass will reflect some, perhaps 10%, with the balance passing through the glass

Now for a mechanical analogy—this time for resonance.
Why is it when a car tire is out of balance it is most pronounced at a “certain speed”?
Why is it when a washing machine is in a spin cycle it vibrates most at its’ “certain speed.
What are these “certain speeds”?
The answer is the speed at which the car frame, or washing machine frame is resonate at.
Resonance is defined as the point at which an object absorbs the most energy.
An example of an electrical resonant circuit is the series combination of a coil of wire and a capacitor. Sometimes called a “tank circuit”.
Antennas also can have a resonant point. The shortest antenna length that is resonant is a half wavelength. Remember, we started by defining what a wavelength is.
In the case of our example, 30 Mhertz, the wavelength is 10 meters. The shortest antenna length that will resonate at 30 Mhz is half of 10 meters or 5 meters long. Check it out!

This slide is the very nature of radio, electromagnetic waves and is MAGIC!!
The top figure shows a dipole antenna with a battery connected in series with it
The second figure shows the same arrangement but a switch is connected between the battery and the other half of the dipole
If we measure the voltage across the top dipole we get the battery voltage but the current meter will read nothing-it’s an open circuit.
Now let’s have you mechanically flip the switch on the second dipole at a rate of 30 million times per second. The volt meter will read the same as the above example, but the ammeter will now read a current. If the dipole is physically positioned at a half wave length above the earth and you perform the ohms law calculation: voltage divided by current equals resistance, you will calculate an impedance (AC resistance) of 72 ohms.
The great Michael Farady, James Clerk Maxwell and Oliver Heaviside are responsible for this discovery and the tools needed to evaluate how this works.