Echoes and reverberations are the phenomena of the reflection of sound. A reflection is created when sound wave incident on a big object.
The Lightning Ball
A minute amount of mercury vapor or another inert gas is introduced to an enclosed evacuated container and an high voltage is applied. The gas, which is electrically conductive, is then partially ionized (known as plasma). Electrical discharge in an ionized gas may emit light (which is similar to the lightning we see in the air during a storm.)
Staying Your Shadow on the Wall
Where there is light, there is shadow. The shadow can only do what the owner does, but here, you can leave your shadow behind for a few seconds. Then you can try to shake hands with your own shadow. Don't believe it? Come try!
Do you need to raise you voice when talking to your friend if you are separated by a big balloon? Is it filled with air? Then it is still possibles! Would the effect be the same if a small balloon was used instead? How is this possible? The answer is…… it is a sound lens!