Singing Glasses
Theme: God gives his children precious comfort in times of trouble.
Bible Verse: But no one says, “Where is God my Maker, Who gives songs in the night?” (Job 35:10)
Materials Needed:
• Several goblets (or glassware with stems)
• Water
Bible Lesson
The passage in Job 35 describes those who do not call upon God for help. They may cry out and plead for aid (v. 9), but they ignore the one source of help available to them. Let us be sure that we do not fail in this way. Instead, we have the privilege of calling upon the name of the Lord in times of trouble.
Nights especially can be times of fear and loneliness. This is when the presence of the Lord is most precious, as he “gives songs in the night.” Such a song may be a Bible verse or an actual chorus or hymn. Perhaps we take Christian music too much for granted. It is a great blessing; our lives would be diminished without it. Remember Paul and Silas, who were comforted with songs at midnight while they were locked in prison (Acts 16:25). The Lord and his disciples also sang a hymn before they went to the Mount of Olives, where Jesus was arrested (Matthew 26:30).
Science Activity
Set out several goblets with water in them. Ordinary drinking glasses may also work but not as well. The secret is to have glassware with thin walls.
By themselves the glasses are silent; they have no ability to make music. However, the touch of a finger can make them sing. Wet your index finger and gently stroke the goblet completely and continuously around the rim while you hold the glass steady by the base of the stem. Very little finger pressure is needed, and the finger must be kept moist while it is moving around the rim. In a few seconds the glass should start to resonate or vibrate, causing a clear, loud sound. The ringing sound is quite pleasant. The pitch, or frequency, depends on the glass size and also on the amount of water it contains. Volunteers may want to help you get several goblets vibrating at the same time. By adjusting the water levels, interesting chords can be produced. It takes a gentle hand to coax the sounds from the glasses. Likewise, it takes the Lord to give comforting “songs in the night.”
A moist finger rubbed around the rim can cause a goblet to ring.
Science Explanation
The ringing of a glass is called a resonance vibration. Sound is always produced by vibrating objects, whether violin strings, vocal cords, or air currents within a flute. For a goblet, the moving finger causes the sides of the glass to vibrate slightly inward and outward. The moistened finger provides a smooth contact, necessary for the continuous sound. It is somewhat like a violin bow moving across a string and causing the string to vibrate.
The sound produced by a goblet has a frequency of several hundred cycles per second. If you watch the water surface closely, you will see tiny standing waves produced by the moving glass walls.
Goblets can sometimes be shattered by high-frequency sounds that originate outside themselves. This is also called resonance, but it is an entirely different type of vibration. The shattering frequency is well above 20,000 cycles per second and very intense. In this case, the atoms within the glass begin to vibrate violently and eventually tear apart, resulting in breakage. There is no danger of broken glass in our activity as long as one holds the goblet upright.
If you have difficulty getting sound from the glassware, it may help to wet your fingertip with vinegar. The vinegar dissolves any oil that may be present and increases the needed friction between your fingertip and the glass rim.