Thursday, Nov 18, 2010

Told students that test on waves has been postponed until next Tuesday.

Handed back RA 11.1a, Name that Sound quiz - asked if there were any questions

Quiz on waves vocabulary

Two ways of classifying waves: transverse vs longitudinal, mechanical vs electromagnetic
The reason we did polarization was to show the difference between transverse and longitudinal waves - ONLY transverse waves can be polarized.

Some waves (mechanical) require a medium, something material to travel through.
For sound waves, the closer together the atoms, the faster the speed.
Speed of sound = 330 m/s in air (about 1 km for every 3 seconds, or 5 seconds for every mile)
If you see the flash of lightning and count the seconds, you can tell how far away the lightning struck.
Water molecules are closer together than air molecules so speed of sound is greater in water: 1500 m/s (5000 ft/sec) in water
closer still in metals: 5000 m/s in aluminum

Electromagnetic waves do not require a medium, they can travel through the vacuum of space. We know this because we can see the stars.

Went through parts of electromagnetic spectrum:
radio - microwave - infrared - visible (ROYGBIV) - ultraviolet - Xray - gamma ray

Showed that TV sets and computer monitors give off radio waves that can be picked up with a radio.
Microwaves have a wavelength of about 1 ft (30 cm) since they have to fit inside the box in the kitchen
Remotes give off IR (that can be seen with cell phone in camera mode). Also, people glow in the IR (detectable by alien in the Predator movie). We are not as hot as the Sun which glows in visible light but we are hot enough to glow in IR.
Visible light (broken into colors by prism). Our eyes are most sensitive to yellow-green light because we evolved that way to be most sensitive to the main light put out by the Sun.
Ultraviolet - enough energy to cause sunburns
X-rays - can show bones
Gamma Rays - formed from reactions within nuclei of atoms or collisions of neutron stars or black holes (gamma ray bursts)

All these waves have the SAME speed in vacuum. (Students often mistakenly think the higher energy waves travel faster.)
Radio waves are NOT sound waves!! Radio waves are electromagnetic transverse waves, sound waves are mechanical longitudinal waves.

Used equation speed = wavelength * frequency to find frequency of microwave, wavelengths of FM and AM radio waves.

Gave students problem worksheet for homework.
Also asked students to complete RA 11.2b if they had not already.