Waves: Mechanical, Pressure and Light

“In space no one can hear you scream”....Alien

Is there energy here?

Force?

Work?

Work = Energy?

Why not one big wave?

Matter is sticky!

How well it sticks depends on the type of material.......

Solids > Liquids > GasesRipples are partly due to water sticking together.

As the first ripple rises, some of the force is resisted by the elastic connections between water molecules and the ripple is “pulled down” creating a trough.

slinky demo

Slinky

Potential energy converted to kinetic energy due to its height.

Energy in elastic bonds cause the up and down vibration.

Basically, we get a conversion and reconversion of energy back and forth between kinetic and potential energy!

What happens to the air as the slinky passes through it?

Slinky cont.

As slinky passes through air it encounters individual molecules.....what happens to these molecules as they get whacked?

Some kinetic energy from slinky is converted into kinetic energy in the molecules.

The end product is that the slinky, being quite heavier than the molecules, will end up pushing the air molecules in the same direction and about the same speed.

Air molecules will continue on in a straight line and at a constant velocity even after slinky is gone until other forces act on them. (sounds like laws of motion to me!)

Pressure waves

What ends up happening is a region of dense air being pushed through the air.

But, no actual movement of molecules along the wave occurs!!!! The energy is transferred from molecule to molecule, kind of like dominoes falling.

Ultimately, as slinky bounces up and down and is losing energy it causes regions of dense and less dense air moving away from slinky.

The rythmic bouncing of slinky creates oscillations along the slinky.....mechanical waves.

Wavelength, Frequency, Amplitude and Speed.

Speed = wavelength * frequency

units are called Hertz or kilohertz

1 Hertz = 1 wavelength per second

Wavelength is measured as distance.

Frequency of a wave is the number of wavelengths that pass a set point per second.

Amplitude, or strength of wave, is measured as the difference between ambient and the peak of the wave. The higher the amplitude the more energy is carried.

Summary of Waves

Carry energy by transferring the energy from one molecule to the next. eg. billiards

They are caused by vibrations in the energetic source

They tend to have a cyclical pattern, with measurable wavelength, frequency, amplitude and speed.

Now for biology

Waves carry information and an organism may just glean some information from this energy which may help them survive and/or reproduce.

Several of our sense are designed just for this.

For example, the pressure waves we described.....any other name for them? SOUND!

Our ears are designed to detect pressure waves ranging from 20 – 20,000 Hz.

Type of energy transfer?

Cochlea

Tympanic membrane

External Auditory Canal

Semicircular canals

Auditory Ossicles(malleus, incus and stapes)

“In space, no one can hear you scream”....Alien

Is this true or false? Given what we have discussed what would you say?

Is there air in space? Therefore there is nothing to transmit the energy from one place to another. No waves = No sound

Conduction: The transfer of heat (kinetic energy) by physical contact between molecules. Includes mechanical waves and pressure waves.

Convection: The transfer of kinetic energy by moving groups of molecules from one area to another.

Radiation: The transfer of energy via electromagnetic waves.

So far both mechanical and pressure waves depend on a fairly dense medium to carry energy from one place to another. Here are three ways energy can be transferred.

Waves - ElectromagneticElectromagnetic waves transfer energy with no physical contact.

Electromagnetic waves are a combination of a electric and a magnetic fields.

Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing, electromagnetic radiation.

Relative sizes of wavelengths to things you know

Properties of waves Reflection

Refraction

Absorption

Retina

Cornea

pupil

Iris

Optic nerve

Rods: Respond unselectively to many wavelengths of light. Primarily detect amplitude.Cones: Respond to a narrow range of wavelengths; different types detect different frequencies; responsible for color vision.Cones require fairly high light levels to function well – thus in dim light, we see primarily in Black-and-white.

Photoreceptors - eyes

You should be able to:

Identify wavelength, period, frequency, amplitude, and speed of a wave.

Explain how a vibrating object creates sound waves.

Describe how a mechanical wave carries energy from one place to another.

Trace the path of sound energy through the ear.

Identify key structures in the ear and how they affect sound waves or are affected by them.

Identify/explain properties of three modes of energy transfer: Conduction, Convection and Radiation.

Identify and explain the properties of electromagnetic waves: reflection, absorption and refraction.

Trace the path of light through the eye.

Identify important structures in the eye and explain how light affects them or they are affected by light.