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## 23 April 2012

### Just the facts -- waves

I've had lots of positive feedback from the "just the facts" series of posts.  Yesterday, my colleague Erik Born asked me if I had such a list of facts for waves and optics.  I've done optics before, at this post.  But I'd not written up waves.

Below are the fundamental facts of physics that Woodberry Honors Physics students need to know for the national Honors Physics exam.  These are all also correct for the AP Physics B exam, but AP B adds in standing waves, and quantitative interference/diffraction for slits and thin films.  I think this is accurate for the New York Regents exam as well; please, New York folks, let me know if I'm missing something.

Enjoy!

Regarding waves, you need to know:
Definition of wavelength, amplitude, frequency, period

v = lf

The speed of a wave depends on the material through which it travels.

When a wave changes materials, the frequency doesn’t change.

Longitudinal waves have particles that vibrate parallel to the wave velocity; transverse waves have particles vibrating perpendicular to the wave velocity.

Sound is a longitudinal wave that moves about 340 m/s through air.

The loudness of a sound is related to the wave’s amplitude; the pitch of a sound is related to the wave’s frequency.

Electromagnetic waves are transverse waves that move through a vacuum, or air, at 3 x 108 m/s.   Visible light is an electromagnetic wave with wavelength in a vacuum of about 400-700 nm.

The brightness of light is related to the wave’s amplitude; the color of light is related to the waves’s frequency.

Electromagnetic waves with frequency lower than visible light are called infrared, microwave, and radio.  Electromagnetic waves with frequency higher than visible light are called ultraviolet, x-rays, and gamma rays.

The index of refraction n is defined as c/v : the speed of light in a vacuum divided by the speed of light in a material.

When two waves collide, they interfere constructively and/or destructively.  The colliding waves’ amplitudes add algebraically.

Two waves whose frequencies are close together will produce a “beat frequency” equal to the difference between the two waves’ frequencies.

The Doppler effect means that a wave source approaching an observer increases the observed frequency; a source receding from an observer decreases the observed frequency.

Refraction is the bending of a wave at a boundary due to the speed change.

Diffraction is the bending of a wave around an obstacle, which happens when an obstacle is about a wavelength in size.