Cracking the SAT Physics Subject Test

Through a vacuum, all electromagnetic waves travel at a fixed speed

c = 3.00 × 10^8 m/s

regardless of their frequency.

Electromagnetic

Wave Speed

All electromagnetic

waves, regardless of

frequency, travel through a

vacuum at this speed.

The most important

equation for waves,

v = λf, is also true for

electromagnetic waves.

For EM waves traveling

through vacuum, v = c, so

the equation becomes

λf = c.

Electromagnetic waves can be categorized by their frequency (or wavelength); the
full range of waves is called the electromagnetic (or EM) spectrum. Types of
waves include radiowaves, microwaves, infrared, visible light, ultraviolet, X-
rays, and γ-rays (gamma rays), and although they’ve been placed in the spectrum
below, there’s no universal agreement on all the boundaries, so many of these bands
overlap. You should be familiar with the names of the major categories and, in
particular, memorize the order of the colors within the visible spectrum (which, as
you can see, accounts for only a tiny sliver of the full EM spectrum). In order of
increasing wave frequency, the colors are red, orange, yellow, green, blue, and
violet, which is commonly remembered as ROYGBV (“roy-gee-biv”). The
wavelengths of the colors in the visible spectrum are usually expressed in
nanometers. For example, electromagnetic waves whose wavelengths are between
577 nm and 597 nm are seen as yellow light.