SAT Physics Subject Test

Chapter 14 Optics

Optics is the study of light and its interaction with devices such as mirrors, lenses, and prisms. Light (or visible light) makes up only a small part of the entire spectrum of electromagnetic waves, which ranges from radio waves to gamma rays. The waves we studied in the preceding chapter needed a material medium to travel through, but electromagnetic waves can propagate through empty space. Electromagnetic waves consist of time-varying electric and magnetic fields that oscillate perpendicular to each other and to the direction of propagation of the wave.

THE ELECTROMAGNETIC SPECTRUM


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

c = 3.00 × 108 m/s

regardless of their frequency.


Electromagnetic waves can be categorized by their frequency (or wavelength); the full range of waves is called the electromagnetic (or EMspectrum. Types of waves include radiowavesmicrowavesinfraredvisible lightultravioletX-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 usuallyexpressed in nanometers. For example, electromagnetic waves whose wavelengths are between 577 nm and 597 nm are seen as yellow light.

  1. What’s the approximate frequency range for orange light?

Here’s How to Crack It

According to the spectrum, light is orange if its wavelength is between about 6 × 10–7 m and 6.25 × 10–7 m. Using the equation λf = c, we find that the upper end of this wavelength range corresponds to a frequency of

while the lower end corresponds to

So the frequency range for orange light is

4.8×1014 Hz ≤ forange ≤ 5 × 10 14 Hz

  2. How would you classify electromagnetic radiation that has a wavelength of 1 cm ?

Here’s How to Crack It

According to the electromagnetic spectrum presented on the previous page, electromagnetic waves with λ = 10–2 m are microwaves.