BioPHYSICAL chemistry

Due to the quantization of energy (Chapter 9), the amount of energy in

a wave of light is proportional to its frequency, with higher frequencies

of light having higher energies:

E=hν (14.2)

The number his called Planck’s constant and has the value of 6.626 ×

10 −^34 Js.

Whereas the speed of light is restricted, light can have almost any energy

(Figure 14.2). Radiowaves are low-energy electromagnetic radiation that

travels through space between the radio station and your radio. Radio-

waves are also used when performing nuclear magnetic resonance (NMR)

experiments (Chapter 16). The frequencies for this type of radiation are in

the order of 1–100 MHz; that is, 10^6 Hz or millions of times per second.

Light with frequencies in the gigahertz range, or billions of oscillations

per second, lies in the microwave region. Microwaves are used to cook

foods in microwave ovens, to detect objects at long distances by radar, and

to measure the properties of electrons in electroparamagnetic resonance

(EPR; Chapter 16). At somewhat higher energies we have infrared light,

with frequencies of roughly 10^14 Hz that are characteristic of molecular

vibrations (Chapter 11), then visible red light, visible orange, yellow, green

blue violet, and the ultraviolet spectrum. At higher frequencies yet, there

are X-rays at 10^17 Hz, gamma rays, and other very high-energy photons.

Of these frequencies, X-rays are useful for determining the structures of

proteins since their wavelengths are about 1 Å and so are comparable with

the size of atoms (Chapter 15).

292 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Type of radiation Radiofrequency

Log (frequency (Hz))

Wavelength 3 km 30 m 30 cm 3 mm 0.03 mm 300 nm 3 nm 0.03 nm

Microwave Infrared

Red

700 620

Orange

580

Yellow

530

Green

470

Blue

420 nm

Violet

Ultraviolet X-rays, γ-rays

Visible light

5 6 7 8 9 1011121314151617181920

Figure 14.2The electromagnetic spectrum of light.