An essential part of modern dentistry is visible-light curing (VLC),
a procedure that hardens the restorative materials used in fillings, veneers, and other applications. These "curing lights" work
by activating molecules known as photoinitiators within the restorative materials. The photoinitiators, in turn, start a process of
polymerization that causes monomers to link together to form a
tough, solid polymer network. Thus, with VLC a dentist can apply
and shape soft restorative materials as desired, shine a bright light
on the result as shown in FIGURE $25-50(a),$ and in 20 seconds have a
completely hardened - or cured- final product.
The most common photoinitiator is camphoroquinone (CPQ).
To cure CPQ in the least time, one should illuminate it with light
having a wavelength that matches the wavelength at which CPQ
absorbs the most light. A rough plot of the relative absorption of
CPQ for light of different wavelengths is shown in FIGURE $25-50$ (b).
Many VLC units use a halogen light, but there are some drawbacks
to this approach. First, the filament in a halogen light is heated to
a temperature of about $3000 \mathrm{K},$ which can cause heat degradation
of components in the curing unit itself. Second, less than 1$\%$ of the
energy given off by a halogen bulb is visible light, so a halogen bulb
must have a high power rating to produce the desired light intensity
at the wavelengths that CPQ will actually absorb.
More recently, VLC units have begun to use LEDs as their light
source. These lights stay cool, emit nearly all of their energy output
as visible light at the desired wavelength, and provide light with
an intensity as high as $1000 \mathrm{mW} / \mathrm{cm}^{2},$ which is about 10 times the intensity of sunlight on the surface of the Earth.
$\cdot$ How much radiation pressure does the beam of light in Problem
110 exert on a tooth, assuming the tooth absorbs all the light?
$$\begin{array}{ll}{\text { A. } 6.67 \times 10^{-8} \mathrm{N} / \mathrm{m}^{2}} & {\text { B. } 2.00 \times 10^{-6} \mathrm{N} / \mathrm{m}^{2}} \\ {\text { C. } 1.33 \times 10^{-5} \mathrm{N} / \mathrm{m}^{2}} & {\text { D. } 4.00 \times 10^{3} \mathrm{N} / \mathrm{m}^{2}}\end{array}$$