We've all used thermometers to measure temperature
but sometimes we need to measure temperature in places you (1) __
can' t put a thermometer -- for example, inside a rocket engine.
One of the best methods for those really hard-to-get-to
place is based on laser spectroscopy. Light from a laser point- (2) __
ing at the region of interest is absorbed by molecules and some (3) __
of it are re-emitted back towards the source where it can be (4) __
measured. The re-emitted fight is shifted in frequency (changed
in wavelength) by amounts that depend on the molecule and
also the temperature and pressure of the surrounding gas be- (5) __
cause of collisions among the molecules. The single, sharp
frequency from the laser is thus smeared out into a whole
spectrum of frequency whose shape can be used to determine (6) __
the temperature provided so that the molecular transitions and (7) __
line shapes are known. Because it is the main component of
the air, it is convenient to use nitrogen since its molecular (8) __
properties are known from room temperature to temperature
of about 1,200 degree Celsius from experimental measurements. (9)
Although this is a large range, it is far short of what it is needed (10) __
for, some applications.