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.
