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.