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Fill in each gap with one of the following words. (There is an extra word.) Then check your answers with the help of the key below.


will / of / into / in / have / is / and / toward / the / but / with / back / because / they / of /





The fact that an atom will emit a photon of radiation when one of its electrons drops to a lower energy state has made possible a powerful new tool - the laser, the acronym for light amplification by stimulated emission of radiation. In its simplest form, a laser consists __________ (1) a rod of transparent crystal or a tube filled __________ (2) gas or liquid. A mirror is placed at one end a half-silvered mirror at the other. The laser is "pumped" by adding energy - for example, by shining another light source __________ (3) it, by adding electrical energy, or by stimulating a chemical reaction. This raises electrons in __________ (4) laser to higher energy states.

Some of the electrons __________ (5) spontaneously fall back to a lower state, emitting photons. The photons that travel ___________ (6) the sides of the laser are quickly lost, __________ (7) those traveling along the length of the rod or tube are reflected __________ (8) by the mirrors. When they strike other excited atoms, __________ (9) stimulate those atoms to release photons of the exact same energy level (or wavelength), which travel __________ (10) the same direction as the stimulating photons. The result is an intense, highly focused beam __________ (11) light escaping through the half-silvered end of the laser.

Because the photon wavelength __________ (12) determined by the characteristics of the atoms in the lasing material, laser light is of a single wavelength. And __________ (13) it travels in a tight beam, it can carry a great deal of energy over a great distance without loss. Newer variations include electron lasers, in which photons are emitted when electrons are wiggled back __________ (14) forth by magnetic fields, and solid state lasers that can be built into microchips. In addition to visible light, lasers can be made that produce infrared and ultraviolet radiation and even x-rays.


Go down to see the key to this exercise.










Key to the exercise:

1: of

2: with

3: into

4: the

5: will

6: toward

7: but

8: back

9: they

10: in

11: of

12: is

13: because

14: and