A laser pointer is a small (often battery-operated) laser device designed for pointing at objects by illuminating them with a collimated visible laser beam. Most laser pointers, particularly the cheap ones, contain a small GalnP/AIGaInP laser diode operating somewhere in the red spectral region. Some significantly more expensive pointers emit green or even blue light and normally contain a small diode-pumped solid state laser with a nonlinear crystal for frequency doubling.
A typical use of a hand-held laser pointer is to point at some screen or chart during a presentation, e.g. a conference talk. This is convenient because it can be done from a large distance and requires only a rather small device. However, the visibility of the generated spot on the screen is often poor (particularly for red laser pointers with relatively long emission wavelength), and a fast moving light spot can have a somewhat nervous appearance. Therefore, some people would prefer an old-fashioned telescopic pointing device for presentations. Laser pointers can be useful e. g. for aligning some machinery, or for certain optical distance measurements.
The apparent brightness of the illuminated spot strongly depends on the wavelength of the emitted light. Most devices operate in the red spectral region, where the sensitivity of the eye rapidly decreases with increasing wavelength. Devices with 650-nm output appear about twice as bright as those emitting the same power at 670 nm, and 635-nm devices still about two times brighter. However, the shorter-wavelength laser pointers are typically more expensive. This is particularly true for green lasers, which are quite significantly brighter than their red counterparts, but still cost quite a lot. They involve a diode-pumped solid state laser and a frequency doubler. Due to the typically poor conversion efficiency of the frequency doubler at low power levels, hundreds of milliwatts of infrared (typically 1064-nm) light are required to generate a few milliwatts in the green, and the batteries will accordingly not last very long, except if they are comparatively heavy.
Lay persons often ask what is the range of a laser pointer, and responding to this interest some producers specify some more or less questionable number. If the question is meant to be how far the light of a laser pointer can propagate, the answer is of course that there is no limit, as long as the light is not absorbed or scattered away in the atmosphere. Of course, much of that light can get from earth to the moon without problems. However, the beam area will eventually become larger due to the beam divergence, so that the intensity e.g. on a screen will be reduced even if the overall power stays constant.

单选题 The word "illuminating" in the passage is closest in meaning to ______.
A. directing B. responding
C. clarifying D. brightening

单选题 Which of the following does NOT explain why some people prefer an old- fashioned telescopic pointing device to a laser pointer for presentations?
A. The laser pointers can only generate weak light on the screen.
B. The laser pointers can cause dizziness.
C. The laser pointers can only be used from a long distance.
D. The red laser pointers can only generate an even weaker light on the screen than other laser pointers.

单选题 The word "generated" in the passage is closest in meaning to ______.
A. illuminated B. engendered
C. lengthened D. adjusted

单选题 According to the passage, what is the relationship between the wavelength of the light and the eye? ______
A. The longer the wavelength, the more sensitive it is to the eye.
B. The longer the wavelength, the less sensitive it is to the eye.
C. The longer the wavelength, the more harmful it is to the eye.
D. The longer the wavelength, the less harmful it is to the eye.

单选题 It can be inferred from paragraph 3 that red lasers ______.
A. are brighter than green lasers
B. are more expensive than green lasers
C. have longer wavelength than green lasers
D. are more sensitive to the eye than green lasers

单选题 The word " counterparts" in the passage is closest in meaning to ______.
A. spare parts B. equivalents
C. components D. conversions

单选题 According to the passage, the range of a laser pointer ______.
A. has no limit
B. has a definite limit
C. is not affected by the beam divergence
D. is a difficult question to answer

单选题 Look at the four squares, , , and

单选题 Which of the following sentences best expresses the essential information in the sentence below?
(Incorrect answer choices omit important information or change the meaning of the original sentence in an important way. )
That kind of range is normally not limited by the beam divergence but by the overall optical power(apart from the wavelength and level of ambient light), since the issue is not the comparatively minor divergence on the way from the laser pointer to the screen, but rather the huge divergence of the scattered light on the way back.
A. The range is limited by the overall optical power besides other factors because the range should cover divergence on both ways.
B. The range is not limited by the beam divergence because it not only covers the divergence to the screen.
C. The range is not only determined by the beam divergence but also by the two-way divergence on the way to and from the screen.
D. The range is not only a minor issue of divergence but an issue of how to calculate the divergence.

单选题 The word "it" in the passage refers to ______.
A. direct staring into a 1-mW beam
B. a 1-mW beam
C. irritating for the eye
D. the eye

单选题 How does the author show that laser can be dangerous? ______
A. By giving warnings under different occasions.
B. By listing some standards that the manufactures should follow.
C. By comparing different types of lasers.
D. By demonstrating the side-effects of lasers.

单选题 According to the passage, what is the safety hazard of the green laser pointers? ______
A. Some cheap green ones do not emit infrared light.
B. Some cheap green ones can emit extremely strong beam in the infrared spectral region.
C. Some cheap green ones can only emit green light.
D. Some cheap green ones are sold without an infrared spectral region.

填空题 Directions: Select phrases from the answer choices and match them to the category to which they relate. THREE answer choices will NOT be used. (This question is worth 3 points. )
Red lasers
·______
·______
·______
Green lasers
·______
·______
·______

A. can only be used in experiment.
B. are more expensive.
C. are less expensive.
D. have shorter wavelength.
E. have a diode-pumped solid state laser.

F. have a frequency doubler.

G. normally can only generate weak light on the screen.

[H] are harmful to the naked eye.

[I] have longer wavelength.