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单选题
A light that has a light period shorter than its dark period is described as（）.
A
flashing
B
pulsating
C
occulting
D
alternating
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更多 “单选题A light that has a light period shorter than its dark period is described as（）.A flashingB pulsatingC occultingD alternating” 相关考题
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Thomas is afraid of the dark, () he sleeps with a light on in his room.
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共用题干
Learn about Light1 .Ancient civilizations were amazed by the existence of light for thousands of years. The Greek philosophers believed that light was made up of countless,tiny particles that enter the human eye and create what we call vision. However,Empedocles and a Dutch scientist named Christian Huygens believed that light was like a wave. According to them,light spread out and travelled like a straight line. This theory was accepted during the 19th century.2 .In 1905,Albert Einstein published a research paper in which he explained what is re-ferred to as the photoelectric effect. This theory explains that particles make up light.The particles Einstein was referring to are weightless bundles（束）of electromagnetic（电磁）energy called photons（光子）.Today,scientists agree that light has a dual（二重）nature一it is part particle and part wave. It is a form of energy that allows us to see things around us.3 .Things that give off light are known as sources of light. During the day,the primary source of light is the sun. Other sources of light include stars,flames,flashlights,street lamps and glowing gases in glass tube.4 .When we draw the way light travels we always use straight lines. This is because nor-mally light rays travel in a straight line. However,there are some instances that can change the path and even the nature of light. They are reflection,absorption,interference（干扰）, etc.5 .Physicists have attempted to measure the speed of light since the early times. In 1849, Hippolyte Fizeau conducted an experiment by directing a beam of light to a mirror located kilometers away and placed a rotating cogwheel（旋转齿轮）between the beam and the mirror. From the rate of rotation of the wheel,number of wheel's teeth and distance of the mirror,he was able to calculate that the speed of light is 313 million meters per second. In a vacuum（真空）,however,the speed of light is 299,792,458 meters per second. This is about a million times faster than the speed of an airplane.Paragraph 5______A: How Is the Nature of Light Explained Today?B: What Are Sources of Light?C: How Did Physicists Measure the Speed of Light?D: How Does Light Travel?E: How Did People Think of Light Years Ago?F: What Causes a Shadow?
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共用题干
第三篇Small but WiseOn December 14,NASA blasted a small but mighty telescope into space.The telescope is called WISE and is about as wide around as a trashcan.Don't let its small size fool you:WISE has a powerful digital camera,and it will be taking pictures of some of the wildest objects in the known universe,including asteroids,faint stars,blazing galaxies and giant clouds of dust where planets and stars are born."I'm very excited because we're going to be seeing parts of the universe that we haven't seen be- fore,"said Ned Wright,a scientist who directs the WISE project.Since arriving in space,the WISE telescope has been circling the Earth,held by gravity in a polar orbit (this means it crosses close to the north and south poles with each lap).Its camera is pointed outward,awayfrom the Earth,and WISE will snap a picture of a different part of the sky every 11 minutes.After six months it will have taken pictures across the entire sky.The pictures taken by WISE won't be like everyday digital photographs,however. WISE stands for "Wide-field Infrared Survey Explorer".As its name suggests,the WISE camera takes pictures of features that give off infrared radiation.Radiation is energy that travels as a wave.Visible light,including the familiar spectrum of light that be- comes visible in a rainbow,is an example of radiation.When an ordinary digital camera takes a picture of a tree,for example,it receives the waves of visible light that are reflected off the tree.When these waves enter the camera through the lens,they're processed by the camera,which then puts the image together.Waves of infrared radiation are longer than waves of visible light,so ordinary digital cameras don't see them,and neither do the eyes of human beings.Although invisible to the eye,longer infrared radiation can be detected as warmth by the skin.That's a key idea to why WISE will be able to see things other telescopes can't. Not everything in the universe shows up in visible light.Asteroids,for example, are giant rocks that float through space一but they absorb most of the light that reaches them.They don't reflect light,so they're difficult to see.But they do give off infrared radiation,so an infrared telescope like WISE will be able to produce images of them.During its mission WISE will take pictures of hundreds of thousands of asteroids.Brown dwarfs are another kind of deep-space objects that will show up in WISE's pictures.These ob- jects are "failed" stars一which means they are not massive enough to jump-start the same kind of reactions that power stars such as the sun.Instead,brown dwarfs simply shrink and cool down.They're so dim that they're almost impossible to see with visible light,but in the infrared spectrum they glow.It is true that infrared radiation_________.A:is not detectable to humansB:looks brighter than visible lightC:is visible light reflected off an objectD:has longer waves than those of visible light
考题
根据以下材料，回答
Using a computer or smartphone at nightcan cause us to 11 on the pounds, new research has revealed.
The study found a link between blue light 12 --blue light is 13 by smartphones and tables--and increasedhunger. It found that exposure to the light increases hunger levels for severalhours and even increases hunger levels 14 eating a meal.
Results of the US study show thatblue-enriched light exposure, compared with 15 light exposure, was 16 with an increase in hunger that began 15minutes after light onset and was still present almost two hours after themeal. Blue light exposure has also already been shown to decreased 17 in the evening, increasing the risk ofinsomnia.
Study co-author Ivy Cheung, of NorthwesternUniversity in Chicago, said, "A single three-hour exposure toblue-enriched light in the evening 18 impacted hunger andglucose metabolism." "These results are important because theysuggest that 19 environmental light exposure for humansmay represent a novel 20 of influencing food intake patterns andmetabolism."
The study group 21 10 healthy adults with regular sleep and eating schedules who received
22 carbohydrate-rich meals. They completed afour-day trial 23 dim light conditions, which involvedexposure to less than 20 lux during 16 hours 24 and less than three lux duringeight hours of sleep. On day three they were exposed to three hours of 260 lux,blue-enriched light starting 10.5 hours after waking up, and the effects werecompared with dim light exposure on day two.
Ms Cheung said more research is needed todetermine the 25 of action involved in the relationshipbetween light exposure, hunger and metabolism.A.operating
B.manipulating
C.conducting
D.making
考题
根据以下材料，回答
Using a computer or smartphone at nightcan cause us to 11 on the pounds, new research has revealed.
The study found a link between blue light 12 --blue light is 13 by smartphones and tables--and increasedhunger. It found that exposure to the light increases hunger levels for severalhours and even increases hunger levels 14 eating a meal.
Results of the US study show thatblue-enriched light exposure, compared with 15 light exposure, was 16 with an increase in hunger that began 15minutes after light onset and was still present almost two hours after themeal. Blue light exposure has also already been shown to decreased 17 in the evening, increasing the risk ofinsomnia.
Study co-author Ivy Cheung, of NorthwesternUniversity in Chicago, said, "A single three-hour exposure toblue-enriched light in the evening 18 impacted hunger andglucose metabolism." "These results are important because theysuggest that 19 environmental light exposure for humansmay represent a novel 20 of influencing food intake patterns andmetabolism."
The study group 21 10 healthy adults with regular sleep and eating schedules who received
22 carbohydrate-rich meals. They completed afour-day trial 23 dim light conditions, which involvedexposure to less than 20 lux during 16 hours 24 and less than three lux duringeight hours of sleep. On day three they were exposed to three hours of 260 lux,blue-enriched light starting 10.5 hours after waking up, and the effects werecompared with dim light exposure on day two.
Ms Cheung said more research is needed todetermine the 25 of action involved in the relationshipbetween light exposure, hunger and metabolism.A.dim
B.dull
C.dark
D.dusky
考题
If you are writing or studying,it makes very much difference where the light comes from.People who use books and pens every day have to be especially careful about the way the light shines on their work.
Every house gets its light either from daylight through the windows--which is the best to use--or from lamps or electricity;but whichever kind of light it is,the way it shines toward our book or work is a matter of great importance to the eyes.
Take a book,sit with your back toward the window,and try to read.Your shadow（影子）falls all over the page and makes it almost as bad for your eyes as if you were in a dark room.
Now turnaround and face the window.The page is in the shadow again,while the bright light is in your eyes.
Try sitting with your right side toward the window.This is very well for reading,but if you were writing,the shadow of your hand would fall across the page and bother（打搅）you a little.
There is just one other way:sit with your left side to the window.Now everything is perfect for reading and for writing,too.
Whatever kind of light is in the room,the rule about the right to sit is always the same.
Which of the following is true?A.How the light shines on our work is of much importance.
B.The way the light shines on your work makes no difference.
C.We needn't care about where the light comes from.
D.People can write or study under a light that comes from any direction.
考题
This cargo light is not （）enough. Please change it for a new one.A、lightB、brightC、tightD、dark
考题
单选题A flashing light is a light that（）.A
flashes at regular intervals at a frequency of 120 flashes or more per minuteB
is yellow in colorC
is visible over an arc of the horizon of not less than 180°nor more than 225°D
All of the above
考题
单选题You see a vessel displaying three lights in a vertical line.The highest and lowest lights are red and the middle light is white.She is also showing a white light at the stern，which is lower than the forward light.It could be a（）.A
Survey vesselB
Vessel not under commandC
Vessel agroundD
Pilot vessel with port side to you
考题
单选题A vessel engaged in fishing must display a light in the direction of any gear that extends outward more than 150 meters. The color of this light is（）.A
whiteB
greenC
redD
yellow
考题
单选题You are underway and pass by a lighthouse. Its light，which was white since you first sighted it，changes to red. This means（）.A
the light is characterized as alternately flashingB
the lighthouse has lost power and has switched to emergency lightingC
it is the identifying light characteristic of the lighthouseD
you have entered an area of shoal water or other hazard
考题
单选题Operating a diesel engine under light loads and at low temperatures for an exhaust for an extended period can result in（）A
formation of carbon on the intake and exhaust portsB
high water jacket temperaturesC
overheated pistons and cylindersD
an increase in lube oil viscosity due to fuel dilution
考题
单选题An occulting light is one in which （）.A
the period of darkness exceeds the period of lightB
there is only a partial eclipse of the lightC
the periods of light and darkness are equalD
the period of light exceeds the period of darkness
考题
单选题If you hear a continuous blast of the ship’s whistle for a period of a period of not less than 10 saconds supplemented by a continuous ringing of the general alarm bells for not less than 10 seconds,you should go to your（）.A
boat stationB
fire stationC
man overboard stationD
collision station
考题
单选题When making landfall at night，you can determine if a light is a major light or an offshore buoy by（）.A
the intensity of the lightB
checking the period and characteristics against the Light ListC
the color，because the buoy will have only a red or a green lightD
Any of the above can be used to identify the light
考题
单选题Increasing the valve clearance between a valve stem and rocker arm, will result in the valve （）A
closing laterB
opening soonerC
staying open for a shorter period of timeD
staying open for a longer period of time
考题
单选题The inside light in an inflatable liferaft is turned on（）.A
automatically as the liferaft inflatesB
with a switch near the boarding handleC
at night because the light has a photosensitive switchD
by screwing the bulb in after the raft is inflated
考题
单选题Planetary aberration is due，in part，to（）.A
refraction of light as it enters the Earth's atmosphereB
rotation of the Earth on it's axisC
the body's orbital motion during the time required for its light to reach EarthD
a false horizon
考题
单选题The period of a lighted aid to navigation refers to the（）.A
date of construction or establishmentB
length of time between flashes of the lightC
time required for the longest flash of each cycleD
time required for the light to complete each cycle
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