QUES: The attainment of high efficiency in blue LEDs was quickly followed by the development of the first white LED. In this device a Y
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12:Ce (known as "YAG") phosphor coating on the emitter absorbs some of the blue emission and produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. However using different phosphors (fluorescent materials) it also became possible to instead produce green and red light through fluorescence. The resulting mixture of red, green and blue is not only perceived by humans as white light but is superior for illumination in terms of color rendering, whereas one cannot appreciate the color of red or green objects illuminated only by the yellow (and remaining blue) wavelengths from the YAG phosphor.

 What does the YAG phosphor coating not produce?
What is the answer?
ANS: unanswerable
QUES: Cotton remained a key crop in the Southern economy after emancipation and the end of the Civil War in 1865. Across the South, sharecropping evolved, in which landless black and white farmers worked land owned by others in return for a share of the profits. Some farmers rented the land and bore the production costs themselves. Until mechanical cotton pickers were developed, cotton farmers needed additional labor to hand-pick cotton. Picking cotton was a source of income for families across the South. Rural and small town school systems had split vacations so children could work in the fields during "cotton-picking."

What was the procedure called which involved workers to work for a share of the profits? 
What is the answer?
ANS: sharecropping
QUES: The political stability and development of hydrocarbon production made Republic of Congo the fourth largest oil producer in the Gulf of Guinea and provided the country with a relative prosperity despite the poor state of its infrastructure and public services and an unequal distribution of oil revenues.

What factors led to the Republic of Congo being the third largest oil producer in the Gulf of Guinea?
What is the answer?
ANS:
unanswerable