Một chất phát quang được kích thích bằng ánh sáng tím có bước sóng 0,4 μm thì phát ra ánh sáng màu lục có bước sóng 0,5 μm. Giả sử công suất của chùm sáng phát quang màu lục bằng 10% công suất của chùm sáng kích thích màu tím. Tỉ số giữa photon ánh sáng phát quang và số photon ánh sáng kích thích trong cùng một khoảng thời gian là:

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Một số câu hỏi khác có thể bạn quan tâm.

• Choose the best answer for the following sentence:

  Last night we wanted to see ___________ film, so we went to ___________ cinema.
   

• Choose the best answer for the following sentence:

  Don’t worry ________ it; I promise it won’t be ____________ problem.
  
  

• Choose the best answer for the following sentence:

  By the end of the 21st century, scientists ___________ a cure for the common cold.
  
  

• Choose the best answer for the following sentence:

  Pessimists think that many large corporations will be ___________ in the future.
  
  

• Choose the best answer for the following sentence:

  It’s likely that domestic chores will no ____________ be a burden.
  
  

• Read the following passage and choose the best answer for each blank:

          About 50 years from now, it will be (1)________ to use virtual reality to bring the dead back to life. People will drag out their old and boring home movies and choose a (2)_____ to bring back to life. The visual will be combined (3)_______ records about that person to create a virtual relative.
        Then the family can have a (4)_______ with that relative, keep it running, and live with it. The experience will be (5)________ living with a ghost.
        People (6)________ died long ago will be resurrected so they can take on new roles. In 2030 the great people of the 20th century will be brought back (7)______ virtually and be presented on television.
  '     The world will look on in awe as they watch the famous (8)_______ at a party. They will watch Richard Nixon make out with Margaret Thatcher, the Three Stooges try to pick a (9)______ with Hulk Hogan, Elvis (the king) Presley trying to (10)_________ Queen Elizabeth, and Albert Einstein exploring reality with Marilyn Monroe.

   

  1. A. possible                 B. able                      C. capable                 D. moveable
  2. A. cousin                    B. friend                    C. relative                 D. ancestor
  3. A. for                          B. on                         C.about                    D. with
  4. A. conference             B. chat                      C. quarrel                  D. forum
  5. A. as                          B. like                        C.such                       D. also
  6. A. who                       B. whom                    C. whose                   D. which
  7. A. live                        B. living                      C. alive                      D. lived
  8. A. cooperate             B. collaborate             C. affect                     D. interact
  9. A. competition           B. fight                       C. battle                    D. war
  10. A. impress               B. conquer                 C. befriend                 D. complement

• Choose the best answer for the following sentence:

  John’s trying to finish a report, _________________ .
  
  

• Choose the best answer for the following sentence:

  He’s too young to be allowed in this bar. _______________ . 
  
  

• Choose the best answer for the following sentence:

  ______________, so she’s struggling to make ends meet.
  
  

• Choose the best answer for the following sentence:

  ______________, but there may be snaps in its practical application.