(d) longitudinal non-mechanical wave
Ans: (b)
62.Ultrasonic wave has its frequency:
   (a) less than 20 Hz
   (b) more than 20 Hz
   (c) more than 20,000 Hz
   (d) 20 Hz - 20,000 Hz
Ans: (c)
63.Infrasonic wave has its frequency:
   (a) less than 20 Hz
   (b) more than 20 Hz
   (c) less than 20,000 Hz
   (d) more than 20,000 Hz
Ans: (a)
64.The noise of 100 db is assumed to be: [IAS (Pre) 2000]
   (a) a properly listened sound
   (b) an ordinary conversation
   (c) a noise of the street
   (d) a noise produced by the machine of a shop and listened at another
       neighbour"s place
Ans: (d)
65.The focal length of a convex lens is – [CDS 2011]
   (a) the same for all colours
   (b) shorter for blue light than for red
   (c) shorter for red light than for blue
   (d) maximum for yellow light
Ans: (b)
66.If the door of a running refrigerator in a closed room is kept open, what will
   be the net effect on the room? [CDS 2011]
   (a) It will cool the room
   (b) It will heat the room
   (c) It will make no difference on the average
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