(c) their eyes are sensitive to darkness
   (d) their eye holes expand in darkness
Ans: (b)
221. When two similar sources vibrate with slightly different frequencies, a
   regular rise and fall occurs in the loudness of tone. This phenomenon Is
   known as
   (a) doppler effect
   (b) beats
   (c) resonance
   (d) echo
Ans: (b)
222. When sound waves travel from one medium to another, the quantity
   which remains unchanged is its
   (a) speed
   (b) frequency
   (c) wavelength
   (d) All the above
Ans: (b)
223. In a concert hall reverberation is controlled by
   (a) reduction of seating capacity in the hall
   (b) increase of sound intensity in the hall
   (c) lining the walls, with leather panels put on over a layer of sound
       absorbent glass fibre
   (d) by opening the doors and windows of concert hall
Ans: (c)
224. When a whistling engine approaches a person standing on the platform,
   the frequency or the note
   (a) appears to increase
   (b) appears to decrease
   (c) does not change at all
   (d) first increases then decreases
Ans: (a)
                                          93