B, if the train is close to the speed of light, l = l0 1? (vc)2。 As we all know, the length of the carriage decreases along the running direction, while the height perpendicular to the running direction remains unchanged. Therefore, B is correct.
C, reading the information recorded on the CD with laser is based on the good parallelism of laser, so C is correct.
D. When the bell in the temple rings, the stone hanging in the monk's meditation room often rings by itself, which is the phenomenon that sound waves ring. So, d is wrong.
So choose BC.
(2) The wave velocity propagates along the positive direction of the X axis. According to the uphill and downhill methods, it is known that the vibration direction of a particle at x=0 is the negative direction of the y axis. The period of particle vibration is equal to the period of wave t = λ V = 2 10s = 0.2s, then ω = 2π T = 10π rad/s, and the amplitude a = 3cm. Because the particle vibrates downward at time 0.
(3)① According to the problem, the incident angle α = 60, the refraction angle γ = 30, and n = sin α sin γ = 3.
② n = cv, v = cn = 3×108m/s =1.73x108m/s.
So the answer is: (1) BC.
(2) the negative direction of y axis y=-3sin( 10πt)
(3)①n=sinαsinγ=3
② 1.73× 108.