英語(yǔ)時(shí)差：穿越隧道

Have you even wondered why you can hear FM radio signals in a tunnel longer than AM? Take a ride with Don and Ya?l to find out why. D: What a great day for a drive! How do you like my new Ferarri?

Y: Very fancy.

D: Best of all, the radio works, so we can still enjoy A Moment of Science. Hey, I think it's coming on now.

[SOUNDS]

Y: Hey, where'd the signal go?

D: It must be because we're driving through a tunnel. Try an FM station.

[SOUNDS]

Y: There it is!

[SOUNDS]

How come we can hear FM longer in a tunnel than AM?

D: It's because FM stations transmit at higher frequencies. They generally go from eighty-eight to one-hundred-and-eight megahertz. That's over a hundred million cycles per second.

Y: Wow! That's pretty fast.

D: As a result, a single radio wave from an FM station is only a few meters long, about the size of a car. An AM station is putting out radio waves that are hundreds of times as long!

Y: So the longer wavelengths can't make it as easily into tunnels?

D: Long wavelengths tend to be absorbed or blocked as they try to enter the smaller tunnel. The higher-energy and shorter-wavelength waves from an FM station aren't absorbed or blocked as much. Instead, they tend to bounce around and get reflected off of surfaces.

Y: Like the inside of a tunnel.

D: Yep. Of course, they only bounce so far. If the tunnel is long enough, even FM stations fade out until you come out the other end.

Y: Here we are!