某电磁波的波长（真空中）为960m，射入某种物质时波速为2×108m/s，求：射入该物质时的波长．（电磁波进入另一种介质时，它的频率不会改变）
电磁波在真空中传播的速度：c=3×108m/s．
电磁波的波长λ=960m，由公式c=λf可知电磁波的频率f=
=c λ
=3.125×105Hz．3×108m/s 960m
∵电磁波进入另一种介质时，它的频率不会改变．
∴f′=f=3.125×105Hz．
电磁波射入某种物质时波速c′=2×108m/s，射入该物质时的波长：λ′=
=c′ f′
=640m．2×108m/s 3.125×105Hz
答：电磁波射入该物质时的波长为640m．
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Here's a shopping receipt (收据) of Linda.
| ||
| 1. What time did Linda go shopping? | ||
[ ] | ||
| A. In the afternoon. B. In the evening. C. In the morning. D. I don't know. | ||
| 2. How many kinds of things did Linda buy? | ||
[ ] | ||
| A. Four. B. Five. C. Three. D. One | ||
| 3. Where did Linda go shopping? | ||
[ ] | ||
| A. Times Supermarket B. Wenfeng Great World C. Golden Eagle D. Star Shopping Mall | ||
| 4. How much money did Linda spend buying them? ￥_______. | ||
[ ] | ||
| A. 150 B. 555 C. 375 D. 20 |