如图5-11所示，S为一点光源，M为一平面镜，光屏与平面镜平行放置.SO是垂直照射在M上的光线，已知SO=L，若M以角速度ω绕O点逆时针匀速转动，则转过30°角时，光点 S′在屏上移动的瞬时速度v为多大？
由几何光学知识可知：当平面镜绕O逆时针转过30°时，则：∠SOS′=60°，
OS′=L/cos60°.
选取光点S′为连结点，因为光点 S′在屏上，该点运动方向不变，故该点实际速度（合速度）就是在光屏上移动速度v；光点S′又在反射光线OS′上，它参与沿光线OS′的运动.速度v1和绕O点转动，线速度v2；因此将这个合速度沿光线OS′及垂直于光线 OS′的两个方向分解，由速度矢量分解图5′—1可得：
v1=vsin60°,v2=vcos60°
又由圆周运动知识可得：当线OS′绕O转动角速度为2ω.
则：v2=2ωL/cos60°
vcos60°=2ωL/cos60°,v=8ωL.
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