物理問題 光學

Suppose the person is at a distance D in front of the mirror, his image will be at distance D behind the mirror. In other words, the image is at a distance of 2D from him.

Draw two light rays, one from the top and the other from the foot of the image, to the eyes of the person. The two light rays will cut the wall where the mirror is placed at two points. The distance between these two points is the required length of the mirror.

The length of the mirror L can be found by similar triangles,
L/1.8 = D/2D
i.e. L = 1.8/2 m = 0.9 m

Notice that the length of the mirror only depends on the height of the person, and is independent on his distance in front of the mirror.

Suppose the person is at a distance D in front of the mirror, his image will be at distance D behind the mirror. In other words, the image is at a distance of 2D from him.

Draw two light rays, one from the top and the other from the foot of the image, to the eyes of the person. The two light rays will cut the wall where the mirror is placed at two points. The distance between these two points is the required length of the mirror.

The length of the mirror L can be found by similar triangles,
L/1.8 = D/2D
i.e. L = 1.8/2 m = 0.9 m

Notice that the length of the mirror only depends on the height of the person, and is independent on his distance in front of the mirror.