Physics F3 Question

2009-12-03 4:37 am

Mary is looking at a bird through a plane mirror. Her eyes are 1.6m from the floor and 2m away from the mirror. The horizontal distance between the mirror and the bird is 4m. The bird is 2.5m above the floor behind Mary

Q：What is the minimum distance between the upper edge of the mirror and the floor so that Mary can see the whole image of the bird?

回答 (2)

天同

2009-12-03 6:32 am

✔ 最佳答案

You need to draw out a diagram.
The image of the bird is at a distance of 4 m behind the mirror and 2.5 m above the floor. Draw out this image.

Then joint the object (bird) and its image by a straight line. This should be a horizontal line with length 8 m

Then joint the image and Mary's eyes by another striaght line. This represents the light ray from the image that enters into Mary's eyes. To see the bird, this line should cut the mirror at its highest point.

By similar triangles, if x is the distance between the light ray that you have drawn and the horizontal line between object and image, then

x/(2.5-1.6) = 4/(4+2)
i.e. x = (4/6) x (0.9) m = 0.6 m

Hence, height of upper edge of mirror = (2.5-0.6) m = 1.9 m

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[匿名]

2009-12-03 5:04 am

6.067124525m

參考: me

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