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QUESTION

This was an experiment I saw in my son's workbook. It said to mark out the top of your forehead and the bottom of your chin on a mirror using a whiteboard marker. Then slowly move backwards, and investigate what happens to the size of the reflection subjective to the two marks made. It actually got me quite flabbergasted. I always thought the reflection would get smaller as you moved away from the mirror.

Why is this?

{ asked by think123 }

ANSWER

The mirror gets proportionally smaller.

The explanation is the similarity of triangles. The eye and the marks on the mirror form a triangle, while the eye and the two points on the image form another triangle. The two triangles are similar, with ratio 1/2, no matter the distance.

Similarity of triangles explains "why does your reflection stay the same size when you move further away from the mirror"

{ answered by Cristi Stoica }
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