# Plane Mirrors

matt72lsu

## Homework Statement

You are 1.9 m tall and stand 3.5 m from a plane mirror that extends vertically upward from the floor. On the floor 1.5 m in front of the mirror is a small table, 0.80 m high.

a) What is the minimum height the mirror must have for you to be able to see the top of the table in the mirror?

pythag. theorem?

## The Attempt at a Solution

i know the mirror has to be half as tall as the person in order to see their body but i'm not sure how to use this knowledge to translate it to the table. thanks for your help!

Homework Helper
Can you draw the ray diagram?
Let h be the height of the table and H be the minimum height of the mirror from the ground.
A ray from the top of the table T falls on the mirror at M and reflects towards your eye E. Draw two parallel lines from T and M to the ground. Parallel line from T meets the mirror at N and parallel line from M meets you at P.
Now you can varify easily that the triangles EPM and TNM are similar. So the sides of the triangles are proportional. With these hints try to find the minimum height of the mirror.

matt72lsu
i got like 1.2m but it said it was wrong.

Homework Helper
The formula becomes
(H - 0.8)/1.5 = (1.9 - H)/3.5)
Solve for H.

matt72lsu
thanks! i was close lol