Chapter 1 - Measurement and Problem Solving - Learning Path Questions and Exercises - Exercises - Page 31: 61

0.866 m.

The triangle is equilateral triangle. So, if a perpendicular is dropped from lamp vertex to opposite side, it would divide the side into half. i.e. 0.5m. So, as per pythagoras theorem, the vertical distance $= \sqrt{1^2-0.5^2} $ =0.866m.