A small mirror is attached to a vertical wall, and it hangs a distance of 1.87 m above the floor. The mirror is facing due east, and a ray of sunlight strikes the mirror early in the morning and then again later in the morning. The incident and reflected rays lie in a plane that is perpendicular to both the wall and the floor. Early in the morning, the reflected ray strikes the floor at a distance of 3.56 m from the base of the wall. Later on in the morning, the ray is observed to strike the floor at a distance of 1.46 m from the wall. The earth rotates at a rate of 15.0o per hour. How much time (in hours) has elapsed between the two observations

 t = 1.62 h

Explanation:

A flat mirror fulfills the law of reflection where the incident angle is equal to the reflected angle.

          θ_i = θ_r

If we use trigonometry to find the angles, the mirror is at a height of L = 1.87 m,   and the reflected rays reach a distance x1 = 3.56 m

         tan θ₁ = x₁ / L

         tan θ₁ =

         θ₁ = tan⁻¹  1.90

         θ₁ = 62.29º

for the second case x₂ = 1.46 m

        tan θ₂ = x₂ / L

        θ₂ = tan⁻¹  

        θ₂ = 37.98º

the difference in degree traveled is

         Δθ = θ₁- θ₂

          Δθ = 62.29 - 37.98

          Δθ = 24.31º

as in the exercise they indicate that every 15º there is an hour

         t = 24.31º (1h / 15º)

         t = 1.62 h

this is poiselle's equation from fluid mechanic in physics. mech eng