Law of reflection

Symmetry of incident and reflected raysThe diagram on the right shows how a flashlight beam reflects from a flat mirror. If the beam approaches at a low angle the reflected beam also leaves the mirror at a low angle. If the flashlight beam approaches nearly perpendicular to the mirror the reflected beam is also nearly perpendicular to the mirror. Reflection shows symmetry of angles between the incident and reflected beams. Read the text aloud
Angle of incidence, angle of reflection, and the normal to the surface in a ray diagram representing the reflection of lightReflection is symmetric in angle. Therefore, to describe reflection we start by defining angles. The normal is an imaginary line drawn perpendicular to the reflecting surface at the point of intersection with the incident ray. All angles are defined relative to the normal.
θi:    The angle of incidence is the angle between the incident ray and the normal.
θr:    The angle of reflection is the angle between the reflected ray of light and the normal.
The law of reflection states that the angle of incidence equals the angle of reflection (equation 20.3). Read the text aloud Show Diffuse reflection and the law of reflection
(20.3) θ i = θ r
θi  = angle of incidence (degrees)
θr  = angle of reflection (degrees)
Law of reflection
  1. Draw the incident light ray,
  2. Draw the normal to the surface at the point of contact,
  3. Determine the angle of incidence,
  4. Draw the reflected ray on the other side of the normal line, such that the angle of reflection equals the angle of incidence. Read the text aloud Show Using the law of reflection
A light ray strikes a flat mirror at an angle of 50° to the mirror surface. Draw a ray diagram for the incident and reflected light rays. Label the angles of incidence and reflection and the normal. Read the text aloud
Ray diagram showing reflection from a flat mirror
Asked: a ray diagram
Given: angle between the incident ray and the (flat) mirror, 50°
Relationships: θi = θr
Solution: From geometry, the angle of incidence is
θi = 90° − 50° = 40°
From the law of reflection, the angle of reflection is also 40°.
Show Scanning mirrors in laser printers
A light ray hits a mirror at an angle of incidence of 35°. What is the angle of reflection?
  1. 35°
  2. 55°
  3. 70°
A light ray perpendicular to the surface of a mirror strikes it. What is the angle of reflection?
  1. 45°
  2. 90°
  3. 180°

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