Construct the reflected ray and explain why a flat-mirror image appears behind the glass.
Select a page to enlarge it. The PDF contains the complete set.
Draw the reflecting surface, mark the point of incidence, and construct a perpendicular normal there. Measure the incident angle between the ray and this normal. The normal is a reference line, not a physical ray, so draw it differently.
For a smooth plane mirror, the angle of reflection equals the angle of incidence. Draw the outgoing ray on the opposite side of the normal with an arrow away from the mirror. If the incident ray is 25° to the mirror surface, it is 65° to the normal.
Reflected light travels into the eye in front of the mirror. Extending those rays backward locates an apparent source behind the mirror. Use dashed extensions to distinguish them from real light paths. For an ideal plane mirror, the image is upright, the same size as the object, and equally far behind the mirror as the object is in front.
A mirror does not send every reflected ray toward every observer. Move the observer and the rays that enter the eye change. A plane mirror reverses the front–back direction relative to its surface; the everyday left–right description depends on how we compare ourselves with the image.
Measure angles from the normal and distinguish a virtual image from a point where real rays meet.
Physics Girl · Demonstration
Compare turning an object to face a mirror with what the mirror itself reverses. Use front–back reasoning.
Move the optical components and follow the resulting light paths. The scene is ready to explore below.
Drag a mirror endpoint to tilt it. Watch which rays still reach the bottom opening. Reset before comparing a second change.
Explain why equal reflection angles matter at both mirrors.
A ray is 35° to the normal. What is its reflection angle in degrees?
The two angles use the same reference.
The reflected angle equals 35°.
An object is 12 cm in front of a plane mirror. How far behind the mirror is its image, in cm?
The two distances from the mirror are equal.
The image is 12 cm behind the mirror.
Sketch the route through two mirrors. Where should the arrowheads point?
From the viewed object toward the upper mirror, then down to the lower mirror, then into the eye. At both mirrors, measure equal incident and reflected angles from the normal.
The image position is found with backward extensions. The actual reflected light remains on the observer’s side of the reflecting surface.
Commit to an answer, then use the feedback to check your reasoning.
