A face appears behind a mirror, although nobody is standing there. Light entering transparent glass, by contrast, travels inside it. Separating an apparent location from an actual light path makes the difference clearer.
Use paper and a pencil: draw a flat boundary as a line and light direction as arrows. The everyday-science guide asks what happens under which conditions. Here, apply that approach to a boundary.
Add a normal before measuring an angle
At the point where light strikes, draw a line perpendicular to the surface. This is the normal. Incidence compares the incoming ray with this line; reflection compares the outgoing ray with it. At a plane mirror, the two angles are equal.
A ray 30° from the normal is 90°−30° = 60° from the mirror surface. Calling 60° its reflection angle switches the reference line. The first example asks you to identify that reference before any calculation.
Separate a reflected ray from its backward extension
Draw solid arrows from one object point to the mirror and onward toward the eye. Extend the reflected paths backward behind the mirror using dashed lines. Their common apparent origin locates the virtual image.
The reflected rays do not actually converge there, so a screen at that apparent position cannot directly collect the image. Virtual does not mean invisible. In the second example, distinguish an apparent origin from a meeting of actual rays.
Name the endpoints of a distance
For an ordinary plane mirror, image and object are equally far from the mirror on opposite sides, and their sizes match. Put the object 30 cm in front: the image appears 30 cm behind. Object-to-image separation is then 30 + 30 = 60 cm.
This comparison concerns positions across the mirror line. Mirror-to-image distance and object-to-image distance start at different points. Mark equal lengths on the two sides, then trace the two endpoints of each distance.
Follow transmitted light toward the normal in glass
The third example sends light obliquely from air into glass with a higher refractive index. The transmitted light slows and bends toward the normal. Again, compare with the normal rather than the surface. Some light can also reflect at the boundary.
A lower speed need not change direction: at normal incidence, light continues along the normal. Reversing the same oblique path from glass to air makes it bend away from the normal. Label both materials and the direction of travel.
Return each answer to its diagram
For reflection, mark the angle's reference line. For the image, distinguish solid rays from dashed extensions. For refraction, label air → glass. After answering the three examples, redraw one diagram for each. A dashed extension turned into a travelling-light arrow, or an angle measured from the surface, identifies a place to revisit.
Next, the related sound guide separates frequency and amplitude, using a different description from a ray path. The Solar System guide distinguishes the source illuminating the Moon; keep that source question separate from the direction question studied here.
TRY & READ
Check your understanding with examples
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Example 1 · Physics: waves and light
From which line is the angle of reflection at a plane mirror measured?
The reflected ray's direction
The normal to the mirror
The mirror surface itself
The incident ray's direction
Read the answer and explanation
Answer: The normal to the mirror
Both the angle of incidence and the angle of reflection are measured from the normal. The normal is perpendicular to the mirror at the point where the ray strikes. Using the mirror surface instead would measure a different angle.
Topics: Physics: waves and light. Range: Broader knowledge. Difficulty: Basic, Standard. These are selected initially. You can change these on the setup screen.
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