You hold a page of text up to a plane mirror and the writing appears reversed. What does the mirror actually invert?
ALeft and right — mirrors swap the horizontal axis
BUp and down — mirrors invert the vertical axis
CFront and back — mirrors swap depth, mapping each point to an equal distance behind the surface
DBoth left-right and up-down, which is why images appear strange
A plane mirror inverts only the axis perpendicular to its surface — the depth axis (front-back). Points in front of the mirror map to points equally far behind it; x (horizontal) and y (vertical) are unchanged. The perceived left-right reversal is a cognitive artifact: when you mentally 'turn around' to face your reflection as if it were another person, that imagined rotation is what swaps left and right in your perception. The mirror itself is indifferent to handedness in the horizontal plane. Hold text up to a window and look at it from outside — you see the same reversal, for the same reason.
Question 2 Multiple Choice
An object is placed 30 cm in front of a plane mirror. Which statement correctly describes the image?
AThe image is 30 cm behind the mirror and can be projected onto a screen placed there
BThe image is 30 cm behind the mirror and cannot be projected onto a screen — it is virtual
CThe image is 60 cm behind the mirror because the light must travel to the mirror and back
DThe image is 30 cm in front of the mirror on the same side as the object
The image forms as far behind the mirror as the object is in front of it — 30 cm in this case. But the image is virtual: the reflected rays diverge as they leave the mirror's surface, and your eye traces them backward as if they originated from a point 30 cm behind the mirror. No light actually reaches that point. A screen placed there would catch nothing. This distinguishes a virtual image (apparent convergence of backward-traced rays) from a real image (actual convergence of forward-traveling rays, as produced by a concave mirror or converging lens).
Question 3 True / False
As you walk toward a plane mirror, the minimum mirror height needed to see your full reflection decreases, because you are closer to the image and it subtends a larger angle.
TTrue
FFalse
Answer: False
The minimum mirror height needed to see your full reflection is always half your height, regardless of distance. This follows from the geometry: as you walk closer, your image also moves closer at the same rate (since image distance equals object distance). The rays from your head and feet to the mirror's edges subtend the same angle at your eyes whether you're near or far. The half-height rule is fixed by your own geometry — the distance from your eyes to your head, and from your eyes to your feet — not by how far you are from the mirror.
Question 4 True / False
You can see a clear image of yourself in a plane mirror, which proves the image is real — if it weren't real, it couldn't be seen.
TTrue
FFalse
Answer: False
This is the central misconception about virtual images. A virtual image can be seen clearly — your brain traces diverging reflected rays backward and perceives them as coming from behind the mirror. What makes an image virtual is not invisibility but the fact that no actual light rays converge at the image location. A real image (like that formed by a concave mirror) actually has light converging at the image point and can be caught on a screen. A virtual image cannot be projected onto a screen, but it is absolutely visible to an eye that receives the diverging rays.
Question 5 Short Answer
Why is the image formed by a plane mirror described as 'virtual,' and what does this mean for where the light rays actually travel?
Think about your answer, then reveal below.
Model answer: The image is virtual because the reflected rays diverge — they never actually meet at the image location. After reflecting off the mirror, the rays spread apart as they travel toward the observer's eye. The eye traces these rays backward along straight lines into the mirror, and those backward extensions converge at a point behind the mirror — the apparent image location. But no light physically reaches that point: the mirror surface blocks it. A 'real' image would require the reflected rays to actually converge in space, which only curved mirrors or lenses can achieve. The plane mirror's flat surface can only produce diverging reflected rays, hence a virtual image.
The virtual/real distinction is fundamentally about where the light actually goes. Real images can be caught on a screen because light physically passes through the image point. Virtual images cannot, because the light never reaches the image location — only the backward extrapolations of the reflected rays do. Understanding this distinction is essential for the next topic (spherical mirrors), where both virtual and real images can form depending on object position and mirror curvature.