1. Core concepts

A plane mirror has a flat, smooth reflecting surface — dressing mirrors, still water, and polished metal plates. Plane-mirror imaging is based on the reflection of light.

• Real image: formed where actual rays converge; can be caught on a screen.
• Virtual image: cannot be caught on a screen; seen only by the eye. Plane mirrors form virtual images.

2. Core lab: exploring plane-mirror imaging

A curriculum-required optics lab. Goal: how image position and size relate to the object’s position and size.

Kit: two identical candles (or matching objects), a glass plate, stand, white/grid paper, screen, ruler, and matches.

Key tip: a glass plate replaces a mirror because it both reflects (to form an image) and transmits light, so you can locate the image.

• Suggested table: trial | u (cm) | v (cm) | size relation (image vs object).

1. Set up: lay paper on a level table and mount the glass plate vertically on it.
2. Locate the image: light a candle in front; move an unlit matching candle behind until it coincides with the image — that is the image position.
3. Check that it is virtual: place a screen at the image location and look without looking through the plate — no image appears.
4. Record & measure: mark object, plate, and image; draw the object–image line and measure distances to the plate.
5. Change object distance and repeat: move the lit candle and record several trials.

3. Conclusions (imaging facts)

Experiments also support the symmetry statement: the image and object are mirror-symmetric about the plane.

• Nature: the image is virtual.
• Size: same as the object (independent of object distance).
• Distance: image distance equals object distance (v = u).
• Symmetry: object and image are mirror images; the joining line is perpendicular to the mirror.

4. Principle — why the image is virtual

Source S emits in all directions; some reflected rays enter the eye. From the habit that light travels in straight lines, we perceive the rays as coming from S′, where their backward extensions meet — the image of S.

There is no real source at S′ behind the mirror; the light does not actually come from S′, so S′ is called a virtual image.

5. Locating the image by construction

Using mirror symmetry, construct the image as follows:

1. From a point A on the object, drop a perpendicular to the mirror MN; foot O.
2. Extend AO to A′ with OA = OA′; A′ is the image of A.
3. Repeat for other points and join the image points with dashed lines (dashes mark a virtual image).

• v = uImage distance equals object distance; ray construction with i = r also meets at the image.

6. Applications

• Daily life: dressing mirrors, dance studios, shops that look larger.
• Medicine: dental inspection mirrors.
• Military: periscopes (two plane mirrors at 45° to the horizontal).
• Energy: heliostat fields reflecting sunlight onto a tower.
• Tech: kaleidoscopes and laser cavity mirrors.

7. Pitfalls & exam habits

• Why a glass plate: reflects to form an image and transmits light so you can locate it.
• Two identical candles: to compare size (coincidence ⇒ same size).
• Size does not change with distance: the image stays the same size as the object.
• Example: 1 m from the mirror → 2 m person-to-image; step back 0.5 m → 3 m apart; image height unchanged.
• A 1.68 m person 3 m from the mirror: person–image = 6 m, image height 1.68 m; moving farther does not change size.
• Eye charts via a mirror: distance to the chart’s image = chart-to-mirror + person-to-mirror (use the given numbers).

8. Extension: spherical mirrors

Spherical mirrors are concave (inner surface) or convex (outer surface).

• Concave: converges light — solar cookers, flashlight reflectors.
• Convex: diverges light — car rear-view mirrors, road bend mirrors (wider field).