6.9 Mirrors

Types of mirrors

A plane mirror produces a virtual, upright, same-size image behind the mirror. A concave mirror (converging) reflects parallel rays inward through a real focal point; it can form real or virtual images depending on object distance. A convex mirror (diverging) reflects rays outward, always producing a virtual, upright, reduced image with a wide field of view.

Mirror equation and magnification

Use 1/f = 1/d₀ + 1/dᵢ, where f is focal length, d₀ is object distance, and dᵢ is image distance. Magnification m = -dᵢ/d₀ = hᵢ/h₀. Positive dᵢ means a real image (in front of mirror); negative dᵢ means virtual (behind mirror). Positive m means upright; negative m means inverted.

Sign conventions & ray diagrams

For concave mirrors, f is positive; for convex mirrors, f is negative. Real images form on the same side as the object and can be projected on a screen; virtual images cannot. Ray diagrams use two key rays: one parallel to the axis reflecting through the focal point, and one through the focal point reflecting parallel to the axis, to locate the image.

Key terms

Plane mirror image

Virtual, upright, same size as object; located as far behind mirror as object is in front.

Concave mirror

Converging mirror with positive focal length; can produce real or virtual images depending on object distance relative to f.

Convex mirror

Diverging mirror with negative focal length; always produces virtual, upright, reduced images.

Mirror equation

1/f = 1/d₀ + 1/dᵢ, relating focal length, object distance, and image distance.

Magnification (mirrors)

m = -dᵢ/d₀ = hᵢ/h₀; positive m = upright image, negative m = inverted image.

Try an AP-style question

A concave mirror has f = 10 cm, and an object is 15 cm away. Where is the image?

1. 6 cm in front of the mirror
2. 30 cm behind the mirror
3. 15 cm behind the mirror
4. 30 cm in front of the mirror