The Mirror Equation

Curved mirrors bend light to form images, just like lenses do. A concave mirror curves inward and can focus light into a real image you could catch on a screen, while a convex or flat mirror always spreads light apart, forming a virtual image that only appears to be behind the mirror.

1/f = 1/dₒ + 1/dᵢ

• f — focal length (cm): the distance from the mirror to the point where reflected light rays converge
• dₒ — object distance (cm): how far the object sits in front of the mirror
• dᵢ — image distance (cm): how far the image forms from the mirror

A concave mirror has a focal length of 10 cm. An object sits 30 cm in front of it. How far from the mirror does the image form?

• f = 10 cm
• dₒ = 30 cm

1. 1/dᵢ = 1/f − 1/dₒ
2. 1/dᵢ = 1/10 cm − 1/30 cm

dᵢ = 15 cm

The same 10 cm focal length mirror now has an object placed 20 cm away instead of 30 cm. Where does the image form?

• 10 cm
• 20 cm
• 40 cm

A convex mirror, like the kind used for a car's side mirror, always forms what kind of image?

• Virtual and upright, always smaller than the object
• Real and inverted, always larger than the object
• Real and upright, always the same size