What are Mirrors?
A mirror is a smooth reflective surface that forms images by reflecting light. Concave mirrors curve inward and can focus light to form real or magnified virtual images, while convex mirrors curve outward and always form reduced virtual images.
The mirror equation 1/f = 1/do + 1/di relates a mirror's focal length to the object and image distances; concave mirrors have positive f, convex mirrors negative f.
- •Curves inward
- •Can form real or magnified virtual images
- •e.g., makeup mirrors, telescopes
- •Curves outward
- •Always forms a reduced, virtual image
- •e.g., car side mirrors, security mirrors
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Step-by-step worked examples
A concave mirror has f = 15 cm; an object sits 45 cm away. Find the image distance.
di = f·do/(do−f) di = (15×45)/(45−15) = 675/30 di = 22.5 cm (real, inverted image)
The same concave mirror (f = 15 cm) has an object 10 cm away, inside the focal length.
di = (15×10)/(10−15) = 150/−5 di = −30 cm → virtual, upright, magnified image (shaving/makeup mirror)
A convex mirror has f = −25 cm (car side mirror); an object is 40 cm away. Find the image distance.
di = (−25×40)/(40−(−25)) = −1000/65 di ≈ −15.4 cm → virtual, upright, reduced image
Flashcards
Quick quiz
Q1.Which mirror always forms a virtual, reduced image?
Q2.For a concave mirror with f=15 cm and do=45 cm, what is di?
Q3.Placing an object inside a concave mirror's focal length produces…
Q4.How does focal length relate to radius of curvature R?
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Common mistakes
Mixing up which mirror is concave and which is convex. — Correct: Concave curves inward (like a cave) and converges; convex bulges outward and diverges.
Thinking convex mirrors can form real images. — Correct: Convex mirrors only ever form virtual, upright, reduced images.
Confusing focal length with radius of curvature. — Correct: Focal length is half the radius of curvature: f = R/2.
Forgetting the sign convention (convex f negative). — Correct: In 1/f = 1/do + 1/di, concave mirrors use positive f, convex mirrors negative f.
FAQ
What is the mirror equation?
1/f = 1/do + 1/di, relating a mirror's focal length to the object and image distances.
What is the difference between concave and convex mirrors?
Concave mirrors curve inward and converge light (real or magnified virtual images); convex mirrors curve outward and diverge light (always reduced virtual images).
How do you calculate image distance for a mirror?
Rearrange the mirror equation: di = f·do/(do−f), using the known focal length and object distance.
What are real-world examples of concave and convex mirrors?
Concave: shaving/makeup mirrors, satellite dishes, telescopes. Convex: car side mirrors, security mirrors in stores.