Short on time? This page is the recap. For explanations, Figures 1–2 and the worked examples, use the full study guide.
Recap
- Concave mirror: rays parallel to the axis reflect through the focal point F, in front of the mirror.
- Convex mirror: rays parallel to the axis reflect as if from F, behind the mirror.
- Plane mirror: the focal point is infinitely far away.
- Real image: reflected rays actually meet; it can be caught on a screen. Virtual image: reflected rays only appear to meet, behind the mirror.
- Principal rays: parallel ray → through (or from) F; vertex ray → reflects symmetrically about the axis; focal ray → reflects parallel.
- Describe every image by location, type, orientation and size.
- Only plane, concave spherical and convex spherical mirrors are in the course.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| f ≈ R/2 | F halfway between vertex and C | get f from the radius of curvature |
| 1/sᵢ + 1/sₒ = 1/f | distances from the vertex | locate the image |
| sᵢ = −sₒ | plane mirror | image as far behind as object is in front |
| |M| = |hᵢ/hₒ| = |sᵢ/sₒ| | size ratio | find image height; decide enlarged or reduced |
| Sign | Positive | Negative |
|---|---|---|
| f | concave | convex |
| sᵢ | real image, in front | virtual image, behind |
| Mirror and object | Image |
|---|---|
| Concave, object beyond C | real, inverted, reduced, between F and C |
| Concave, object between C and F | real, inverted, enlarged, beyond C |
| Concave, object inside F | virtual, upright, enlarged, behind |
| Convex, anywhere | virtual, upright, reduced, between vertex and F |
| Plane, anywhere | virtual, upright, same size, same distance behind |
Assumptions behind the numbers
- Rays stay close to the axis and the mirror is small compared with R, so f ≈ R/2 and all parallel rays meet at one F.
- All distances are measured from the vertex, in the same unit.
Mistakes to avoid
- Using R instead of f.
- Giving a convex mirror a positive f.
- Forgetting that 1/sᵢ is not sᵢ: invert at the end.
- Saying a plane-mirror image is “on the mirror”.
- Drawing virtual-ray extensions as solid lines.
- Describing size but not type or orientation.
Quick self-check
- A spherical mirror has R = 30 cm. What is |f|? (15 cm)
- An object is 30 cm from a concave mirror with f = 10 cm. Describe the image. (15 cm in front; real, inverted, |M| = 0.50, reduced)
- An object is 15 cm from a convex mirror with f = −15 cm. Where is the image? (7.5 cm behind; virtual, upright, |M| = 0.50)
Next: practice questions.