The two formulas
| Quantity | Mirror | Lens |
|---|---|---|
| Formula | 1/v + 1/u = 1/f | 1/v − 1/u = 1/f |
| Magnification | m = −v/u | m = v/u |
| Power | – | P = 1/f (f in metres), in dioptres |
Sign convention (New Cartesian)
- Measure everything from the pole (mirror) or optical centre (lens).
- Light travels left to right, so the object distance u is always negative.
- Distances in the direction of light are positive; against it are negative.
- Focal length: concave mirror and concave lens are negative; convex mirror and convex lens are positive.
- Heights above the axis are positive. So a negative magnification means the image is inverted.
Quick read of the answer. For a mirror, negative v means the image is in front (real). For a lens, positive v means the image is on the other side (real). A real image is always inverted, and a virtual one is always upright.
All of ray optics is on the physics formula sheet.
Questions students ask
Where is the image when an object is at 2F of a concave mirror?
Also at 2F (the centre of curvature), real, inverted and the same size. m = −1.
Can a convex mirror form a real image?
Not of a real object. A convex mirror always forms a virtual, upright, smaller image behind the mirror. That's why it's used as a rear-view mirror.
What happens when the object is at the focus?
The reflected or refracted rays come out parallel, so the image forms at infinity.
What is the power of a lens with focal length 25 cm?
P = 1/f = 1/0.25 m = +4 D for a convex lens. A concave lens of the same focal length is −4 D.