Ray diagrams for mirrors show how light reflects to form images. They are important because they let you predict where an image appears, how large it is, whether it is upright or inverted, and whether it is real or virtual. Concave mirrors can form several different image types depending on object position, while convex mirrors always form smaller upright virtual images.
These diagrams are used in telescopes, headlights, shaving mirrors, security mirrors, and many optical instruments.
The method uses a few principal rays whose paths are easy to draw. For spherical mirrors, the focal point F is halfway between the mirror and the center of curvature C, so f = R/2. A ray parallel to the principal axis reflects through the focal point for a concave mirror, while for a convex mirror it reflects as if it came from the focal point behind the mirror.
Where reflected rays actually meet gives a real image, and where their backward extensions appear to meet gives a virtual image.
Key Facts
- Mirror equation: 1/f = 1/do + 1/di
- Magnification equation: m = hi/ho = -di/do
- For a spherical mirror, f = R/2, where R is the radius of curvature.
- Concave mirrors have a real focal point in front of the mirror and can form real or virtual images.
- Convex mirrors have a virtual focal point behind the mirror and always form upright, reduced, virtual images.
- A real image forms where reflected rays actually meet, while a virtual image forms where reflected rays only appear to originate.
Vocabulary
- Principal axis
- The straight reference line that passes through the mirror's vertex, focal point, and center of curvature.
- Focal point
- The point where rays parallel to the principal axis converge after reflection, or appear to diverge from after reflection.
- Center of curvature
- The center of the sphere from which a spherical mirror is a small section.
- Real image
- An image formed where reflected light rays actually meet and that can be projected onto a screen.
- Virtual image
- An image formed where reflected rays appear to come from and that cannot be projected onto a screen.
Common Mistakes to Avoid
- Drawing convex mirror rays as if they meet in front of the mirror is wrong because convex mirrors make reflected rays diverge, so the image is found by extending rays backward behind the mirror.
- Forgetting that the focal point is halfway to the center of curvature is wrong because spherical mirrors follow f = R/2, not f = R.
- Using the same image description for every concave mirror object position is wrong because a concave mirror changes image type when the object moves inside F, between F and C, at C, or beyond C.
- Ignoring sign conventions in the mirror equation is wrong because the sign of f and di determines whether the mirror and image are real or virtual.
Practice Questions
- 1 A concave mirror has focal length 10 cm. An object is placed 30 cm in front of the mirror. Use 1/f = 1/do + 1/di to find the image distance, then state whether the image is real or virtual.
- 2 A convex mirror has focal length -15 cm. An object 6 cm tall is placed 45 cm in front of the mirror. Find the image distance and magnification, then calculate the image height.
- 3 A student places an object between the focal point and a concave mirror. Explain why the image is virtual, upright, and enlarged using the behavior of reflected rays.