Why does a convex mirror always make a smaller, upright image?

The outwardly curved surface of a convex mirror causes reflected light rays to diverge. When our eyes or a camera trace these diverging rays backward, they appear to originate from a single point behind the mirror, forming a virtual image. This image is always upright and smaller than the object because the divergence spreads the light, making the image appear closer and diminished. This wide field of view is why convex mirrors are used for security and vehicle side mirrors.

What does a negative image distance (di) mean in the mirror equation?

A negative image distance indicates that the image is virtual and located behind the mirror. Virtual images cannot be projected onto a screen because light rays do not actually converge there; they only appear to originate from that point when traced backward. In the mirror equation, a negative di corresponds to a positive magnification (M) for upright images, which is typical for flat mirrors and convex mirrors, and for concave mirrors when the object is inside the focal point.

How is the focal length of a concave mirror related to where you can start a fire with sunlight?

The focal length is the distance from the mirror to its focal point, where parallel rays of light (like those from the distant sun) converge. To start a fire, you would aim a concave mirror at the sun and place a flammable material at its focal point. All the reflected sunlight concentrates at this small spot, intensely heating it. This real-world application directly demonstrates the mirror's ability to focus energy, which is the same principle used in solar furnaces and some telescope designs.

Does this simulator show all the light rays? What are the limitations of the model?

No, it shows a simplified principal ray diagram using typically two or three special rays whose paths are easy to trace (e.g., through the focal point or parallel to the axis). In reality, every point reflects light in all directions. The model also uses the paraxial approximation, assuming all rays are near the principal axis and the mirror is a perfect sphere. Real mirrors can suffer from spherical aberration, where rays far from the axis focus at different points, blurring the image.

Reflection

Flat, concave, and convex mirrors with auto-drawn ray diagrams.

About this model

Mirrors transform the path of light through the principle of reflection, where the angle of incidence equals the angle of reflection. This interactive simulator models this core behavior for three fundamental mirror geometries: flat (plane), concave (converging), and convex (diverging). It visualizes the formation of images by automatically constructing principal ray diagrams. For a spherical mirror, the model uses the mirror equation, 1/f = 1/do + 1/di, and the magnification equation, M = hi/ho = -di/do, where f is the focal length, do is the object distance, di is the image distance, ho is the object height, and hi is the image height. The focal point for a spherical mirror is at half the radius of curvature (f = R/2). Key simplifications include using the paraxial approximation (rays close to the principal axis) and treating mirrors as perfectly reflective, thin surfaces with no aberrations. By moving the object and switching mirror types, students learn to predict image characteristics—real or virtual, upright or inverted, magnified or diminished—based on object position. They directly observe how concave mirrors can focus light to a point or project images, while convex mirrors always produce virtual, upright, and reduced images, explaining their use in security and vehicle side-view mirrors.

Who it's for: High school and introductory college physics students studying geometric optics, as well as educators seeking a dynamic tool to demonstrate mirror ray diagrams and image formation.

Key terms

Law of Reflection
Focal Point
Virtual Image
Real Image
Mirror Equation
Magnification
Concave Mirror
Convex Mirror

How it works

Geometric reflection: angle of incidence equals angle of reflection (measured from the surface normal). Plane mirrors give a left–right reversed virtual image. Concave and convex spherical mirrors are drawn with paraxial principal rays and focal point F at half the radius of curvature.

Key equations

θᵢ = θᵣ (specular reflection)

Spherical mirror: f = R / 2

Frequently asked questions

Why does a convex mirror always make a smaller, upright image?: The outwardly curved surface of a convex mirror causes reflected light rays to diverge. When our eyes or a camera trace these diverging rays backward, they appear to originate from a single point behind the mirror, forming a virtual image. This image is always upright and smaller than the object because the divergence spreads the light, making the image appear closer and diminished. This wide field of view is why convex mirrors are used for security and vehicle side mirrors.
What does a negative image distance (di) mean in the mirror equation?: A negative image distance indicates that the image is virtual and located behind the mirror. Virtual images cannot be projected onto a screen because light rays do not actually converge there; they only appear to originate from that point when traced backward. In the mirror equation, a negative di corresponds to a positive magnification (M) for upright images, which is typical for flat mirrors and convex mirrors, and for concave mirrors when the object is inside the focal point.
How is the focal length of a concave mirror related to where you can start a fire with sunlight?: The focal length is the distance from the mirror to its focal point, where parallel rays of light (like those from the distant sun) converge. To start a fire, you would aim a concave mirror at the sun and place a flammable material at its focal point. All the reflected sunlight concentrates at this small spot, intensely heating it. This real-world application directly demonstrates the mirror's ability to focus energy, which is the same principle used in solar furnaces and some telescope designs.
Does this simulator show all the light rays? What are the limitations of the model?: No, it shows a simplified principal ray diagram using typically two or three special rays whose paths are easy to trace (e.g., through the focal point or parallel to the axis). In reality, every point reflects light in all directions. The model also uses the paraxial approximation, assuming all rays are near the principal axis and the mirror is a perfect sphere. Real mirrors can suffer from spherical aberration, where rays far from the axis focus at different points, blurring the image.

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