How Does a Mirror Work?

Why do mirrors create perfect reflections, while most surfaces scatter light in different directions? The answer lies in how light waves interact with different materials.

This article explores how mirrors work, including the science behind plane, concave, and convex mirrors.

Image Credit: Neeqolah/Shutterstock.com

Diffuse vs. Specular Reflection

As we look around a room, we see most objects because light waves bounce off their surfaces and reach our eyes. However, not all surfaces reflect light in the same way.

Figure 1. Light reflected from objects

Diffuse reflection occurs when the surface of an object is uneven, causing the incident light to scatter in multiple directions rather than forming a clear image.

Figure 2. Diffuse reflection

When light encounters a smooth surface, such as a glass mirror or a plane mirror, it follows the law of reflection. This principle states that the angle of incidence (the angle at which the light ray strikes the mirror) is equal to the angle of reflection.

Figure 3. Law of reflection

In this case, specular reflection occurs, meaning that the light reflecting off the surface remains well-organized, producing a clear mirror image rather than a scattered effect. This is why mirrors reflect light so precisely.

How Do Flat Mirrors Work?

We see an object because light bouncing off it reaches our eyes, either from a light source (such as the Sun or a flashlight) or through reflected light from its surface.

In the diagram below, an object placed in front of a mirror surface reflects light waves according to the laws of geometric optics. The ray of light coming from the object reflects in a predictable manner, making the image appear as if it is located behind the mirror.

This results in a virtual image, meaning that although the rays seem to converge behind the mirror, no actual light passes through that point.

Figure 4. Image formed by reflection in a flat mirror

How Do Concave Mirrors Work?

A concave mirror is a type of spherical mirror, where the reflecting surface curves inward. The focal length is approximately half the radius of curvature. Different points on the curved surfaces of a concave mirror will reflect light waves differently.

A ray of light that is both parallel and very close to the optical axis will be reflected by the mirror so that it crosses the optical axis at the paraxial focal point.

The paraxial focal point is located one-half the radius of curvature from the point where the optical axis intersects the mirror. The word “paraxial” comes from the Greek para, meaning “at the side of” or “beside,” and axial, referring to the axis itself.

Another light beam that is parallel to the optical axis, but not close to it, will be reflected light by the mirror so that it crosses the optical axis at a point slightly closer to the mirror. This variation in crossover points is known as spherical aberration.

If the mirror has a parabolic cross-section instead of a circular one, all incoming light that is parallel to the optical axis will cross at the same point. This is why a parabolic mirror does not produce spherical aberration.

Astronomical telescopes, such as the Newtonian telescope (invented by Isaac Newton), use parabolic mirrors to eliminate distortions and improve image accuracy.

For simplicity, we can approximate that spherical mirrors behave similarly to parabolic mirrors, determining that the focal length of a spherical mirror is about one-half the radius of curvature.

Figure 5. Image formed by a concave mirror

When an object is located between the focal point and the mirror, meaning the object distance is less than the focal length, a virtual, upright, and enlarged image is obtained. This effect is commonly seen when using a curved mirror such as a make-up mirror.

• A ray (1) that appears to come from the focal point strikes the mirror and is reflected parallel to the optical axis.
• A ray (2) that is parallel to the optical axis is reflected light so that it passes through the focal point.
• A ray (3) striking the mirror at the optical axis is reflected at an angle equal to the angle of incidence.

The ray diagram below uses three reflected beams to illustrate how the image can appear magnified and upright. The image formed in this case is a virtual image because the light rays do not physically converge at the image location.

Figure 6. Ray diagram with a concave mirror

How Do Convex Mirrors Work?

A convex mirror produces a diverging effect, meaning that it causes light rays to spread outward instead of focusing. The image formed by a convex mirror is always virtual, upright, and smaller than the actual object.

In the diagram below, three light rays illustrate how a convex mirror reflects light:

• A ray (1) that is parallel to the optical axis is reflected as if it came from the focal point (f).
• A ray (2) directed toward the focal point is reflected parallel to the optical axis.
• A ray (3) striking the mirror at the optical axis is reflected so that the angle of reflection is equal to the angle of incidence.

Figure 7. Convex mirror ray diagram

The result is an image that appears smaller, upright, and located behind the mirror, which is why convex mirrors are widely used in applications requiring a wide-angle view, such as vehicle side mirrors, telescopes, and security cameras.

Key Takeaways

• Flat mirrors create virtual, same-size images through specular reflection.
• Concave mirrors focus light rays, producing either real or virtual images depending on the object’s position.
• Convex mirrors cause light to diverge, always forming a virtual, smaller image.

Want to learn more? Watch this video!

For a visual explanation of real and virtual images and how reflection of light works in plane, concave, and convex mirrors, check out this video from Infinity Learn: