The power of a plane mirror is zero dioptres.
A plane mirror is a flat reflective surface. Its optical power, measured in dioptres, is inversely related to its focal length. The formula for optical power (P) is P = 1/f, where 'f' is the focal length in meters. For a plane mirror, parallel light rays striking its surface reflect as parallel rays, meaning they neither converge nor diverge. This characteristic implies that a plane mirror has an infinite focal length. Since power is the reciprocal of focal length (1/∞), the power of a plane mirror is zero.
Understanding Optical Power and Focal Length
Optical power indicates the degree to which a mirror or lens converges or diverges light.
- Large Power, Small Focal Length: If a mirror or lens has a short focal length, it brings light to a focus (or appears to bring it to a focus) over a short distance, indicating a high optical power.
- Small Power, Large Focal Length: Conversely, if the focal length is long, the power is low. A plane mirror, with its infinite focal length, represents the extreme end of this spectrum, possessing zero optical power.
The S.I. unit for optical power is the dioptre (D), which is equivalent to one inverse meter (m⁻¹).
| Mirror Type | Conceptual Focal Length (f) | Optical Power (P = 1/f) | Light Behavior |
|---|---|---|---|
| Plane Mirror | Infinite ($\infty$) | 0 Dioptres | Neither converges nor diverges |
| Concave Mirror | Finite (Positive) | High (Positive) | Converges light |
| Convex Mirror | Finite (Negative) | Low (Negative) | Diverges light |
Key Characteristics of Plane Mirrors
Beyond their zero optical power, plane mirrors possess several distinct optical properties:
- Law of Reflection: For light striking a plane mirror, the angle of reflection always equals the angle of incidence. Both angles are measured with respect to the normal (an imaginary line perpendicular to the mirror surface at the point of incidence). This fundamental principle governs how light behaves upon reflection.
- Image Formation: Plane mirrors always form images that are:
- Virtual: The light rays do not actually converge to form the image; they only appear to originate from it.
- Erect: The image is upright, not inverted.
- Same Size: The image is the same size as the object.
- Laterally Inverted: The image is reversed from left to right (e.g., your left hand appears as the right hand of your reflection).
- Same Distance Behind Mirror: The image appears to be as far behind the mirror as the object is in front of it.
Practical Applications of Plane Mirrors
Due to their unique properties, plane mirrors are widely used in various everyday and scientific applications:
- Household Mirrors: The most common use, allowing us to see our reflections.
- Periscopes: Used in submarines to view objects above the water's surface, employing two plane mirrors arranged to reflect light.
- Kaleidoscopes: Create beautiful, symmetrical patterns through multiple reflections between several plane mirrors.
- Optics Experiments: Fundamental tools for demonstrating the laws of reflection and understanding basic light behavior.
- Solar Furnaces and Reflectors: While large-scale solar concentrators often use curved mirrors, smaller, simple solar heaters might incorporate plane mirrors to direct sunlight.
Understanding the zero power of a plane mirror is crucial for distinguishing its optical behavior from that of curved mirrors or lenses, which possess non-zero power due to their ability to converge or diverge light.
