Why a mirror reflects a band of wavelengths rather than all light
A mirror works by bouncing light off a surface or a stack of thin layers, and how well it bounces depends on the wavelength relative to the structure. In a high-contrast grating mirror, the geometry of the grating is tuned so that a chosen wavelength reflects strongly; the optimized design reported a 250 nm wide bandwidth centered near 2.65 micrometers, a mid-infrared wavelength used in vertical-cavity lasers [1]. That number is the width of the reflecting band, not the whole spectrum, which is why the same paper notes the design can be scaled for other wavelengths [1].
Multilayer mirrors make the wavelength dependence even more explicit. They stack alternating materials so that reflections from many interfaces add up, but only for wavelengths near the design value. A rubidium-based multilayer design targets 11.4–17 nm, a range useful for lithography and X-ray astronomy, and the authors model how layer choices change the reflection coefficient at specific wavelengths such as 11.4, 13.5, and 17 nm [5]. In the soft X-ray water window, a Cr/Sc multilayer achieved 35% peak reflectance at a photon energy of 396 eV, which corresponds to a narrow slice of that extreme range [4].
What ranges do real mirrors actually cover?
For everyday optics, the visible and near-infrared region is the common working band. A four-mirror space camera design covers visible and near-infrared wavelengths while maintaining high-quality imaging [3]. Near-infrared mirrors used in gas sensing were calibrated across an 80 nm span, with an effective reflectivity of 0.9982 at 1531.6 nm; the reflectivity uncertainty was smallest in the center of that range and larger at the edges [2]. That pattern—best performance in the middle, falling off at the edges—is a useful way to picture any mirror's band.
At shorter wavelengths, the numbers look different. The water-window multilayer mirror reached 35% reflectance at 396 eV, described as the highest reported in that spectral range [4]. That is far below the near-unity reflectivity of the infrared mirror [2], showing that moving to extreme wavelengths costs reflectivity even when the mirror is optimized. The rubidium multilayer work is theoretical, and the authors note that experimental verification of their boron-carbide barrier idea is still required [5], so its predicted performance should be read as a design target rather than a measured result.
How to read a mirror's wavelength range
When a mirror is described as covering a range, the useful details are the center wavelength, the bandwidth, and how reflectivity behaves across that band. The mid-infrared grating mirror's 250 nm bandwidth is tied to a 2.65 micrometer center [1]; the near-infrared calibration covered 80 nm with the best uncertainty in the middle [2]. These are different kinds of ranges—one is a design bandwidth, the other a measurement span—so they should not be compared as if they were the same specification.
The practical lesson is that a mirror is matched to its job. A mirror for a visible-light camera, a mirror for a mid-infrared laser, and a mirror for soft X-rays are built on different principles and cover different slices of the spectrum [3][1][4]. If you need to know whether a particular mirror will reflect your light, the answer lies in its design wavelength and bandwidth, not in the word "mirror" alone.
Sources used in this answer
Optimized Si/SiO2 high contrast grating mirror design for mid-infrared wavelength range: robustness enhancement
An optimized high-contrast grating mirror for the mid-infrared achieved a 250 nm bandwidth centered near 2.65 micrometers and can be scaled for other wavelengths.
Direct readout of mirror reflectivity for cavity-enhanced gas sensing using Pound-Drever-Hall signals
Near-infrared cavity mirrors were calibrated over an 80 nm range, with effective reflectivity of 0.9982 at 1531.6 nm and uncertainty smallest in the center of the range.
Starting-point design method for unobscured four-mirror reflective imagers based on multi-aberration evaluation
A four-mirror reflective space camera design operates across visible and near-infrared wavelengths with high-quality imaging.
At wavelength metrology of high-reflectance multilayer mirror in the water window
A Cr/Sc multilayer mirror for the water window reached 35% peak reflectance at 396 eV, the highest reported in that spectral range.
Upgraded Rubidium-Based Multilayer Mirrors for Using with Radiation in Wavelength Range 11.4–17 nm
Rubidium-based multilayer mirrors are modeled for the 11.4–17 nm range, with boron carbide layers predicted to raise reflection but still needing experimental verification.