Reflected light is light that bounces off a surface instead of passing through it. In everyday life, it’s how we see almost everything around us, because most objects don’t glow themselves; they reflect light from the sun or a lamp. In technology it’s used in mirrors, periscopes, headlights, optical sensors and solar concentrators. In microscopy, reflected light is used to examine opaque specimens such as metals, minerals, coins, circuit boards and insect surfaces, which light can’t shine through.
Reflected light in everyday life and technology
- Vision. Apart from light sources such as the sun, lamps and screens, we see objects by the light they reflect. Their color depends on which wavelengths they reflect and which they absorb.
- Mirrors and periscopes. Flat mirrors redirect light to let us see around corners or behind us.
- Reflectors. Curved reflectors in flashlights, headlights and lamps gather light and direct it into a beam.
- Telescopes. Many astronomical telescopes use curved mirrors rather than lenses to gather and focus light (see the difference between telescopes).
- Sensing and measurement. Barcode scanners, optical mice and many industrial sensors detect light reflected from a surface.
- Retroreflectors. Road signs and safety strips send light back toward its source, which is why they glow in headlights.
Specular and diffuse reflection, and why they matter under a microscope
There are two broad kinds of reflection:
- Specular reflection comes from smooth surfaces such as mirrors, polished metal and still water. Light bounces off at the same angle it arrived, so you get a clear, mirror-like reflection.
- Diffuse reflection comes from rough surfaces such as paper, rock and fabric. Light scatters in many directions, so the surface looks evenly lit from many viewing angles.
This matters in microscopy. A polished metal sample under coaxial (through-the-objective) light reflects strongly back into the objective and looks bright, while scratches and pits throw light away and look dark. A rough mineral or an insect’s body scatters light diffusely, so the angle and spread of your light source change the image dramatically.
What reflected light is used for in microscopy
Microscopes that light the specimen from above are called reflected light, incident light or epi-illumination microscopes. They’re used for:
- Metallography: checking grain structure, phases, inclusions, welds and cracks in polished (often etched) metal samples
- Electronics and semiconductors: inspecting circuit boards, solder joints, chips and wafers for defects
- Geology: studying opaque ore minerals in polished sections, and surfaces of rocks and mineral grains
- Materials and manufacturing: coatings, paints, ceramics, plastics, machined surfaces and quality control
- Forensics: tool marks, fibers on surfaces, documents and inks
- Biology at low power: whole insects, seeds, shells and other specimens too thick for light to pass through, usually under a stereo microscope
For a side-by-side comparison with ordinary slide microscopes, see transmitted vs reflected light microscopes.
Reflected light contrast techniques
As with transmitted light, there are several ways to light a surface for contrast:
- Brightfield: coaxial light makes flat areas bright and features dark.
- Darkfield: light arrives only at steep angles, so flat areas go dark and scratches, edges and particles glow. It’s excellent for finding defects. The transmitted version is described in dark field microscopy.
- Oblique or raking light: light from a low angle on one side throws texture into relief, which is ideal for coins, engraving and tool marks.
- Polarized light: reduces glare from shiny surfaces and reveals some anisotropic metals and ore minerals. See polarized light microscopy.
- DIC: reveals tiny height differences on polished surfaces. See DIC explained.
- Fluorescence: most fluorescence microscopes use reflected-light geometry, sending excitation light down through the objective. See how fluorescence microscopy works.
Hobby subjects for reflected light
A stereo microscope with a top light, or the top lamp on a dual-light student microscope, opens up lots of subjects (more ideas in what everyday things look like under a microscope):
- Coins, medals and stamps
- Mineral specimens, sand grains and fossils
- Insects, spiders, feathers and seeds
- Circuit boards, watch movements and small mechanisms
- Fabric weaves, paper fibers and printed dots in magazines
- Leaf surfaces, flower parts and moss
Limits of reflected light
- You only see the surface. Internal structure needs thin sections and transmitted light.
- Glare from shiny surfaces can wash out detail unless you diffuse or polarize the light.
- At high magnification you need a proper coaxial epi-illuminator, because a side lamp can’t reach a surface that the objective almost touches.
- Rough or tilted surfaces reflect light unevenly, so you’ll often need to adjust the light angle or use a ring light.