There are two forms of the following can be broadly divided into the phenomenon of reflection of light (including infrared).
(1)Reflection by the workings of the free electron: reflection by the metal surface. It is strong reflection directivity along the law of reflection.
(2)Reflection white cloth or white paper, such as wheat flour: reflection by small transparent body. To become and diffuse reflection (diffuse) usually.
Reflected by the metal surface is carried out in very surface.
Since the reflection law of reflection as expected, it is necessary to uneven surface in order to diffuse.
However, if the reflectance is lowered and do this often.
When fine powder or fibrous metal, absorption increases that repeated many times reflected by the metal surface between with diffuse reflection increases.
For example, it is (0.9) ^ 5 = 0.59 and reflecting five times in the aluminum surface of the reflection factor of 0.9.
Since repeated many times reflected by the fiber in powder or curvature of the surface decreases as the fine metal and until the outside is reflected, the reflection factor is reduced, it will become blackish.
Since the work of the free electrons is largely responsible, tend reflectance good material with good electrical conductivity generally to reflection on the metal surface.
Such as silver, gold, copper, material having good electrical conductivity is such as aluminum, but these are excellent reflective material.
Gold, copper colored because the reflectivity fall in the short wavelength region of visible light, visible light reflectance is slightly less, but have an excellent reflectance in the infrared region.
And to use a gold-plated mirror and light heating spot heater is because is the most excellent infrared region reflectance.
Copper is also of excellent, but it does not use so susceptible to degradation due to oxidation.
There is a drawback silver also easily turned black due to sulfur compound in the air.
As an aside, the action of the free electrons are involved significantly to the transfer of heat. Thermal conductivity is also excellent material of electric conductivity for that. (Silver, copper, gold, aluminum, etc.)
It find gold mirror is better for the purpose from the figure above, that use the radiant energy of the halogen lamp.
Reflection efficiency will fall about 10% compared to the gold-plated mirror aluminum mirror.
It can not use reflectance (blue area) short wavelength side is poor, because the colored as for visible light gold-plated mirror.
Gold-plated mirror is useless as for ultraviolet.
Aluminum is ideally suited for use as a visible light region for mirror and short wavelength region.
There is a trend of reverse reflection by small transparent body for reflection on the metal surface.
For example rock sugar is transparent and does not reflect the most light.
It is only a small boundary surface reflection due to the difference in the refractive index of air and rock sugar.
However, light is refracted within the complex fine particles As you fine, the light that is reflected, is returned to the original of which come more.
Become whitish and is reflected fairly Not really, but it becomes much granulated sugar, coarse to pure white diffuse reflection becomes that much higher reflectance when it comes to white sugar.
The same applies with ice. Chunks of ice’s transparency, but snow is a fine crystals of this is a high reflectance is white.
It is seen to be composed of a transparent fiber when viewed under a microscope also white cloth or paper.
Reflection principle of the white paper is the same as in the example of ice and the sugar, is refracted complex as light passes through a transparent thin fibers, some of which is reflected at the interface, many times in the fiber it light reversion by being repeated increases, it is highly reflective.
That’s of highly reflective material in the metal than is the transparent body as a material, and that moreover it is a fine or fibrous powder, the incident light is irregularly reflected repeatedly complex refractive therein, the interface reflection is. Reflectance, the higher the refractive index is high also, about the transparency of the material is high. They are therefore not reflective highly directional, such as metal.
Because the transmittance of light of fiber material that make up the white cloth or white paper is good in the visible light – near infrared, but it will become generally opaque as consisting of (transparency is high) to the far infrared region, reflecting also less likely to occur.
Far-infrared heater Suitable for heating white cloth or white paper, such as white pigment, because the absorption rate increases reflected light or transmitted light is reduced in such a principle.