Reflection, absorption and transmission effects

5.10.1 Reflection, absorption and transmission effects

The optical properties of a material are related to the interaction of the material with electromagnetic radiation, particularly visible light. The electromagnetic spectrum is shown in Figure 5.1, from which

Physical properties 285 it can be seen that the wavelength λ varies from 10 4 m for radio waves down to 10 −14 m for γ-rays

and the corresponding photon energies vary from 10 −10 to 10 8 eV.

Photons incident on a material may be reflected, absorbed or transmitted. Whether a photon is absorbed or transmitted by a material depends on the energy gap between the valency and conduction bands and the energy of the photon. The band structure for metals has no gap and so photons of almost any energy are absorbed by exciting electrons from the valency band into a higher energy level in the conduction band. Metals are thus opaque to all electromagnetic radiation from radio waves, through the infrared, the visible to the ultraviolet, but are transparent to high-energy X-rays and γ-rays. Much of the absorbed radiation is re-emitted as radiation of the same wavelength (i.e. reflected). Metals are both opaque and reflective, and it is the wavelength distribution of the reflected light, which we see, that determines the color of the metal. Thus, copper and gold reflect only a certain range of wavelengths and absorb the remaining photons, i.e. copper reflects the longer-wavelength red light and absorbs the shorter-wavelength blue. Aluminum and silver are highly reflective over the complete range of the visible spectrum and appear silvery.

Because of the gaps in their band structure, non-metals may be transparent. The wavelength for vis- ible light varies from about 0.4 to 0.7 µm so that the maximum band-gap energy for which absorption of visible light is possible is given by E = hc/λ = (4.13 × 10 −15 ) × (3 × 10 8 )/(4 × 10 −7 ) = 3.1 eV.

The minimum band-gap energy is 1.8 eV. Thus, if the photons have insufficient energy to excite

electrons in the material to a higher energy level, they may be transmitted rather than absorbed and the material is transparent. In high-purity ceramics and polymers, the energy gap is large and these materials are transparent to visible light. In semiconductors, electrons can be excited into acceptor levels or out of donor levels and phonons having sufficient energy to produce these transitions will be

absorbed. Semiconductors are therefore opaque to short wavelengths and transparent to long. 3 The band structure is influenced by crystallinity and hence materials such as glasses and polymers may

be transparent in the amorphous state but opaque when crystalline. High-purity non-metallics such as glasses, diamond or sapphire (Al 2 O 3 ) are colorless but are changed by impurities. For example, small additions of Cr 3 + ions (Cr 2 O 3 ) to Al 2 O 3 produce a ruby color by introducing impurity levels within the band gap of sapphire which give rise to absorption of specific wavelengths in the visible spectrum. Coloring of glasses and ceramics is produced by addition of transition metal impurities which have unfilled d-shells. The photons easily interact with

these ions and are absorbed; Cr 3 + gives green, Mn 2 + yellow and Co 2 + blue–violet coloring. In photochromic sunglasses the energy of light quanta is used to produce changes in the ionic structure of the glass. The glass contains silver (Ag + ) ions as a dopant which are trapped in the disordered glass network of silicon and oxygen ions: these are excited by high-energy quanta (photons) and change to metallic silver, causing the glass to darken (i.e. light energy is absorbed). With a reduction in light intensity, the silver atoms re-ionize. These processes take a small period of time relying on absorption and non-absorption of light.