PN 350-010000-05
P a g e
| 7
The spectrum below is a reflection-absorption spectrum of the plastic film. The sample is placed on the
accessory and a mirror is placed on top of the sample. The spectrum shows no sign of interference
fringes. The band shapes are correct and have twice the absorption of the transmission case, consistent
with the double passage of the infrared beam through the sample. The technique of using a mirror
behind the sample works well as long as the sample is smooth, flat and non-scattering.
Effect of Dispersion
The spectra of many samples are distorted by anomalous dispersion in the sample. The reflectivity of
a sample is dependent upon the refractive index of the sample and is governed by the Fresnel
equations. For light hitting a sample at normal incidence, the reflection at the sample is given by the
following equation:
For example, the amount of reflection at the first glass surface is equal to R= (0.5/2.5)
2
= 4% for light
passing through a pane of glass, which has a refractive index of about 1.5.
Refractive Index n1
Sample,
Refractive Index n2
(n1 – n2)
2
(n1 + n2)
2
R=
Figure 6: R-A spectrum of polyethylene film
