Download Absorption, Reflection and Transmission

OPAC203
PHYSICAL AND MODERN OPTICS
LABORATORY MANUAL
Absorption, Reflection, and Transmission
of the soda-lime glass
Dr. Ahmet Bingül
Gaziantep University
Department of Optical & Acoustical Engineering
Course Web page:
http://www1.gantep.edu.tr/~bingul/opaclab
Oct 2016
Some parts of the following pages are extracted from “Callister Jr W.D., Rethwisch D.G. Materials science
and engineering (8ed., Wiley, 2010)(ISBN 0470419970)” and METU PHYS 222 lecture notes.
PURPOSE
To measure reflectance, transmittance and absorbance of a soda lime glass by using laser diodes.
EQUIPMENT
Basic Optics Bench
Red (650 nm) and Green (532 nm) Diode Laser
Soda-lime glass whose thickness is 20 mm
High Sensitivity Light Sensor
DataStudio or Capstone Software
INTRODUCTION
When light proceeds from one medium into another (e.g. from air to a glass), several things happen. Some of
the light radiation may be transmitted through the medium, some will be absorbed, and some will be
reflected at the interface between the two media. The intensity I0 of the beam incident to the surface of the
solid medium must equal the sum of the intensities of the transmitted, absorbed, and reflected beams,
denoted as IT, IA, and IR respectively, Figure 1. For the conservation of energy:
IT + IA + IR = I0
(1)
Radiation intensity, expressed in watts per square meter, corresponds to the energy being transmitted per
unit of time across a unit area that is perpendicular to the direction of propagation. Alternatively Equation 1
can be written as:
T+A+R=1
(2)
where
T = IT / I0 = transmissivity = transmittance
A = IA / I0 = absorptivity
R = IR / I0 = reflectivity
Figure 1
Materials that are capable of transmitting light with relatively little absorption and reflection are transparent.
Materials that are impervious to the transmission of visible light are termed opaque. In the lab, we will try to
measure T, A, and R for the transparent materials by using laser diodes.
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THEORY
Reflectance
The reflectivity R represents the fraction of the incident light that is reflected at the interface:
(3)
When the light is transmitted from a vacuum (or air) into solid and the light is normal (or perpendicular) to
the interface then
(4)
where n is the index of refraction of the material.
Notes
The higher the index of refraction of the solid, the greater is the reflectivity.
Reflectivity is approximately 4-5% for typical silicate glasses.
Reflectivity varies with wavelength.
Reflection losses for lenses and other optical instruments are minimized significantly by coating the
reflecting surface with very thin layers of dielectric materials such as magnesium fluoride (MgF2).
Absorbance
Materials may be opaque or transparent to visible light. The light radiation is absorbed in non-metal
materials by two basic mechanisms: electronic polarization and valence band-conduction band electron
transitions (See lecture notes for details).
The intensity of the net absorbed radiation is dependent on the character of the medium as well as the path
length. The intensity of the transmitted light decreases with distance x that the light traverses, Figure 2.
(5)
where
β = absorption coefficient (1/cm)
x = sample thickness (cm)
Note that, Equation (5) is valid when the reflections are ignored. However, if we consider the reflections at
two surfaces then we have:
(6)
where R is the reflectivity of reflection coefficient, Figure 2.
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Figure 2: Intensity variation as a function of sample thickness for of soda lime glass.
For λ = 650 nm, β = 0.16184/cm and R= 0.042715. Red line represents Eqn (5), Blue line represents Eqn (6)
Absorbance is a logarithmic measure of the amount of light absorbed (at particular wavelength) as the light
passes through a sample. When the reflections are ignored it is defined as:
(7)
Transmittance
Transmittance is the fraction of incident light at a specified wavelength that passes through a sample. In
other words, the transmittance of a sample is the ratio of the intensity of the light that has passed through the
sample to the intensity of the light when it entered the sample as shown in Figure 1:
(8)
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EXPERIMENT (for Soda-lime glass)
[Intensity measurements has to be done as described in inverse square lab sheet. Data must be saved in
suitable files.]
Procedure
1. Turn on computer and red laser diode (650 nm).
2. Wait 10 minutes until the laser diode becomes stable.
3. Measure the intensity (I0) the laser diode by using high sensitive light detector. (Collect at least 10
seconds data. Compute mean value of I0 and corresponding standard deviation)
4. Place a slab of soda lime glass between laser diode and light sensor and measure the intensity of
reflected (IR) and transmitted (IT) beams. (Again for each of them, collect at least 10 seconds data.
Compute mean values of IR and IT and their corresponding standard deviations).
5. If possible, repeat above procedure for different size of glasses and the green laser (532 nm).
QUESTIONS
Evaluate T, A, R and β and associated errors.
Compare your R and β values with the values in literature (e.g. visit refractiveindex.info)
Derive Eqn (6).
What are the possible errors in the experiment?
What kind of approximations did you take into consideration while you were obtaining the physical
quantities and how do they affect your results?
6. What discrepancies did you encounter between the calculated quantities and theoretical or literature
values?
7. What is your overall conclusion?
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