What is Absorbance: Definition and 45 Discussions

Absorbance is commonly defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative logarithm of one minus absorptance, as measured on a uniform sample". The term is used in many technical areas to quantify the results of an experimental measurement. While the term has its origin in quantifying the absorption of light, it is often entangled with quantification of light which is “lost” to a detector system through other mechanisms. What these uses of the term tend to have in common is that they refer to a logarithm of the ratio of a quantity of light incident on a sample or material to that which is detected after the light has interacted with the sample.
The term absorption refers to the physical process of absorbing light, while absorbance does not always measure only absorption: it may measure attenuation (of transmitted radiant power), caused by absorption, but also reflection, scattering, and other physical processes.

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  1. DariusP

    I Transmission and absorbance of materials in the deep UV

    I wanted to ask - why do some materials with very high transmittance inevitably start to absorb in deep UV? Is there an explanation?
  2. D

    Atmospheric absorbance of CO2 and impact of increaseing concentration

    This is a serious question, and it is not meant as an attempt to "debunk" greenhouse warming of the earth. Wikipedia has the below image, which shows the atmospheric transmittance as a function of wavelength. Notice the red vertical line through one of the CO2 absorption bands. We see that the...
  3. M

    I Gold's absorbance of blue light

    I have a LED light, which has a strong spectral peak around 470nm, and a broad emission spectrum in the deep red and infrared. I want to block most of the blue peak, and gold will absorb around 62% of this blue light. Will a polished gold foil surface of 1 cm2 be able to absorb 1 watt of this...
  4. R

    I Absorbance of a finite waveband

    For a single wavelength, the absorbance is given by: $$d=\log_{10}\left(\frac{I_{0}}{I_{t}}\right), \tag{1}$$ where ##I_0## is the light intensity incident on the material and ##I_t## is the transmitted intensity (so that ##I_t/I_0## represents the fraction transmitted). Many lasers don't...
  5. R

    What is the difference between absorptance and absorbance in spectrophotometry?

    I want to know what exactly is the physical significance of absorptance versus absorbance. I have not come across a reference that clearly explains the distinction between the two quantities. From the definitions: When light is incident on a material, by finding the fraction that gets...
  6. Irfan Nafi

    Does Beer-Lambert's law calculate the absorbance of a solute

    I'm currently doing a project that involves beer-lambert's law and am confused as to what it actually calculates. So far, my intuition goes as follows: The molar extinction coefficient is for the solvent at a specific concentration and wavelength. In this case, it is blood. And if you have to...
  7. hilbert2

    Infrared spectrum of n-heptane

    I have a work assignment where I'm supposed to calculate the transmittance of blackbody radiation through liquid n-heptane, and I have data of the heptane's absorption coefficient on the wavelength interval from 2.6 to 15 micrometers (it has significant peaks at about 3.5 ##\mu##m, 7.0 ##\mu##m...
  8. J

    NAD+ / NADH Question: Absorption peaks and spectra

    Hi guys, Im new here so I am going to get right to it. Excuse my grammer I am on my mobile. So I am trying to understand the difference in absorbance spectra of NAD+ and NADH. I have seen this touched on on this site but not directly answered. NAD+, with an apparent two aromatic structures...
  9. ReidMerrill

    Ligand Field Strength and UV absorbance

    I have a lab report for an experiment in which we used ion exchange chromatography to separate three Chromium complexes, CrCl2(OH2)4+, CrCl(OH2)52+, and Cr(OH2)63+ We used a UV vis to find the absorbance of each compound and used that to calculate the mole fraction of each ion in a mixture. One...
  10. RoboNerd

    Question about light absorbance with liquid solutions

    Homework Statement A solution of Co+2 ions appears red when viewed under white light. Which of the following statements is true? A) a spectrophotometer set to the wavelength of red light would read a high absorbance b) if the solution is diluted, the amount of light reflected by the solution...
  11. O

    Please help, looking for certain material

    Hello all and thanks from advance, 1.I'm looking for a certain kind of material for a project of mine that can quickly absorb heat and in the same time quickly realesing (I guess that's the right term) heat. Of course it's all based on diffrent parameters, but I'm sure there is a better...
  12. J

    Why Is Water Transparent to Visible Light but Absorbs Microwaves?

    Folks, I am looking for a more intuitive explanation of why water is transparent in the visible range. I am looking for the mechanism -- clearly it is transparent because photons are not absorbed. However, I am clueless as to why water should strongly absorb microwaves due to its polarity but...
  13. K

    Understanding Absorbance Values in UV Spectroscopy

    Hi guys, please can you kindly help me. I am getting an absorbance of over 6.0Abs for my experiment. However, when I am reading posts they mention that an absorbance above 1 is not reliable. Please can you explain so that I can understand. Thanks
  14. K

    Difference between: Absorbance and Absorption Cross section

    Hello, hello, I ask you to tell me the difference between the "absorbance" calculated from the dielectric function, and the "absorption cross section" calculated from Real-time TDDFT. is that they can give the same appearance spectrum? Thank you very much
  15. D

    Why is light attenuation not linear?

    I was wondering why light transmission through a solution decreases exponentially and not linear with the concentration of the absorbing compound in solution. I know the Beer-Lambert law, but I do not really understand the reason for this logarithmic rather than linear dependence. Can anyone...
  16. S

    Absorbance Simulation of Gold Octahedra

    I want to simulate the absorption spectra of gold octahedra when irradiated by a light source, according to the information I have gathered I can use discrete dipole approximation technique because octahedra is not spherical (in contrast to Mie Theory which is applicable to spherical particles)...
  17. ~christina~

    Advantage/Purpose of Transmission vs Absorbance in calibration curve

    Homework Statement The problem involves a calibration curve which was created and where % T vs concentration was favored for use over the usual Absorbance vs concentration and a question of why would this be used in favor over the latter relationship curve was presented. other details...
  18. K

    Relating absorbance wavelengths and refractive index

    Dear All, thank you for your attention. I am a Biologist and I was wondering if it is possible to translate absorbance λ values into a refractive index value? I found out about the sellmeier equation however it appears to require coefficients that have to be experimentally determined...
  19. P

    Is there a way of measuring reflectance vs absorbance

    I am looking at EMI Shielding effectiveness testing and I would like to measure how a material is shielding Electromagnetic waves, qualitatively. Is there a way to measure how a material is shielding qualitatively and quantitatively (in dB)?
  20. T

    Can FGR and the fine structure constant explain high intensity in dark field?

    Let's say I irradiate a sample that has an absorbance β with light of a wavelenght λ1. Is there a way to relate the initial λ1 and the diffracted/scattered λ2 using β?
  21. throneoo

    Light absorbance & color of materials

    it's known that for the visible part of EM wave(electromagnetic wave) , black materials absorb more visible light than white materials do . a friend of mine asked me if in general , black materials absorb more EM radiation(whose frequencies do not lie within the visible range) than white...
  22. A

    Calculating Absorbance of Iron (III) Solution at 580nm for 2.50-cm Cell

    Homework Statement 2. A 5.00 mL aliquot of a solution that contains 5.94 ppm iron (III) is treated with an appropriate excess of KSCN and is diluted to 50.0 mL.What is the absorbance of the resulting solution at 580 nm in 2.50-cm cell? Homework Equations The Attempt at a Solution...
  23. R

    Concentration problem using absorbance

    Homework Statement Hi. A patient produced 1122 mL of urine in 24 hours. A 1.00-mL aliquot of the sample was treated with Mo(Vl) and ascorbic acid and was diluted to a volume of 50.00 mL. A calibration curve was pre- pared by treating 1.00-mL aliquots of phosphate standard solutions...
  24. R

    Calibration curve (absorbance)

    Homework Statement Hi. I have a table where there is an absorbance for a 0.00 concentration (blank). I now that I have to substract that value to to each of the other absorbances that I have, but my question is... Do I have to graph (concentration 0.00, absorbance 0.00) in the calibration...
  25. R

    Question about absorbance (beer's law)

    Homework Statement Hi. I know this will sound silly, but I really don't get it.Let's say that I want to determine phosphate in a given sample by using molybdate. . After plotting the calibration curve using the absorbance and concentration of the phophate standard, I calculate the...
  26. C

    Soultion exhibits a transmittance value how do I convert this value to absorbance?

    1. A solution exhibits 58.8 of %Trasmittance value. Convert this value to absorbance. Report with Three significant figures. Not sure how to do this?? Do I subtract 100 percent from 58.8 I have no clue what to do lol..
  27. P

    Henderson Hasselbach to absorbance derivation

    Homework Statement Did a lab where I had to determine the Ka value of an indicator using spectrophotometry. This is a question I need to answer in my lab report: Derive the equation pH = pKa + log([A-Aa]/[Ab-A]) from the equation pH = pKa + log( [In-]/[HIn]) using Beer's...
  28. C

    Predicting IR Absorbance Peaks: C4v Point Group

    Lets say I need to predict the number of infrared absorbance peaks that will appear on the IR spectrum of a given molecule. As an example, I'll use this molecule: I can see that the point group of this molecule is C4v. I can look up this point groups character table...
  29. P

    Finding amount of iron from absorbance

    So I had a lab where we are supposed to determine the amount of iron present in a vitamin. I have a calibration curve already constructed, and it is telling me that the concentration of the iron is about 3.20 ppm. The vitamin was boiled with acid, filtered, and diluted to 100mL. Then...
  30. C

    Absorbance and cross-sections

    Hi, This isn't a homework question but it is quite basic so thought I should post it here. I am trying to confirm the calculation of absorbance of a gas using the Beer-Lambert law and the gas's absorption cross-section. Is the equation? A = 2.303 * absorption cross-section *...
  31. M

    Spectrophotometry: Iron in Vitamin tablets and Absorbance

    This is the experiment that I did in class, please read the whole thing before posting: http://www.chem.uic.edu/fetzer/chem222/Exp_19.pdf How do I calculate the molarity of Fe(o-phenanthroline) in each solution? So I was thinking of doing this: According to the pdf there are 2.5 grams of...
  32. C

    Converting Transmission to Absorbance (Optics)

    I have taken data for the transmission vs. wavelength for several types of glasses in the IR. I want to convert this to absorbance so that I can generalize transmission to different thickness glasses. I found an equation online that stated A = 2-Log(T%) (where Log is base 10). But I do not...
  33. R

    What Does a Negative Absorbance Reading Indicate?

    hi i know this is kind of a basic question..but what in reality does a negative absorbance reading means?? why can't the spectrometer just say its out the reading range or something?? does a negative reading really say something can we look at it and detect something...like if the negative...
  34. M

    UV Spectroscopy - Absorbance vs transmission

    Hi In UV spectrscopy, is the definition of absorbance (A = ecl) derived from the definition of transmission (T = It/Io) where It is the transmitted light and Io is the incident light, or is the definition of transmission defined from absorbance. Also A = -log(T) and T = 10^-A = 10-ecl...
  35. H

    Absorbance of diluted solution

    Solution A, while in a 1.0 cm cell, has an absorbance of 0.390 and a [Cu2+] = 0.0283 M. Then, 6.57 mL of solution A is diluted with pure water to 100.0 mL. What is the absorbance of this diluted solution? The below is what I did. But I am not sure about the answer. Can you please check...
  36. G

    Absorbance for Purple Solution

    Hi everyone. I don't have access to a visible spectrophotometer to do this so I'm looking for results on the internet. I just did the persulfate variation of the iodine clock and I need absorbance spectroscopy results for the blue/black complex that is formed with the iodine and starch at...
  37. T

    What is the Difference Between Absorbance and Transmittance?

    Dear Forumers, As a non-English speaker, I am confused about absorbance and absorptance. I found some definitions on the Internet, but I am not sure if understand them well. Oxford dictionary says the following (Oxford Dictionary of Physics, 4th ED. ISBN0-19-860759-8) for abroptance: "Symbol...
  38. J

    Effect of ph on uv absorbance of benzoic acid

    i was recently reading about uv absorbance, and i understand why species such as the phenoxide ion and aniline absorb uv radiation at longer radiation and with more efficiency than their protonated counterparts. i have spent long hours, however, trying to figure out why a benzoate ion might...
  39. N

    Absorbance of UV light by glass

    Hi, I'm doing an investigation into the absorbance of UV light by glass of varying thickness (constant density and standard type of glass) for an assigment. However, we're not actually supposed to carry out the investigation due to time constraints, so we're meant to make up the numbers...
  40. H

    Dissociation constant from absorbance

    Homework Statement A 0.120 M solution of sodium picrate ina 1 M sodium hydroxide solution was observed to have an absorbance of 0.335, due only to the absorption by the picrate anion. In the same spectrophotometer cell and at the same wavelength as in the previous measurement, a 0.300 M...
  41. L

    Waters 440 Absorbance Detector for HPLC?

    Not sure whether or not this should go here, but this seemed like the most accurate place since my question is about how an instrument works. I have an old unused Waters 440 Absorbance Detector for HPLC that I've scavenged for parts to try to digitize an old polarimeter. What I'm trying to...
  42. S

    Heat absorbance: PLS PLS HELP

    Heat absorbance: PLS PLS HELP ASAP! Homework Statement A quantity of a monatomic ideal gas undergoes a process in which its pressure is increased by a factor of n = 3 and its volume is increased by a factor of m = 6 as shown in Figure P12.12. What is the heat absorbed by the gas during this...
  43. F

    Absorbance and Concentration Question

    Homework Statement The protein oxyhemoglobin has the following molar absorbance coefficients: \epsilon276 nm = 3.44 x10^{4}M^{-1}cm^{-1} \epsilon415 nm = 1.25 x 10^{5}M^{-1}cm^{-1} If a solutioin of oxyhemoglobin has an absorbance at 415nm of 0.234, a) What is the concentration at 415nm...
  44. A

    Absorbance and Stray light helppp

    Hi, I've a chem assignment in which I've to calculate the stray light, Ps as a percentage of the incident light, Po. Given the apparent absorbance A(app) 0.047, concentration of the solution 2.00 x 10pwr-6, cell length of 1cm, activity coefficients = 1. I calculated the transmittance to be...
  45. S

    Effect of pH changes on UV absorbance

    So I have an chem assignment which I've been going through for the past week or so, but the last question has provided somewhat of a stumbling block. Here it is, So from the question, I have gathered the following, High pH solutions absorb at higher wavelenghts. Whereas low pH solutions...