REVIEW 4 major objections 4 minor 5 references
Using a remote control to determine the infrared absorption coefficient in water
T0 review · 4 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read A TV remote, a solar cell, and a smartphone yield the infrared absorption coefficient of water as (0.113 ± 0.002) cm⁻¹, matching literature 0.119 cm⁻¹.
desk verdict Simple, cheap Beer-Lambert lab with a remote control; the audio-chain linearity is never validated, so the quantitative alpha should be taken as illustrative, and the x<5 cm exclusion needs explanation. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the Beer-Lambert law, $I(x)=I_0 e^{-\alpha x}$, together with its logarithmic linearization. The experimental machinery is a transduction chain: the remote's infrared pulse passes through water, is converted to a voltage by a solar cell, drives a speaker, is captured by a smartphone microphone, and is read off as audio peak amplitude in arbitrary units. The argument treats each peak amplitude as proportional to $I(x)$, so the slope of $\ln I$ versus water thickness directly gives $-\alpha$. Nothing else in the setup needs calibration, which is what makes the experiment low-cost and portable.
What would settle it
Play a fixed remote-control pulse while inserting calibrated neutral-density filters or known attenuators into the beam; if the audio peak height does not fall exponentially with added attenuation, the proportionality assumption fails and the fitted alpha would be biased. Also examine the recorded waveform for flat-topped peaks at small water heights, which would indicate clipping.
Extended reading notes
Core claim
The paper's central claim is that the audio amplitude recorded by a smartphone is a faithful proxy for infrared intensity, so the Beer-Lambert law $I(x)=I_0 e^{-\alpha x}$ can be verified and quantified without purpose-built optical instruments. Taking logarithms turns the exponential decay into a straight line, $\ln I = -\alpha x + \ln I_0$, and a linear fit of the measured points gives $\alpha = (0.113 \pm 0.002)$ cm$^{-1}$ with a correlation coefficient of 0.9969. The fit's intercept, $\ln I_0 = 8.87$, differs by under 1% from the value measured directly with an empty column ($\ln 7480 = 8.92$), and the fitted $\alpha$ agrees with the published 0.119 cm$^{-1}$ for this wavelength. The authors therefore assert that the Beer-Lambert law is demonstrated and that the experimental absorption coefficient is in good agreement with the literature.
Load-bearing premise
The recorded audio peak height is treated as directly proportional to the infrared intensity reaching the solar cell, with no reported check for clipping, compression, or nonlinearity in the speaker, phone microphone, or audio editor.
Editorial extensions
If this is right
- The same audio-amplitude chain can be reused to measure absorption coefficients of other liquids or transparent materials whenever a suitable infrared LED is available.
- A student laboratory can obtain the absorption coefficient to within about 5% of the reference value using only 2.5 cm thickness steps and three repeated pulses per height.
- The internal consistency check—comparing the fitted intercept with a direct empty-cell measurement—provides a built-in way to catch large systematic errors without extra equipment.
- Because the exponential decay appears directly in the audio-editor trace, the experiment makes the Beer-Lambert law visible before any fitting is performed.
Reading between the lines
- If the smartphone or speaker compresses or clips strong signals, the small-height amplitudes will be underestimated and the fitted $\alpha$ will be biased low; inspecting waveforms for flat-topped peaks at small $x$ would reveal this.
- The same setup could be extended to a coarse infrared spectroscopy experiment by swapping infrared LEDs of different wavelengths and comparing the fitted absorption coefficients.
- A direct test of the proportionality assumption would be to insert calibrated neutral-density filters into the beam and verify that audio peak height decays exponentially with added attenuation.
- The authors' $\alpha$ is about 5% below Kou's value; one plausible cause consistent with the data is mild nonlinearity at the largest amplitudes, which a student could check by reducing the source intensity rather than the water path.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a low-cost undergraduate experiment in which infrared light from a TV remote control (λ ≈ 929 nm) is passed through water columns of increasing thickness, and the transmitted intensity is measured indirectly by recording, with a smartphone, the sound produced by a solar-cell-driven speaker. From a linear fit of ln(I) versus water thickness x, the authors obtain α = (0.113 ± 0.002) cm⁻¹ for the absorption coefficient, compare it with a literature value of 0.119 cm⁻¹, and also check the fitted intercept ln(I₀) = 8.87 against a direct empty-column measurement (ln(I₀) = 8.92). The paper claims good agreement with the Beer–Lambert law and with the literature, and it is positioned as a simple, accessible demonstration for physics courses.
Significance. If the quantitative claims are sound, the paper offers a genuinely low-cost and portable way to demonstrate the Beer–Lambert law and to measure an absorption coefficient using equipment available in many households. The direct intercept check against the empty-column measurement is a good internal consistency test, and the paper reports its tabulated data, which facilitates independent analysis. However, the central quantitative result depends on an unvalidated assumption that the recorded audio amplitude is proportional to the optical intensity, and the paper omits per-point uncertainties. Since the reported discrepancy with the literature is about three times the stated fit uncertainty, the claim of agreement is not yet supported without additional calibration and error analysis. The pedagogical value is real, but the manuscript as written does not fully establish the reliability of the measurement.
major comments (4)
- [Section 2 and Section 3, Eq. (1), Table 1, Fig. 4] The intensity values in Table 1 are audio amplitudes obtained by 'editing the audio file' of a speaker driven by a solar cell, but the paper provides no calibration or linearity check for the chain solar-cell → speaker → smartphone microphone. Because the Beer–Lambert fit is performed in log space, any compressive nonlinearity (e.g., speaker distortion, phone automatic gain control, or clipping at large signals) will directly bias the fitted slope α. The authors should validate that the recorded amplitude is proportional to the incident optical power over the measured range, or at least quantify the nonlinearity and its effect on α.
- [Section 3, Table 1] The decision to omit all data below x = 5 cm is unexplained and, as written, contradictory: for smaller water thickness the transmitted signal should be larger, so the statement that 'the signal is not clearly detected' below 5 cm suggests saturation or clipping of the recording rather than weak detection. The authors need to clarify what happened at those thicknesses. If the recording saturated at high intensity, the surviving largest-amplitude points (e.g., x = 5.0, 7.5, 10.0 cm) may also be affected, possibly biasing the fitted slope.
- [Section 3, Table 1 and Fig. 4] The paper gives no per-point uncertainties, and the reported fit uncertainty of ±0.002 cm⁻¹ appears inconsistent with the scatter in Table 1. For example, the drop in ln(I) between consecutive thicknesses varies from 0.38 (between x = 5.0 and 7.5 cm) to 0.19–0.22 (between later points), implying local slopes ranging from about 0.15 cm⁻¹ down to 0.08 cm⁻¹. This variation is far larger than the quoted uncertainty in α. The authors should report standard deviations from the three repeated pulses per thickness, provide a residual plot, and discuss whether the deviations reflect random error, neglected systematic effects, or a non-exponential attenuation.
- [Section 3, last paragraph] The comparison with the literature is under-specified. The value α = 0.119 cm⁻¹ is attributed to Kou et al. (1993), but that reference is a refractive-index database, not a direct measurement of the absorption coefficient. The authors should either state how the literature absorption coefficient was derived from those data or cite a direct absorption-coefficient source, and should specify the wavelength and temperature at which the value applies. Also, the difference between 0.113 and 0.119 cm⁻¹ is about 5% and roughly three times the reported fit uncertainty, so the claim of 'good agreement' should be made more cautiously, acknowledging that systematic uncertainty likely dominates.
minor comments (4)
- [Abstract and Section 1] In the abstract, 'Beer Lambert law' should be hyphenated as 'Beer–Lambert law'. In Section 2, the absorption coefficient is denoted 'a' in Eq. (1) but as 'α' elsewhere; the notation should be unified.
- [Section 3, Table 1 and Fig. 3] The description of how the audio file was edited is insufficient for reproducibility: the authors should specify what quantity was extracted (peak amplitude, RMS amplitude, etc.), which software was used, and how the arbitrary units in Table 1 are defined.
- [Section 3, Fig. 4] Figure 4 shows the data points but does not include the fitted straight line; adding the best-fit line with the fit parameters displayed would make the linearity of the fit easier to judge.
- [Section 2, experimental setup] The paper states that the distance between the remote control and the solar cell was kept constant but does not report the actual distance; please state this distance, as it affects the incident intensity and the reproducibility of the experiment.
Circularity Check
No circularity: the absorption coefficient is obtained by a Beer–Lambert fit to measured audio amplitudes and independently compared with a literature value.
full rationale
The paper's central result, α = (0.113 ± 0.002) cm⁻¹, comes from a linear least-squares fit of ln(I) versus water-column thickness x using the data in Table 1, with I obtained from recorded audio amplitudes. This is a direct measurement-to-fit procedure, not a derivation that assumes its own conclusion. The fitted intercept ln(I₀) = 8.87 is checked against a separately measured empty-column value ln(I₀) = 8.92, so the fit is not being validated with the same quantity it was designed to reproduce. The literature comparison with α = 0.119 cm⁻¹ (Kou et al., 1993) is an independent external benchmark, and the agreement is stated as a check rather than used to construct the fit. The only self-citation, Marín-Sepúlveda (2024), concerns the earlier use of a remote control's infrared signal for teaching the inverse-square law and supplies the general apparatus concept, not the absorption coefficient or any fitted parameter. Concerns about whether the smartphone audio amplitude is a linear proxy for transmitted infrared intensity, and about possible clipping or compression, are experimental-validation issues that affect accuracy; they are not circular steps because the model is not defined in terms of the measured amplitudes nor fitted to the literature value. No step in the derivation chain reduces to its own inputs, and no load-bearing argument depends on a self-citation or an imported uniqueness claim.
Assumptions & free parameters
free parameters (2)
- water absorption coefficient alpha =
0.113 ± 0.002 cm^-1
- fitted intercept ln(I0) =
8.87 (arbitrary units)
assumptions (4)
- domain assumption Beer-Lambert law I(x) = I0 exp(-alpha x) holds for the remote infrared light passing through water
- domain assumption Recorded audio amplitude is proportional to transmitted infrared intensity
- domain assumption Reflection and scattering losses at container and water surfaces are constant or negligible over the measured thickness range
- domain assumption Total source-to-detector distance stays fixed as water height changes
Cite this review
Pith. "Pith review of Using a remote control to determine the infrared absorption coefficient in water." pith.science (2026). https://pith.science/paper/7HVSG6IT
@misc{pith2026250102257,
author = {Pith},
title = {Pith review of: Using a remote control to determine the infrared absorption coefficient in water},
year = {2026},
howpublished = {\url{https://pith.science/paper/7HVSG6IT}},
note = {Machine review of arXiv:2501.02257}
}
read the original abstract
In this work, we present a simple and low cost experiment designed to determine the infrared water absorption coefficient. We used a TV remote control as a point source of infrared light. The intensity after passing the light through different heights of a water column is measured with a solar cell connected to a speaker. The recorded signal, captured with a smartphone sound recorder, provides a practical demonstration of the Beer Lambert law. The collected data were fitted to the theoretical model, obtaining a very good agreement between the value of the water absorption coefficient obtained experimentally and that reported in the literature.
Figures
Reference graph
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Reviewed August 10, 2026 · model on record in the stance chip above.
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