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arxiv: 2605.26148 · v1 · pith:CWUCKSXPnew · submitted 2026-05-23 · ⚛️ physics.ed-ph

A Simple Method of Demonstration of Characteristics of Rainbows Using a Glass of Water and a Few Laser Sources

Soumen Sarkar , Sanjoy Kumar Pal , Pradipta Panchadhyayee , Shinjinee Das Gupta , Debapriyo Syam This is my paper

Pith reviewed 2026-06-30 12:39 UTC · model grok-4.3

classification ⚛️ physics.ed-ph
keywords rainbowminimum deviationdispersionlaserrefractionclassroom demonstrationwater cylinderexperimental verification
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The pith

A cylindrical glass of water with red, green and blue lasers lets students measure minimum-deviation angles that match theoretical predictions for rainbow dispersion.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper sets out to show that a common cylindrical glass filled with water can serve as the dispersing element in a quantitative classroom experiment on rainbow formation. Three laser beams of different wavelengths are sent through the glass and the angles of minimum deviation are read directly from graph paper. The measured angles for each color agree with values calculated from the known refractive indices of water. This approach replaces qualitative prism demos with a low-cost setup that lets students connect observed angles to the wavelength dependence of refraction. A sympathetic reader would care because it turns an abstract optical process into something students can set up and verify with ordinary equipment.

Core claim

Using a cylindrical glass of water as the refracting medium, together with semiconductor lasers emitting at red, green and blue wavelengths and graph paper for angle measurement, the angles of minimum deviation can be determined experimentally; these observed angles closely match the theoretical values obtained from the refractive indices of water at the respective wavelengths.

What carries the argument

Measurement of the angle of minimum deviation for light traversing the water-filled cylinder, treated as an analogue of prism dispersion.

If this is right

  • Classroom groups can obtain numerical confirmation that shorter wavelengths deviate more than longer ones.
  • The same apparatus can be used to test other transparent liquids by substituting them for water.
  • Students can calculate refractive index from measured angles and compare it with tabulated values.
  • The method links the single-reflection rainbow ray path to the measured deviation angle.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the method works for a cylinder, similar low-cost setups might allow angle measurements for other curved surfaces or multiple internal reflections.
  • The approach could be adapted to measure dispersion in solids by replacing the water with a glass rod or block of known shape.

Load-bearing premise

The curved surface of the glass cylinder produces clean, measurable minimum-deviation angles that can be compared directly to prism-based theory without large geometric distortions or scattering effects.

What would settle it

If repeated trials with the same lasers and glass yield minimum-deviation angles that differ from the calculated values by more than the uncertainty of the graph-paper readings, the quantitative match would fail.

Figures

Figures reproduced from arXiv: 2605.26148 by Debapriyo Syam, Pradipta Panchadhyayee, Sanjoy Kumar Pal, Shinjinee Das Gupta, Soumen Sarkar.

Figure 1
Figure 1. Figure 1: ). Several scientists, including Newton, Young, Airy, and Mie, have contributed towards the explanation of the various aspects of the rainbows [1- 4]. In some papers, mathematical equations and modelling are employed to explain the formation of the rainbows [5,6]. In recent times, Dragia T. Ivanov and Stefan N. Nikolov proposed an alternative method for demonstrating the rainbow. They used a large glass sp… view at source ↗
Figure 2
Figure 2. Figure 2: Schematic diagram showing the propagation of light through a spherical bulb of water. It shows a ray of light passing through a transparent spherical container, centred at O, filled with water. BA is the incident ray. Note that, by one of the laws of refraction, the plane AOD of refraction coincides with the plane of incidence, ABC. Again, the plane DOE defined by the reflected ray and OE coincides with th… view at source ↗
Figure 5
Figure 5. Figure 5: Setup for analysing laser beam refraction and minimum deviation in a [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Schematic diagram showing the effect of the thickness of the wall of the container Finally, the perpendicular distance between the rays JK and HI is 2. tan r.cos 𝒊. If i=45° and μglass ≡ μ=1.5, sin 𝑟 = sin 𝑖 𝜇 = 28.43; Therefore tan 𝑟 = 0.5346 2.𝑡.tan 𝑟.cos 𝒊 = 2 ∗ 0.1 ∗ 0.5346 ∗ 0.707 = 0.0756 𝑐𝑚 The width of the LASER beam is ~ 0.1 cm. Thus, the width of the LASER beam is a little more than or about the … view at source ↗
read the original abstract

A rainbow is a captivating natural phenomenon resulting from the refraction, dispersion, and reflection of sunlight within water droplets. Traditional classroom demonstrations often focus on qualitative explanations of the formation of rainbows using prisms or water bowls. This study presents a simple experimental approach to analysing the process of rainbow formation through quantitative analysis using a cylindrical glass filled with water, graph paper, and three semiconductor laser sources emitting red, green, and blue light. By measuring the angles of minimum deviation for different wavelengths, we have found that the experimental values closely match the theoretical predictions. This method offers a hands-on, cost-effective approach to enhance students' understanding of the physics behind rainbows.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a low-cost classroom demonstration in which a cylindrical glass filled with water is illuminated by red, green, and blue semiconductor lasers; angles of minimum deviation are measured with graph paper and asserted to agree closely with theoretical values calculated from known refractive-index dispersion, thereby illustrating the refraction and dispersion underlying rainbow formation.

Significance. If the measurements prove valid, the method supplies an inexpensive, accessible route for students to obtain quantitative data on wavelength-dependent deviation without specialized prism equipment, potentially strengthening introductory optics instruction.

major comments (2)
  1. [Experimental method / abstract claim of close match] The central claim rests on the applicability of the standard minimum-deviation relation (derived for plane-faced prisms) to a cylindrical water container. Refraction occurs at curved surfaces whose local normals vary with impact parameter, producing position-dependent ray paths and possible cylindrical focusing absent from the flat-face derivation; no justification or ray-trace verification is supplied that these effects remain negligible for the claimed agreement.
  2. [Results / abstract] The abstract asserts that 'experimental values closely match the theoretical predictions,' yet supplies neither tabulated angle values, uncertainties, measurement protocol (e.g., how the minimum is located on graph paper), nor any statistical comparison. Without these data the support for the quantitative claim cannot be assessed.
minor comments (2)
  1. A labeled diagram of the laser–cylinder–graph-paper geometry would clarify the alignment procedure and the definition of the measured deviation angle.
  2. The text should state explicitly whether the observed deviation includes an internal reflection (as required for primary rainbows) or is limited to single refraction through the cylinder.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments. We address each major point below and will revise the manuscript accordingly to improve clarity and rigor.

read point-by-point responses

  1. Referee: [Experimental method / abstract claim of close match] The central claim rests on the applicability of the standard minimum-deviation relation (derived for plane-faced prisms) to a cylindrical water container. Refraction occurs at curved surfaces whose local normals vary with impact parameter, producing position-dependent ray paths and possible cylindrical focusing absent from the flat-face derivation; no justification or ray-trace verification is supplied that these effects remain negligible for the claimed agreement.

    Authors: We acknowledge that the minimum-deviation formula is strictly derived for plane-faced prisms and that the cylindrical geometry introduces curved surfaces. The original manuscript applies the formula directly without explicit justification. In the revision we will add a brief discussion of the approximation, noting that for a narrow beam passing near the cylinder axis the local surface can be treated as approximately planar over the relevant impact parameters, together with a simple geometric estimate or reference to ray-tracing results showing that deviations from the prism case remain small for the reported angles. revision: yes

  2. Referee: [Results / abstract] The abstract asserts that 'experimental values closely match the theoretical predictions,' yet supplies neither tabulated angle values, uncertainties, measurement protocol (e.g., how the minimum is located on graph paper), nor any statistical comparison. Without these data the support for the quantitative claim cannot be assessed.

    Authors: We agree that the supporting numerical data and procedural details are not presented with sufficient detail. The revised manuscript will include a table of measured minimum-deviation angles for the three laser wavelengths, estimated uncertainties based on graph-paper resolution, a step-by-step description of how the minimum-deviation position was identified, and a quantitative comparison (e.g., absolute and percentage differences) to the theoretical values. revision: yes

Circularity Check

0 steps flagged

No circularity; experimental match to independent refractive-index theory

full rationale

The paper reports direct angle measurements from a water-filled cylindrical glass illuminated by three fixed-wavelength lasers and states that the observed minimum-deviation angles agree with standard theoretical values calculated from known wavelength-dependent refractive indices of water. No parameters are fitted to the present data, no self-citations supply load-bearing uniqueness theorems or ansatzes, and the comparison is presented as an empirical check against externally tabulated dispersion data rather than a derivation that reduces to its own inputs. The central claim therefore remains a straightforward experimental verification and receives the default non-circularity finding.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard optical theory and experimental measurement; no new free parameters or entities are introduced.

axioms (1)
  • domain assumption Standard laws of refraction and dispersion apply to the water cylinder setup.
    The comparison to theoretical predictions assumes the applicability of Snell's law and wavelength-dependent refractive indices without additional effects from the cylindrical geometry.

pith-pipeline@v0.9.1-grok · 5659 in / 988 out tokens · 43398 ms · 2026-06-30T12:39:39.288593+00:00 · methodology

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Reference graph

Works this paper leans on

13 extracted references

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