Wavelength-Selective Attenuation with Red Cellophane Filters — Study Notes
Overview of wavelength-selective attenuation using layered red cellophane filters and its biomedical relevance. The experiment uses a lamp as the light source and stacked red cellophane sheets as an attenuating medium to model how tissues and blood absorb and filter light. The intensity of transmitted light is measured with an Arduino Science Journal light sensor in lux, under a dark environment to minimize ambient light. The light source and detector are kept at a fixed distance throughout, establishing a controlled setup and serving as the baseline (control) measurement with no filter. The protocol proceeds by recording the lux value without any red cellophane, then adding one layer and recording again, followed by additional layers, each time measuring the lux at the detector. In the first run, the readings were: 698 lux with no filter, then 264 lux after one red cellophane layer, 218 lux after two layers, and finally 88 lux after stacking 10 layers. These results show that the presence and thickness of the absorbing medium dramatically reduce transmitted light, particularly the non-red wavelengths blue and green, leaving primarily red light but also subject to some absorption and scattering. A second trial confirmed the decreasing trend, indicating good reproducibility.
The optics behind the experiment are explained in terms of light filtering and absorption. Shorter wavelengths (violet, blue, green) are absorbed by the red cellophane, while the longer red wavelength passes through more readily, which makes the transmitted light appear red and dimmer. Each additional layer increases the depth the light must traverse in a more absorbing medium, causing greater attenuation (greater loss of intensity). This attenuation concept parallels biomedical devices such as pulse oximeters and near-infrared spectroscopy (NIRS). In pulse oximetry, light passes through tissue and is absorbed differently depending on blood oxygenation, allowing measurement of oxygen saturation. NIRS similarly relies on tissue thickness and blood oxygen levels to determine tissue oxygenation using near-infrared light.
Other biomedical technologies tied to light attenuation include optical coherence tomography (OCT), which relies on backscattered light and measures intensity loss as light penetrates tissue to reveal internal structures; the deeper the light travels, the greater the attenuation, revealing depth-dependent information. Phototherapy for neonatal jaundice uses blue light to break down bilirubin, with effectiveness depending on how well skin transmits the therapeutic wavelength, illustrating how filtering and transmission affect treatment efficacy. The experiment thus connects a simple filtration setup to a suite of real-world biomedical applications, emphasizing how material properties, thickness, and wavelength all influence light transmission.
To ground the discussion in the physics of light, we review the concept of visible light. The visible spectrum ranges from violet (shortest wavelength) to red (longest wavelength). A lamp emitting white light is actually a mixture of wavelengths. When filtered with red cellophane, shorter wavelengths such as violet, blue, and green are absorbed, while red wavelengths are transmitted. Consequently, the light on the far side appears red and is dimmer due to absorption and scattering. The red cellophane acts as a basic optical filter: its material structure blocks wavelengths shorter than red while allowing red wavelengths to pass, creating a strong red tint in transmitted light. Stacking multiple layers amplifies this filtering effect, yielding a visible deepening of red and a pronounced reduction in overall brightness.
As the experiment progresses, removing sheets makes the filter effectively thinner, allowing more wavelengths and light to pass and increasing the Lux readings. The measured lux values rise as layers are removed, illustrating how thickness and the absorbing properties of the sheet govern transmission. This observation aligns with the Beer–Lambert law, which describes how light attenuation depends on the absorbing properties of the material, its thickness, and the amount of absorbing species present.
Beer–Lambert law provides the quantitative framework for the observed attenuation. One common formulation relates the transmitted intensity to the incident intensity via the absorbance, A, and the molar extinction coefficient, ε, path length, l, and concentration, c, of the absorbing species: A = oldsymbol{ackslashvarepsilon} \, c \, l. The transmitted intensity is then related by I = I{0} \, 10^{-A} = I{0} \, 10^{-oldsymbol{ackslashvarepsilon} \, c \, l}. In another common form, attenuation is expressed with the attenuation (or absorption) coefficient α and thickness x: For the red cellophane used in the experiment, the “absorbing species” is the dye embedded in the sheet, and increasing thickness (more layers) increases the effective path length, leading to greater attenuation and lower transmitted intensity. This explains why the final reading with 10 layers is as low as , compared with the initial The transcript notes a common misnaming at the end as “Beard Lambert law,” but the standard and widely accepted formulation is the Beer–Lambert law.
In summary, the experiment demonstrates wavelength-selective attenuation via simple red-cellophane filters and connects the observed optical behavior to foundational principles and a range of biomedical technologies. It emphasizes how material properties (dye concentration), thickness, and wavelength determine transmission, with practical implications for diagnostic devices, imaging techniques, and phototherapy. The approach also highlights the importance of controlled experimental design—dark environment, constant geometry, a baseline measurement, replicated trials—to isolate attenuation effects and support reliable conclusions about light-matter interactions in bioengineering contexts.