The smoke rising from a burning cigarette sometimes appears a faint blue as it drifts into the air. This visual difference from the dense, white clouds typically exhaled prompts curiosity about the underlying physical process. The explanation lies not in the tobacco’s chemical composition, but in the size of the tiny particles suspended in the smoke. The color we observe depends entirely on how these microscopic particles interact with and scatter the light around them.
How Particle Size Affects Light Scattering
Light scattering is the mechanism by which we perceive the color of smoke, determined by the size of the suspended particles. Visible light consists of a spectrum of colors, each corresponding to a different wavelength; blue light has shorter waves and red light has longer waves. When a particle is much smaller than the wavelength of light hitting it, a process called Rayleigh scattering dominates. In contrast, when a particle is approximately the same size as or larger than the light wavelength, the interaction is known as Mie scattering.
The wavelengths of visible light range from about 0.4 micrometers (blue) to 0.7 micrometers (red). For Rayleigh scattering to occur, the smoke particles must be significantly smaller than this range, typically less than 0.1 micrometers in diameter. These extremely small particles act as highly efficient scatterers for the shorter, bluer wavelengths of light, determining the perceived color deflected toward the observer’s eye.
The Scientific Explanation for the Blue Hue
The blue color is characteristic of “sidestream” smoke, the plume that drifts from the lit end of the cigarette between puffs. This smoke is less concentrated than inhaled smoke. The particles in this ambient plume are ultra-fine, often measuring below 0.1 micrometers in diameter. Because these particles are so small relative to the wavelengths of visible light, they scatter light preferentially according to the inverse fourth power of the wavelength.
This strong dependence on wavelength means that blue light, which has a shorter wavelength, is scattered far more intensely than the longer, red wavelengths. When viewing the sidestream smoke, you are seeing the short-wavelength blue light redirected toward your eye by these minute particles. The longer wavelengths, such as red and yellow, mostly pass straight through the plume, causing the smoke to appear distinctly blue.
Why Smoke Appears White or Gray
The white or gray appearance is commonly observed in the dense cloud of “mainstream” smoke, which is the smoke inhaled and then exhaled by the smoker. This smoke is visually different because the physical properties of the particles change dramatically. As the smoke cools and passes through the moist environment of the lungs, the tiny solid and liquid particles coagulate and absorb water vapor, causing the original ultra-fine particles to grow significantly in size.
The resulting mainstream smoke particles are much larger, typically ranging from 0.2 to 0.5 micrometers. Since these particles are now closer to or larger than the wavelength of visible light, the scattering behavior shifts to Mie scattering. Mie scattering is nearly independent of the light’s wavelength, meaning the larger particles scatter all colors of the visible spectrum almost equally. When all wavelengths of light are scattered uniformly, the human eye perceives the resulting combination of colors as white or gray.