Experimental observation of total-internal-reflection rainbows

Area of Science:

• Optics
• Fluid dynamics
• Light scattering

Background:

• Rainbows are optical phenomena typically observed when light interacts with water droplets.
• Standard rainbows (primary and secondary) result from external illumination and specific angles of refraction and reflection.
• Internal illumination of droplets can lead to novel optical effects not observed with external light sources.

Purpose of the Study:

• To investigate a new class of rainbows formed by internal illumination of a droplet.
• To experimentally determine the position of these novel rainbows and compare them with standard rainbows.
• To analyze the influence of the light source's position and the droplet's refractive index on rainbow formation.

Main Methods:

• Experimental setup involving a single droplet illuminated internally by a point light source.
• Precise measurement of rainbow positions (transmission, one-internal-reflection, total-internal-reflection, and primary rainbows).
• Systematic variation of the internal light source's position relative to the droplet's center.

Main Results:

• Observed and measured the position of a new class of rainbows generated by internal droplet illumination.
• Demonstrated that rainbow position is dependent on the light source's position within the droplet.
• Identified a critical proximity of the light source to the droplet center beyond which the rainbow vanishes.

Conclusions:

• Internal illumination of droplets creates a distinct class of rainbows.
• The position and existence of these internal rainbows are sensitive to the light source's position.
• These findings expand the understanding of light-droplet interactions and rainbow phenomena.