colours seen in soap bubbles are due to
Soap bubble When light shines onto a bubble it appears to change colour. Unlike those seen in a rainbow, which arise from differential refraction, the colours seen in a soap bubble arise from interference of light reflecting off the front and back surfaces of the thin soap film. Depending on the thickness of the film, different colours interfere constructively and destructively.
Fire hydrant Hydrant coloring may be due to either purely practical criteria or more artistic. In the United States, the AWWA and NFPA recommend hydrants be colored chrome yellow[1] for rapid identification apart from the bonnet and nozzle caps which should be coded according to their available flow. Class AA hydrants (>1500 gpm) should have their nozzle caps and bonnet colored light blue, Class A hydrants (1000–1499 gpm), green, Class B hydrants (500–999 gpm), orange, Class C hydrants (0–499 gpm), red and inoperable or end-of-system (risking water hammer) black. This aids arriving firefighters in determining how much water is available and whether to call for additional resources, or find another hydrant. Other codings can be and frequently are used, some of greater complexity, incorporating pressure information, others more simplistic. In Ottawa, Ontario, hydrant colors communicate different messages to firefighters; for example, if the inside of the hydrant is corroded so much that the interior diameter is too narrow for good pressure, it will be painted in a specific scheme to indicate to firefighters to move on to the next one. In many localities, a white or purple top indicates that the hydrant provides non-potable water. Where artistic and/or aesthetic considerations are paramount, hydrants can be extremely varied, or more subdued. In both instances this is usually at the cost of reduced practicality.