September 23, 2022
Moon under a Rainbow - Photo from Sarasota, Florida by Briana Bryant
First, in order to see a rainbow we need both sunshine and rain. We must look in the opposite direction of the Sun. In order to have the rain, most likely the rain would come from a cloud that is in the distance causing the rain. The raindrops reflect the sunlight by means of the light entering the drops, and internally reflected from the far side of the raindrops, and back to the observers. As the light enters and leaves each surface the light is bent, and the amount of bend in the light depends on the index of refraction* of the water drop. Different colors of light exhibit a different index of refraction in water. That means that the different colors emerge at slightly different angles to the initial direction and the colors are separated with slightly different angles. A zoomed in photo appears below.
All fine and dandy! But, where is the rain? We see blue sky in the background "beyond" the rainbow. Most likely the source of the rain was the clouds in the top of the photo. Where the cloud appears to lie in front of the rainbow, any rain from that cloud just to the left of the light pole, is falling below the cloud at the wrong angle for the rainbow. The rain of the rainbow is most likely coming from the clouds in the top of the photo. There is clear sky beyond the cloud as well as behind the observer to enable the sunshine. The rain shower is small and highly localized.
The time for this photo can almost be determined by the relative positions of the Sun (from shadows of cars) and the height of the rainbow (rather low indicating a rather high Sun. If the Moon is nearly full, we can see the Moon in mid-afternoon a few days before full Moon or in mid-morning a few days after the full Moon. The time-stamp on the digital photo's header indicates the photo was taken Sept. 13 (a few days after the full Moon of Sept. 9, 2022). From knowledge of the Moon's orbit, this is mid-morning and the camera is looking west. Indeed the time-stamp indicates slightly past 9 AM. The time stamp (EXIF data) is not available on photos copied from web postings.
The apparent distance of a rainbow is also of interest. Even though the rainbow is caused by raindrops between the observer and background objects, the angle of the rainbow relative to the anti-solar point - the direction the Sun's rays are directed to - is always about 42 degrees, which is the center point of the rainbow's arc. If the observers move to one side, the rainbow moves with them relative to the objects on the ground. Thus we can never reach that "pot of gold".
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* The index of refraction for a transparent substance is the ratio of the speed of light in air divided by the speed of light in the substance. For water (raindrops) the index of refraction is about 1.3. Blue light in water (or glass) has a slightly larger index than red light, hence the blue light is bent more and separated from the red light.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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