Primary rainbow

•  A primary rainbow appears when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, is refracted again as it escapes back into the air, and then travels towards an observer.
• The colours in a primary rainbow are always arranged with red on the outside of the bow and violet on the inside.
• The outside (red) edge of a primary rainbow forms an angle of approx. 42.4⁰ from its centre, as seen from the point of view of the observer. The inside (violet) edge forms at an angle of approx. 40.7⁰.
• To get a sense of where the centre of a rainbow might be, imagine extending the curve of a rainbow to form a circle.
• If your shadow is visible as you look at a rainbow its centre is aligned with your head.
• A primary rainbow is only visible when the altitude of the sun is less than 42.4°.
• Primary bows appear much brighter than secondary bows and so are easier to see.
• The curtain of rain on which sunlight falls is not always large enough or in the right place to produce both primary and secondary bows.

Remember that:

• The centre of a rainbow is always on an imaginary straight line (the axis of the rainbow) that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
• The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
• The axis of a rainbow is an imaginary line passing through the light source, the eyes of an observer and the centre-point of the bow.
• The space between a primary and secondary rainbow is called Alexander’s band.