Rainbows occur in Richard's pictures from the early 20s up into his 80s. This universal symbol of hope for him (like Wordsworth) brings back a childhood delight in how it can transform a scene.
Richard must have seen this effect when out for a walk and then gone back to the studio to try to capture the scene. The shaft of sunlight through the centre of the trees seems to echo the light of the rainbow.
The rainbow changes everything. The left half of the picture is down-to-earth Yorkshire and then under the shadow of the rainbow there is suddenly a sense of something strange happening as if the solid ground is dissolving.