Step 1: Understanding the Concept:
This question refers to the powerful imagery used in the poem 'Koel' by Puran Singh.
Step 2: Detailed Explanation:
The poet is deeply affected by the fiery and passionate song of the Koel (Indian cuckoo). He feels that the bird's song is so intense that it has a physical, burning effect on the nature around it.
He uses a metaphor to describe this, stating that the bird's "high-pitched strain" has set "the shades of mango leaves on fire." This is not a literal fire, but a metaphor for the song's burning passion and the way it seems to ignite the poet's soul and the world around him.
Step 3: Final Answer:
In the poet's metaphorical vision, the mango leaves are on fire.