Light in the shadow of a flame

When a burning candle is exposed to sunlight, it casts a shadow on the window sill or on the wall in front of it. Interestingly, not only the outline of the wax body is visible there, but also the flame itself. Are we dealing here with the shadow of light? The process can be examined more closely by looking through the flame at a small object. Then it turns out that the flame has different levels of transparency at different points.

In the area of ​​the luminous zone in the middle, an object lying behind it is hardly recognizable. This region is the least permeable and largely responsible for the penumbra on the wall. The outer edge and the core of the flame surrounding the wick, on the other hand, are almost transparent and leave almost no obscuration on the wall.

© H. Joachim Schlichting (detail)

Breaking Flame | A burning candle stands in the sunlight. On the white wall it can be seen that the shadow of the flame is framed by two vertical bands of light. Above the wick is a similar bright spot.

Surprisingly, there are not only darker zones in the projection, but also bright areas that appear even more intense than direct sunlight. Two vertical stripes light up symmetrically on both sides of the flame, and a comparably bright spot is noticeable in the area of ​​the shadow of the wick. In particular, these amplifications indicate that we are not just dealing with a mere shadow image of the candle flame, but with a complex interaction of the incident light with the hot environment.

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