Problem 13.1.2

Statement

13.1.2. A frosted electric light bulb in the shape of a sphere $$6$$ cm in diameter illuminates a globe $$26$$ cm in diameter. Determine the diameters of the full shadow and penumbra of the globe on the wall. The distance from the globe to the bulb is $$1$$ m, and to the wall $$2$$ m.

Solution

A small note on the solution: the author means that the distance from the light bulb to the globe is the distance between their centers, and the distance from the globe to the wall is also measured from the center.

For problem $13.1.2$ — For problem $$13.1.2$$

Answer

Diameter of the penumbra $d = 60 \ cm$

Diameter of the full shadow $D = 90 \ cm$

Contributed by Adler Last edited Jun 25, 2026 All edits

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13.1: Rectilinear propagation and reflection of light

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