The gnaphosid spider Drassodes cupreus has evolved a pair of lensless eyes for d

Question

The gnaphosid spider Drassodes cupreus has evolved a pair of lensless eyes for detecting polarized light. Each eye is sensitive to polarized light in perpendicular directions. Near sunset, the spider leaves its nest in search of prey. Light from overhead is linearly polarized and indicates the direction the spider is moving. After the hunt, the spider uses the polarized light to return to its nest. Suppose that the spider orients its head so that one of these two eyes detects light of intensity 750 W/m^2 and the other eye detects zero light intensity. What intensity does the first eye (intensity 750 W/m^2) detect if the spider now rotates its head 20 degree from the previous orientation? Express your answer to two significant figures and include the appropriate units. What intensity does the first eye (zero intensity) detect if the spider now rotates its head 20 degree from the previous orientation? Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

(A) now theta = 20 deg

I = Io (cos(theta))^2

I = 750 (cos20)^2 = 662.27 W/m^2

(b) for this eye, theta = 90 - 20 = 70 deg

I = 750 (cos70)^2

I = 87.7 W/m^2

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