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In a Young's double-slit experiment the slit separation is 0.320 mm and the wavelength of the incident light is lambda = 500 nm. How many bright fringes (maxima) are observed in the angular range -30 deg <= theta <= 30 deg?

1. 641
2. 320
3. 321
4. 640

Correct answer: 641

Solution

Condition for bright fringe: d sin(theta) = m lambda. Largest m: m_max = d*sin(30)/lambda = (0.320e-3 * 0.5)/(500e-9) = 1.6e-4/5e-7 = 320. Orders range from -320 to +320 inclusive, giving 2*320 + 1 = 641 bright fringes.

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