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A laser beam can be focused on an area equal to the square of its wavelength. A He-Ne laser radiates energy at the rate of 1mW and its wavelength is 632.8 nm. The intensity of focused beam will be

a
1.5×1013 W​/​m2
b
2.5×109 W​/​m2
c
3.5×1017 W​/​m2
d
None of these

detailed solution

Correct option is B

Area through which the energy of beam passes= (6.328×10−7)2=4×10−13m2 ∴I=PA=10−34×10−13=2.5×109 W/m2

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