Q.

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 5mW and its wavelength  5000A0.The intensity of focused beam will be

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a

2×109W/m2

b

2×108W/m2

c

2×1010W/m2

d

2×1011W/m2

answer is C.

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Detailed Solution

A=λ2 Here A= area    λ=wavelength of laser beam  ⇒A=λ2=5×10−72=25×10−14m2 Now, I=PA here  P=power    I= Intensity of focussed beam ⇒I=PA=5×10−325×10−14=15×10+14×10−3=0.2×1011=2×1010Wm2
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