Question

A laser beam can be focussed 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 $632.8nm$. The intensity of focussed beam will be:

• A. $1.5\times {10}^{12}W/m^2$
• B. $2.5\times {10}^{9}W/m^2$
• C. $3.5\times {10}^{17}W/m^2$
• D. $4.5\times {10}^{15}W/m^2$

Answer 1

Answer: (B)

$2.5\times {10}^{9}W/m^2$

Focussed area, $A={\lambda }^2$

Given: $\lambda =632.8nm$

Intensity of focussed beam, $I=\frac{1mW}{{\lambda }^2}=\frac{{10}^{-3}}{(632.8\times {10}^{-9}{)}^2}$

$⟹I=2.5\times {10}^{9}W/m^2$