A laser beam can be focussed on an area equal to the square...

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 $1 \mathrm{nW}$ and its wavelength is $632.8 \mathrm{~nm}$. The intensity of focussed beam will be (A) $1.5 \times 10^{13} \mathrm{~W} / \mathrm{m}^{2}$ (B) $2.5 \times 10^{9} \mathrm{~W} / \mathrm{m}^{2}$ (C) $3.5 \times 10^{17} \mathrm{~W} / \mathrm{m}^{2}$ (D) None of these

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 $1 \mathrm{nW}$ and its wavelength is $632.8 \mathrm{~nm}$. The intensity of focussed beam will be
(A) $1.5 \times 10^{13} \mathrm{~W} / \mathrm{m}^{2}$
(B) $2.5 \times 10^{9} \mathrm{~W} / \mathrm{m}^{2}$
(C) $3.5 \times 10^{17} \mathrm{~W} / \mathrm{m}^{2}$
(D) None of these

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Key Concepts

Unit Conversion and Dimensional Consistency

When performing calculations in physics, it is crucial to convert all quantities to consistent units, typically in the International System of Units (SI). Ensuring accurate unit conversion is necessary to guarantee that computed results, such as intensity, are dimensionally correct and meaningful.

Intensity

Intensity is a measure of the power delivered per unit area. It is calculated by dividing the energy per unit time (power) by the area over which the energy is distributed. This concept is fundamental in optics and electromagnetism as it relates to how concentrated the energy is in a beam or wave.

Minimum Focal Spot Size

In high-resolution optical systems, the smallest area to which a beam can be focused is governed by the properties of the light involved, particularly its wavelength. The concept that the area is approximately equal to the square of the wavelength stems from the diffraction limit, which sets a fundamental bound on the focusing capability of optical systems.

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