PRK Surgery. Photorefractive keratectomy (PRK) is a...

PRK Surgery. Photorefractive keratectomy (PRK) is a laser-based surgical procedure that corrects near- and farsightedness by removing part of the lens of the eye to change to change its curvature and hence focal length. This procedure can remove layers 0.25$\mu \mathrm{m}$ thick using pulses lasting 12.0 $\mathrm{ns}$ from a laser beam of wavelength 193 nm. Low-intensity beams can be used because each individual photon has enough energy to break the covalent bonds of the tissue. (a) In what part of the electromagnetic spectrum does this light lie? (b) What is the energy of a single photon? (c) If a 1.50 -mW beam is used, how many photons are delivered to the lens in each pulse?

PRK Surgery. Photorefractive keratectomy (PRK) is a laser-based surgical procedure that corrects near- and farsightedness by removing part of the lens of the eye to change to change its curvature and hence focal length. This procedure can remove layers 0.25$\mu \mathrm{m}$ thick using pulses lasting 12.0 $\mathrm{ns}$ from a laser beam of wavelength 193 nm. Low-intensity beams can be used because
each individual photon has enough energy to break the covalent bonds of the tissue. (a) In what part of the electromagnetic spectrum does this light lie? (b) What is the energy of a single photon? (c) If a 1.50 -mW beam is used, how many photons are delivered to the lens in each pulse?

Key Concepts

Electromagnetic Spectrum

The electromagnetic spectrum spans all types of electromagnetic radiation arranged by wavelength or frequency. Key sections include radio waves, microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays. Understanding where a particular wavelength falls within the spectrum, such as determining that a wavelength of 193 nm lies in the ultraviolet region, is essential in many applications including laser surgery and spectroscopy.

Photon Energy Calculation

Photon energy is determined by the equation E = hc/?, where h is Planck’s constant, c is the speed of light, and ? is the wavelength. This relation highlights the quantum nature of light, showing that the energy of a photon is inversely proportional to its wavelength. This concept explains why even low-intensity beams can induce chemical changes if each individual photon carries sufficient energy to break specific molecular bonds.

Laser Beam Power and Pulse Duration Relation

A laser’s power, which is the energy delivered per unit time, when combined with pulse duration, allows calculation of the total energy delivered in a pulse. By dividing the total energy of the pulse by the energy per photon, one can determine the number of photons in each pulse. This principle is crucial in applications where precise energy delivery is required, such as in laser-based surgical procedures.

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