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$\cdot$ Removing birthmarks. Pulsed dye lasers emit light of wavelength 585 $\mathrm{nm}$ in 0.45 $\mathrm{ms}$ pulses to remove skin blemishes such as birthmarks. The beam is usually focused onto a circular spot 5.0 $\mathrm{mm}$ in diameter. Suppose that the output of one such laser is 20.0 $\mathrm{W}$ . (a) What is the energy of each photon, in eV? (b) How many photons per square millimeter are delivered to the blemish during each pulse?

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$1.34 \times 10^{15}$ photons/mm $^{2}$

Physics 102 Electricity and Magnetism

Physics 103

Chapter 28

Photons, Electrons, and Atoms

Electromagnetic Waves

Atomic Physics

Simon Fraser University

University of Sheffield

McMaster University

Lectures

02:42

Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change. The theory of quantum mechanics, a set of mathematical rules that describe the behaviour of matter and its interactions, provides a good model for the description of atomic structure and properties.

03:50

In atomic physics, the Rutherford-Bohr model or Bohr model or Bohr model of the atom was first proposed by Niels Bohr in 1913. It was the first quantum theory of the atomic structure, applying the principles of quantum mechanics to the hydrogen atom. The Rutherford-Bohr model is a planetary model in which the electron is assumed to revolve around the nucleus in circular orbits. The Rutherford-Bohr model of the atom was based on Max Planck's quantum theory of radiation, which explained the spectrum of blackbody radiation, and Albert Einstein's theory of the photoelectric effect, which explained the origin of the spectrum of line emission from atoms. The Bohr model of the atom was the first to explain the discrete spectrum of hydrogen and how the spectral lines of the hydrogen atom were split into multiple spectral lines in a phenomenon called the fine structure. It introduced the Bohr radius and the concept of quantum levels to explain the discrete nature of the spectrum of the hydrogen atom. The Bohr model of the atom had a significant influence on the development of quantum mechanics, and was instrumental in the development and validation of quantum mechanics.

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BIO Removing Birthmarks. P…

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Pulsed dye lasers emit lig…

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01:36

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A pulsed dye laser emits l…

05:44

02:15

photons pulses A laser use…

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Lasers are used extensivel…

05:09

Removing Vascular Lesions.…

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Cp Blo Removing Vascular L…

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$\bullet$ Removing vascula…

10:28

A laser used to weld deta…

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A pulsed laser emits light…

all right in part a were given that these laser pulses have a wavelength of 585 centimeters. We want to know what energy this is associated with. So as usual, he is related to the wavelength through equals H C over Lambda. So if we want to know the energy and E V, we express planks constant in evey dot seconds, which is 4.1 for 10 center the negative 15 multiplied by the speed of light. Yeah, and divide by 585 intermediates. This comes out to 2.1 to evey. And if we want to convert this to Jules, we simply multiply by 1.6 times 10 to the negative 19. And that comes out to three point for times 10 to the negative 19. Jules in part B. We want to know how many photons are released in each. Fools. We know that the power output is 20 watts, so we have 20 Jules per second and we want to multiply by the duration of the pulse, which is your appoint five times 10 to the negative three seconds. So this expression here is now giving us the total energy and each pulse. Yeah, so now we want to divide by on the energy that's in one photon. So that's our answer to part A good and altogether. This comes out to 2.65 times 10 to the 16 Oh, tons. Yeah, in part. See, we want to know how many photons are delivered per square millimeter. The diameter of the patches. Five millimeters. So the area of the patch on Excuse me? The radius of the patch will be 2.5 millimeters and the patches circular so is equal to pi r squared. So we simply divide the number of photons by the area of the patch. And this comes out of 1.35 times 10 to the 15 photons per square millimeter. It the radius here was expressed in millimeters. So we get some square millimeters out

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