# 17.3 The electromagnetic spectrum  (Page 8/33)

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Living things—plants and animals—have evolved to utilize and respond to parts of the electromagnetic spectrum they are embedded in. Visible light is the most predominant and we enjoy the beauty of nature through visible light. Plants are more selective. Photosynthesis makes use of parts of the visible spectrum to make sugars.

## Integrated concept problem: correcting vision with lasers

During laser vision correction, a brief burst of 193-nm ultraviolet light is projected onto the cornea of a patient. It makes a spot 0.80 mm in diameter and evaporates a layer of cornea $0\text{.}\text{30}\phantom{\rule{0.25em}{0ex}}\mu \text{m}$ thick. Calculate the energy absorbed, assuming the corneal tissue has the same properties as water; it is initially at $\text{34º}\text{C}$ . Assume the evaporated tissue leaves at a temperature of $\text{100º}\text{C}$ .

Strategy

The energy from the laser light goes toward raising the temperature of the tissue and also toward evaporating it. Thus we have two amounts of heat to add together. Also, we need to find the mass of corneal tissue involved.

Solution

To figure out the heat required to raise the temperature of the tissue to $\text{100º}\text{C}$ , we can apply concepts of thermal energy. We know that

$\text{Q}=\text{mc}\Delta T,$

where Q is the heat required to raise the temperature, $\Delta T$ is the desired change in temperature, $m$ is the mass of tissue to be heated, and $c$ is the specific heat of water equal to 4186 J/kg/K.

Without knowing the mass $m$ at this point, we have

$Q=m\left(\text{4186 J/kg/K}\right)\left(\text{100º}\text{C}–\text{34º}\text{C}\right)=m\left(\text{276,276 J/kg}\right)=m\left(\text{276 kJ/kg}\right).$

The latent heat of vaporization of water is 2256 kJ/kg, so that the energy needed to evaporate mass $m$ is

${Q}_{\text{v}}={\mathrm{mL}}_{\text{v}}=m\left(\text{2256 kJ/kg}\right).$

To find the mass $m$ , we use the equation $\rho =m/\text{V}$ , where $\rho$ is the density of the tissue and $\text{V}$ is its volume. For this case,

$\begin{array}{lll}m& =& \rho \text{V}\\ & =& {\text{(1000 kg/m}}^{3}\right)\left(\text{area}×\text{thickness}{\text{(m}}^{3}\text{))}\\ & =& \text{(1000 kg/}{\text{m}}^{3}\right)\left(\pi \left(0.80×{\text{10}}^{–3}\phantom{\rule{0.25em}{0ex}}\text{m}{\right)}^{2}/4\right)\left(0\text{.}\text{30}×{\text{10}}^{–6}\phantom{\rule{0.25em}{0ex}}\text{m}\right)\\ & =& 0.151×{\text{10}}^{–9}\phantom{\rule{0.25em}{0ex}}\text{kg.}\end{array}$

Therefore, the total energy absorbed by the tissue in the eye is the sum of $\text{Q}$ and ${\text{Q}}_{\text{v}}$ :

${\text{Q}}_{\text{tot}}=m\left(c\Delta \text{T}+{\text{L}}_{\text{v}}\right)=\left(0.151×{\text{10}}^{-9}\phantom{\rule{0.25em}{0ex}}\text{kg}\right)\left(\text{276 kJ/kg}+\text{2256 kJ/kg}\right)=\text{382}×{\text{10}}^{-9}\phantom{\rule{0.25em}{0ex}}\text{kJ}.$

Discussion

The lasers used for this eye surgery are excimer lasers, whose light is well absorbed by biological tissue. They evaporate rather than burn the tissue, and can be used for precision work. Most lasers used for this type of eye surgery have an average power rating of about one watt. For our example, if we assume that each laser burst from this pulsed laser lasts for 10 ns, and there are 400 bursts per second, then the average power is ${\text{Q}}_{\text{tot}}×\text{400}=\text{150 mW}$ .

Optics is the study of the behavior of visible light and other forms of electromagnetic waves. Optics falls into two distinct categories. When electromagnetic radiation, such as visible light, interacts with objects that are large compared with its wavelength, its motion can be represented by straight lines like rays. Ray optics is the study of such situations and includes lenses and mirrors.

When electromagnetic radiation interacts with objects about the same size as the wavelength or smaller, its wave nature becomes apparent. For example, observable detail is limited by the wavelength, and so visible light can never detect individual atoms, because they are so much smaller than its wavelength. Physical or wave optics is the study of such situations and includes all wave characteristics.

what is variations in raman spectra for nanomaterials
I only see partial conversation and what's the question here!
what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
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Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
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LITNING
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what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how did you get the value of 2000N.What calculations are needed to arrive at it
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How we can toraidal magnetic field
How we can create polaidal magnetic field
4
Because I'm writing a report and I would like to be really precise for the references
where did you find the research and the first image (ECG and Blood pressure synchronized)? Thank you!!