# 8.3 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.

#### Questions & Answers

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 to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
how did you get the value of 2000N.What calculations are needed to arrive at it
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