# HM1 FINAL VERSION

```Homework 1 BIEN210,
Winter 2021
Due date: February 2nd, 2021
1. Thermal radiation. Calculate total energy radiated by humans in a day via
thermal radiation. How does it compare with basal metabolic rate?
As calculated above, the total energy lost by EM radiation by humans in a day is roughly
7 000 kJ per day. The basal metabolic rate depends on each human, but its average is
approximately 80W (for men), which means 6 900 kJ per day.
In this particular case, it doesn’t make any sense because the energy lost by EM
radiation would be more than the basal metabolic rate. However, we can conclude that
the basal metabolic rate is used almost exclusively to conter the energy loss by
radiation. In other words, in order to stay alive and at the good temperature, one must
have a basal metabolic rate at least equivalent to the energy lost by radiation.
2. Lasers. Femtosecond lasers are pulsed lasers with pulse duration between
1 femtosecond and 999 femtoseconds. Describe one application of
femtosecond pulsed lasers in medicine. For this application, please
highlight at least two advantages of femtosecond lasers vs lasers with
longer pulses (such as picosecond or nanosecond lasers) or continuum
wave lasers.
Femtosecond pulsed lasers are widely used in laser medicine. For example, one
application of femtosecond pulsed lasers is the cutting of the corneal flap and the
performing the lens fragmentation during cataract surgery.
As femtosecond lasers pulses have ultra-short period of energy emission in the form of
light, less heat is produced, which results in less residual debris. A continuous lasers
heats up the material too much and damages it, which is not the case with femtosecond
pulsed laser. This is a great advantage in the application of cataract, because it means
the corneal flap will have minimal damage after the cut.
Also, femtosecond lasers pulses are so short in duration that only the tissues that are
meant to be changes (in this case the corneal flap) will change. In other words, there is
no time for adjacent tissue (that we want to remain unchanged) to be affected by the
laser1. This allows for the laser to have great target accuracy, which is essential while
working on the human eye because we don’t want tissues adjacent to the cornea to be
damaged.
Another advantage is that the laser itself can fragment the denser pieces of the
cataract, which means less ultrasound energy is needed to emulsify the residual pieces
of the cataract2. This leads to an easier recovery for the patient of cataract surgery.
1
OcalaEyeFL. “Benefits of the Femtosecond Laser.” YouTube, uploaded by OcalaEyeFL, 5 Aug. 2015,
2
R B, Goyal. “MANAGING DIFFICULT CATARACT CASES BY SMALL INCISION CATARACT SURGERY.” Journal of
Evolution of Medical and Dental Sciences, vol. 3, no. 49, 2014, pp. 11695–97. Crossref,
doi:10.14260/jemds/2014/3536.
3. Bioluminescence. Bioluminescence has found many uses in the biomedical
field. Describe how it works and what are the main advantages over
fluorescence.
Light is created by the return of an exited electron to its ground state. There are many
ways to excite an electron out of its ground state. Chemiluminescence is a case of light
emission where the energy to excite the electrons comes from the potential energy
stored inside the chemical bonds of unstable molecules. In other words, it’s the
emission of light due to chemical reaction. When chemiluminescence happens inside a
living organism (like fireflies), we call that bioluminescence.
In the biomedical field, bioluminescence it used to study biological structure and
processes by taking pictures of them via bioluminescence. It is used in particular to
study processes of gene transcriptions. We can introduce the enzymes and chemicals
(namely luciferin and luciferase) responsible for bioluminescence inside the organism
we want to study, making them bioluminescent even if there are not naturally so.
We can also take some images of minute biological structure with fluorescent
technology (where the light emission is triggered by the energy coming from another
photon), bioluminescence contains a few advantages over fluorescence.
For example, bioluminescence is an energy consuming process, and the reaction
responsible for exciting the photon consumes ATP. Therefore, bioluminescence can be
used for ATP detection in a cell (and therefore to measure cell viability).
Also, bioluminescence doesn’t need an initial light source to stimulate the electrons.
Fluorescent imaging technology does and not all the photons introduced are going to be
converted to fluorescent light. In bioluminescence, as no photons need to be introduced
in the sample, the samples observed are less “noisy” because there is a less bright
background, which result in more precise images. Also, it is possible to have
interference of other weak fluorophores (other than the structures you want to observe)
while using fluorescent imaging technology. For example, collagen is a weak
fluorophore and will cause interference and background noise when studied with
fluorescent technology. For these reasons, bioluminescent imaging technology are
considerably more precise and sensitive, resulting in better imaging.
4. Optics. What are practical applications of the Brewster’s angle? Calculate
the Brewster’s angle at the interface water-glass.
The Brewster’s angle is the particular angle at which all the reflected light lies a in a
single polarized plane. That has a lot of relevant applications in technologies requiring
polarized light. For example, some photographers use the principle of Brewster’s angle
to reduce the glare in a picture due to the reflected light on water. They do so by using a
polarizing filter – that uses the Brewster’s angle – in the lens to reduce the glare due to
reflection.
Below are the calculations of the Brewster’s angle for a water – glass interface.
5. Open-ended question. Design an experiment that uses light to measure
brain function. What wavelengths of light will you use? Why? (Answer this
question after the lecture on Jan. 28th).
In the brain, like in all tissue, there are absorbers as well as scatterers of light. I would
focus on absorbers and use the level of absorption of light to measure the level of brain
activity.
The level of oxygenated blood will be my indicator of the level of brain activity. In order
to measure the level of oxygenated blood, I will measure the level of light loss
(absorption) at the wavelength know to be an absorption peak of oxygenated
hemoglobin (i.e., 0.4um in wavelength). The less photons that come out of the sample,
the more it was absorbed, so the more oxygenated hemoglobin there is in a sample, as
0.4um was exactly the absorption peak of oxygenated hemoglobin – which means it
must have been largely absorbed by oxygenated hemoglobin. I would therefore shine a
light of wavelength 0.4um on one side of the brain tissue, and measure the quantity of
light that was absorbed with a light receptor placed on the opposite side of the light
projector.
0.4 um in wavelength is at the limit of the visible spectrum (just at the junction between
ultraviolet light and visible light). As the frequency of this wave is not that high, it should
not be too dangerous if there is no prolonged exposure, but secondary effect and
potentially harmful consequence of shinning this kind of light on the brain should be
looked into.
Also, as light does not penetrate well into the skull, we can’t measure absorption on
solid and opaque tissues because light won’t go through and we won’t be able to
measure light output on the other side of the tissue. Therefore, this technique should be
used on tissues not yet completely developed so that they are more transparent. In
other words, this technique would only be useful on embryos and fetuses brain tissues.
```