# Search

## wave optics

### (7 points)4. Series 37. Year - 4. a perfect passage?

A polarized beam of light coming from a material with refractive index $n_1$ is incident on a planar interface of a material with refractive index $n_2$ such that it does not lose intensity after passing through. It then reaches the parallel interface with refractive index $n_3$, again passing through without any loss, and so on. Find a sequence $n_i$ that satisfies this.

Marek J. met the Brewster's angle

### (10 points)3. Series 37. Year - S. weighted participants

1. According to definitions by International System of Units, convert these into base units
• pressure $1 \mathrm{psi}$,
• energy $1 \mathrm{foot-pound}$,
• force $1 \mathrm{dyn}$.
2. In the diffraction experiment, table salt's grating constant (edge length of the elementary cell) was measured as $563 \mathrm{pm}$. We also know its density as $2{,}16 \mathrm{g\cdot cm^{-3}}$, and that it crystallizes in a face-centered cubic lattice. Determine the value of the atomic mass unit.
3. A thin rod with a length $l$ and a linear density $\lambda$ lies on a cylinder with a radius $R$ perpendicular to its axis of symmetry. A weight with mass $m$ is placed at each end of the rod so that the rod is horizontal. We carefully increase the mass of one of the weights to $M$. What will be the angle between the rod and the horizontal direction? Assume that the rod does not slide off the cylinder.
4. How would you measure the mass of:
• an astronaut on ISS,
• a small asteroid heading towards Earth?

Dodo keeps confucing weight nad mass.

### (12 points)2. Series 37. Year - E. light at the end of a tunel

Measure the illumination intensity of light passing through a cola as a function of the drink's thickness. Determine the absorption coefficient by curve fitting the measured data.

A wasp flew into Jarda's soda can.

### (12 points)5. Series 36. Year - E. disappearing CD

Use diffraction on the grating to determine the density of data written to the CD. Try to compare the results with the DVD.

Káťa still has a lot of CDs at home. Pepa envies her.

### (10 points)5. Series 36. Year - S. ethanol or methanol?

The binding energy of a fluorine molecule is approximately $37 \mathrm{kcal/mol}$. Assuming the range of binding interactions to be approximately $3 \mathrm{\AA }$ from the optimum distance, what (average) force do we have to exert to break the molecule? Calculate the „stiffness“ of the fluorine molecule if such an average force was applied in the middle of this range. What would be the vibrational frequency of this molecule? Compare this with the experimental value of $916{,}6 \mathrm{cm^{-1}}$. ($4 \mathrm{pts}$)

Using Psi4, calculate the dissociation curve $\mathrm {F_2}$ and fit a parabola around the minimum. What value will you get for the energy of the vibrational transitions this time? ($3 \mathrm{pts}$)

You are given two bottles of alcohol that you found suspicious, to say the least. After taking them to the lab, you obtain the following Raman spectra from them. Using the Psi4 program, calculate the frequencies at which the vibrational transitions of both the methanol and ethanol molecules occur. Use this to determine which bottle contains methanol and which one contains ethanol. You can use the approximate geometries of ethanol and methanol, which are included in the problem statement on the web. ($3 \mathrm{pts}$)

Alcohol from Mikuláš?!

### (10 points)3. Series 36. Year - S. quantum of orbital

1. Similarly to the series, use the Hückel method to create the Hamiltonian matrix for the cyclobutadiene molecule and verify that its eigenvalues are $\alpha +2\beta$, $\alpha$, $\alpha$, $\alpha -2\beta$. Sketch the diagram of the final energies in the resulting orbitals. And show how the electrons will occupy them. $(4~b)$
Bonus: What is the main difference in the characterics of these orbitals and their occupancy compared to a benzene molecule we showed in the series? What are the consequences for the cyclobutadiene molecule? $(2~b)$
2. Try going back to the beta-carotene molecule and calculate again at what wavelength it should absorb using the Hückel method. What should the value of the parameter $\beta$ be equal to in order to be consistent with the experimental results
Alternative: If you encounter a problem with the diagonalisation of the hamiltonian, solve the problem statement with the hexa-1,3,5-triene molecule. The experimentally determined absorption value in this case is at a wavelength of $250 \mathrm{nm}$. $(4~b)$
3. What happens to a molecule (a molecule with only simple bonds is sufficient) if we use UV light to excite an electron from the $\sigma$ to the $\sigma ^\ast$ orbital? $(2~b)$

Mikuláš gives presents again, this time at the right time of the year, almost.

### (10 points)2. Series 36. Year - S. counting the quanta

1. Find a beta-carotene molecule and calculate what color should it have or rather what wavelength it absorbs. Use a simple model of an infinite potential well in which $\pi$ electrons from double bonds are „trapped“ (i.e., two electrons for each double bond). The absorption then corresponds to such a transition that an electron jumps from the highest occupied level to the first unoccupied level.
Compare the calculated value with the experimental one. Why doesn't the value obtained by our model come out the way we would expect? (5b)
2. Let's try to improve our model. When studying some substances, especially metals or semiconductors, we introduce the effective mass of the electron. Instead of describing the environment in which the electrons move in a complex way, we pretend that the electrons are lighter or heavier than in reality. What mass would they need to have to give us the correct experimental value? Give the result in multiples of the electron's mass. (2b)
3. If we produce microscopic spheres (nanoparticles) of cadmium selenide ($\ce {CdSe}$) with a size of $2{,}34 \mathrm{nm}$, they will glow bright green when irradiated by UV light with a wavelength of $536 \mathrm{nm}$. When enlarged to a size of $2{,}52 \mathrm{nm}$, the wavelength of the emitted light shifts to the yellow region with a wavelength of $570 \mathrm{nm}$. What would the size of spheres need to be to make them emit orange with a wavelength of $590 \mathrm{nm}$? (3b)

Hint: $\ce {CdSe}$ is a semiconductor, so it has a fully occupied electron band, then a (narrow!) forbidden band, and finally an empty conduction band. Thus, we must consider that the emitted photon corresponds to a jump from the conduction band (where such states are as in the infinite potential well) to the occupied band. Therefore, all the energies of the emitted photons will be shifted by an unknown constant value corresponding to the width of the forbidden band.

Finally, a bonus for those who would be disappointed if they didn't integrate – the 1s orbital of the hydrogen atom has a spherically symmetric wave function with radial progression $\psi (r) = \frac {e^{-r/a_0}}{\sqrt {\pi }a_0^{3/2}}$, where $a_0=\frac {4\pi \epsilon _0\hbar ^2}{me^2}$ is the Bohr radius. Since the orbitals as functions of three spatial variables would be hard to plot, we prefer to show the region where the electron is most likely to occur. What is the radius of the sphere centered on the nucleus in which the electron will occur with a probability of $95 \mathrm{\%}$? (+2b)

An early Mikuláš's package.

### (10 points)6. Series 35. Year - S. lasering

1. How big must an aperture in a spatial filter be if we created it from a lens with a diameter of $40 \mathrm{cm}$ and its focal length is $4 \mathrm{m}$? Our Gaussian laser beam has an input diameter $30 \mathrm{cm}$ and a wavelength $1~053 nm$. The radius of the focus (parameter $\sigma$) of the Gaussian beam can be obtained using

$\begin{equation*} r = \frac {2}{\pi }\lambda \frac {f}{D} \end {equation*}$ where $D$ is the diameter of the beam, $f$ is the focal length of the lens and $\lambda$ is the wavelength of the laser.

1. The laser beam is focused on a surface of a nuclear fuel pellet of a $1 \mathrm{mm}$ diameter. What energy should it have in order for the intensity in its focus to reach $10^{14} W.cm^{-2}$? The radius of the focus is $25 \mathrm{\micro m}$ and a pulse lasts $10 \mathrm{ns}$. How many beams do we need to equally cover the surface of a pellet? What is their total energy?
What energy must the laser beam have if it is not focused on a surface of a nuclear fuel pellet, but the beam diameter matches exactly the diameter of the pellet and the density is its focus reaches $10^{14} W.cm^{-2}$? Assume that we have one such beam and it shines homogenously on the pellet „from all directions“.

### (10 points)5. Series 35. Year - S. stabilizing

1. What intensity must a laser with a wavelength of $351 \mathrm{nm}$ have in order to stabilize a Rayleigh-Taylor (RT) instability using the surface ablation of a fuel pellet? Suppose the boundary between the ablator and DT ice is corrugated with a wavelength of
1. $0,2 \mathrm{\micro m}$,
2. $5 \mathrm{\micro m}$.
1. How will the intensity of the laser change if we also apply a magnetic field with magnitude $5 \mathrm{T}$?
2. What else can help us minimize the RT instability?

### (10 points)4. Series 35. Year - S. shining

1. How far from the surface of the target (suppose it is made of carbon and the laser has wavelength of $351 \mathrm{nm}$) is critical surface situated and how far does two-plasmon decay occur, if the characteristic length of plasma1)
1)
The density of plasma $n_e$ is typically expressed as a funciton $n_e = f$\frac {x}{x_c}$$, where $x$ is the distance from the target and $x_c$ is so called characteristic length of plasma, which represents scale parameter for the distance from the target.))is~$50 \mathrm{\micro m}$? Next assume
1. that the density of the plasma decreases exponentially with distance from the target,
2. that the density of the plasma decreases linearly with distance from the target.
1. What energy must electorns have in order to go through the critical surface to the real surface of the target? To calculate the distance electron travels in carbon plasma use an empirical relationship $R = 0{,}933~4 E^{1{,}756~7}$, where $E$ has units of \jd {MeV} and $R$ has units of \jd {g.cm^{-2}}.
2. What is the distance that an electron has to travel in the electric field of the plasma wave in order to reach the energies determined in second exercise?
3. Which wavelengths of scattered light are present in the case of stimulated Raman scaterring for laser with wavelength of $351 \mathrm{nm}$?

# Organizers and partners

Organizer

Organizer

General Partner

Main Partner

Partner

Media Partner

Created with <love/> by ©FYKOS – webmaster@fykos.cz