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## wave optics

### (8 points)1. Series 34. Year - 4. solar sail

A solar sail with the surface area of $S = 500 \mathrm{m^2}$ and area density $\sigma =1,4 \mathrm{kg\cdot m^{-2}}$ is located at the distance of $0,8 \mathrm{au}$ from the Sun. What force does the solar radiation act on the sail at the beginning of the sail's motion? What is the acceleration of the sail at that moment? The luminosity of the Sun is $L_{\odot } =3,826 \cdot 10^{26} \mathrm{W}$. Assume that the radiation approaches the sail from a perpendicular direction and scatters elastically. **Hint:** We recommend you find the acceleration for small initial velocity $v_0$ and then let $v_0 = 0$.

### (9 points)5. Series 33. Year - 5. optically relativistic

Calculate the phase shift $\Delta \Phi $ when an optical beam with a wavelength $\lambda _0$ goes through a glass plate with thickness $h$ and the index of refraction $n$ that is moving along the beam with constant speed $v$ relative to a case when the plate is stationary. We are interested mainly about the first nonzero term of Taylor series of $\Delta \Phi (v)$.

Dodo at optic lab.

### (6 points)1. Series 33. Year - 3. infra sauna

Dano continues with equiping of his mansion with another sauna—this time an infra sauna. He wants to place a tube lamp right underneath the ceiling of the sauna which is $H=2,5 \mathrm{m}$ above the ground. Suppose the source of radiation emits energy with the power per unit length of $p = 1,2 \mathrm{kW\cdot m^{-1}}$, a radiation of what intensity and total energy would reach the skin of a person situated approximately $h=50 \mathrm{cm}$ above ground? The lamp is a straight tube, shines in a homogeneous manner and reaches from wall to wall just under the middle of the ceiling.

**Hint:** For simplicity, approximate the sauna to be a room where the sides touching the lamp and the ceiling are mirrors and the other two sides and the floor absorb the light without remitting it back into the room.

Karel visited wellness in Slovakia.

### (9 points)1. Series 33. Year - 5. generally relativistic

Before he set off on his flight towards Mars, the Starman in his Tesla Roadster arranged with Musk that once he reaches the distance $r=5 \cdot 10^{6} \mathrm{km}$ from the centre of mass of the Earth, Musk will shine a powerful green laser at him. The wavelength of the laser increases under the influence of the gravitational field of Earth. Compare this change of the wavelength to the electromagnetic Doppler effect. Study each of these effects separately. Assume that the Starman is moving away from Earth with velocity $v=4 \mathrm{km\cdot s^{-1}}$.

Vašek wants to go on a trip with Starman.

### (6 points)3. Series 32. Year - 3. heat in the Dyson sphere

What would be the diameter of a Dyson sphere that would surround a star with the luminosity of the Sun, so the temperature on the outer surface of the sphere is $t= 25 \mathrm{\C }$?. Don't consider the presence of the atmosphere in the Dyson sphere. A Dyson sphere should be a relatively thin concave structure of spherical shape surrounding the star.

Karl likes Dyson spheres.

### (12 points)5. Series 31. Year - E.

*We are sorry. This type of task is not translated to English.*

### (3 points)3. Series 30. Year - 1. long film

You are downloading your favourite film with file size 12 GB at 10 MB ⁄ s. Assuming the signal travels along a twisted-pair wire at the speed of light and modulation spreads the transmission speed evenly, that is at 1 b ⁄ s we would have to receive 1 second of the signal to acquire 1 bit of information, determine the length of cable filled by the film's data if it travels along a sufficiently long cable.

Michal's colleague claimed that 100Gb Ethernet frames are smaller than a chip.

### (7 points)3. Series 30. Year - 4. free radar

A red marker is placed on every bollard along a road (wavelength of the red colour used is $λ_{r}=630nm)$. When the driver of a passing car sees the marker as blue (wavelength $λ_{b}=450nm)$, she knows that she is speeding. What is the car's speed when this happens? What is the momentum and kinetic energy of a typical passenger car at this speed?

Kuba found a futuristic-looking photo on the internet.

### (5 points)2. Series 29. Year - 5. round it up

Mirek felt that during winter it is a little bit too dark for reading in his room. So he decided to make a hole in his wall for another window. He went to glass-works first to buy the glass panes. There was one nice round piece, but before he would buy it, he needed to check whether it's not too uneven (specifically convex). He placed the pane on the glass desk of the glass-works and saw rainbow circles around the centre of the pane caused by interference of the perpendicularly coming white light in the thin space between the two glasses. Mirek randomly chose two red circles ($λ≈700nm)$ and measured with a ruler their diameters $d_{k}=(10,5±0,5)\;\mathrm{mm}$ and $d_{k+1}=(13,0±0,5)\;\mathrm{mm}$. Based on these measurements he managed to determine the radius of curvature of the pane. Calculate it as well and think about it's errors.

Mirek si nechce zkazit oči.

### (8 points)6. Series 27. Year - E. gelatinous speed of light

Determine the speed of light in a translucent gelatinous cake that you will make yourself. Don't forget to describe what its composed of.

**Hint:** Get yourself a microwave or a laser

Karel was going through different physical websites on the internet and found http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p009.shtml