Propagation of Waves

Q. Pure $Si$ at $500\text{K}$ has equal number of electron ($n_e$) and hole ($n_h$) concentrations of $1.5\times {10}^{16}{\text{m}}^{-3}$. Doping by indium increases nh to $4.5\times {10}^{22}{\text{m}}^{-3}$. The doped semiconductor is of :

1. $n$-type with electron concentration $n_e=2.5\times {10}^{23}{\text{m}}^{-3}$
2. $p$-type with electron concentration $n_e=5\times {10}^9{\text{m}}^{-3}$
3. $n$-type with electron concentration $n_e=5\times {10}^{22}{\text{m}}^{-3}$
4. $p$-type with electron concentration $n_e=2.5\times {10}^{10}{\text{m}}^{-3}$

Q. Figure shown plane waves refracted from air to water using Huygen's principle $a,b,c,d,e$ are lengths on the diagram. The refractive index of water with respect to air is equal to

1. $\frac{a}{e}$
2. $\frac{b}{e}$
3. $\frac{b}{d}$
4. $\frac{d}{b}$

Q. The equation alpha=(D-d)/(n-1)d is correctly matched for Where D=Theoretical vapour density d= Observed vapour density 1) A=> nB/2 + nC/3 2) A=> nB/3 + (2n/3)C 3) A=>(n/2)B + (n/4)C 4) A=> (n/2)B + C

Q. In a line of sight radio communication, a distance of about $50\text{km}$ is kept between the transmitting and receiving antennas. If the height of the receiving antenna is $70\text{m}$, then the minimum height of the transmitting antenna should be:
(Radius of the Earth $=6.4\times {10}^6\text{m}$).

1. $40\text{m}$
2. $32\text{m}$
3. $20\text{m}$
4. $51\text{m}$

Q.

In an optical communication system operating at $1200mm$, only $2\%$ the source frequency is available for TV transmission having a bandwidth of $5MHz$. The number of TV channels that can be transmitted is

1. $2million$

2. $10million$

3. $0.1million$

4. $1million$

5. $0.5million$

Q. A transmitting antenna at the top of a tower has a height 32 m and the height of the receiving antenna is 50 m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth 6.4 $\times {10}^6$m.

1. 40 km
2. 45.5 km
3. 44 km
4. 48.5 km

Q. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion : Television signals are received through sky-wave propagation.
Reason : The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

1. If assertion is true but reason is false.
2. If both assertion and reason are true but reason is not the correct explanation of the assertion.
3. If both assertion and reason are true and the reason is the correct explanation of the assertion.
4. If the assertion and reason both are false.
5. If assertion is false but reason is true.

Q. The critical angle of a medium for a specific wavelength if the medium has relative permittivity $3$ and relative permeability $\frac{4}{3}$ for this wavelength, will be:

1. ${15}^{\circ }$
2. ${30}^{\circ }$
3. ${45}^{\circ }$
4. ${60}^{\circ }$

Q. Frequencies in the UHF range normally propagate by means of: (a) Ground waves. (b) Sky waves. (c) Surface waves .(d) Space waves.

Q.

In which frequency range, space waves are normally propagated

1. SHF

2. HF

3. UHF

4. VHF

Q. The T.V. transmission tower in Delhi has a height of $240\text{m}$. The distance up to which that the broadcast can be received, (taking the radius of earth to be $6.4\times {10}^6\text{m}$) is :-

1. $100\text{km}$
2. $55\text{km}$
3. $60\text{km}$
4. $50\text{km}$

Q. A laser beam transfers an energy of $5\text{J}$ across an area of $1\times {10}^{-2}{\text{m}}^2$ in a time of ${10}^{-9}\text{s}$. What is the intensity of the beam?

Q.

A transmitting station releases waves of wavelength $960m$. A capacitor of $256\mu F$ is used in the resonant circuit. The self inductance of coil necessary for resonance is _____ $\times {10}^{-8}H$.

Q. A ground receiver station is receiving a signal at (i) 5 MHz and transmitted from a ground transmitter at a height of 300 m, located at a distance of 100 km from the receiver station. The signal is coming via - (Radius of earth $=6.4\times {10}^{6}m$. $N_{max}$ of isosphere $={10}^{12}{m}^3$)

1. Space wave
2. Sky wave propagation
3. Satellite transponder
4. All of these

Q. Two light waves having the same wavelength $\left(\lambda \right)$ in vacuum are in phase initially. Then, the first ray travels path length $L_1$ through a medium of refractive index ${\mu }_1$. The second ray travels a path of length $L_2$ through a medium of refractive index ${\mu }_2$. The two waves are then combined to observe interference effects. The phase difference between the two, when they interfere-

1. $\frac{2\pi }{\lambda }(L_1-L_2)$
2. $\frac{2\pi }{\lambda }({\mu }_1{L}_1-{\mu }_2{L}_2)$
3. $\frac{2\pi }{\lambda }({\mu }_2{L}_1-{\mu }_1{L}_2)$
4. $\frac{2\pi }{\lambda }(\frac{L_1}{{\mu }_1}-\frac{L_2}{{\mu }_2})$

Q. is the loss of strength of a signal while propagating through a medium.

1. Amplification
2. Modulation
3. Attenuation

Q. Young's double-slit experiment is conducted in water $\left({\mu }_1\right)$ as shown in figure, and a glass plate of thickness $t$ and refractive index ${\mu }_2$ is placed in the path of $S_2$. Find the magnitude of the phase difference at $O$. [Assume that $\lambda$ is the wavelength of light in air]

1. $\left|(\frac{{\mu }_2}{{\mu }_1}-1)t\right|\frac{2\pi }{\lambda }$
2. $|({\mu }_2-{\mu }_1)t|\frac{2\pi }{\lambda }$
3. $\left|(\frac{{\mu }_1}{{\mu }_2}-1)t\right|\frac{2\pi }{\lambda }$
4. $|({\mu }_2-1)t|\frac{2\pi }{\lambda }{\mu }_1$

Q.

The inner part of the fibre optics which carries light is called________.

Q. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion :The electromagnetic waves of shorter wavelength can travel longer distances on earth's surface than those of longer wavelengths.
Reason : Shorter the wavelength, the larger is the velocity of wave propagation.

1. If both assertion and reason are true but reason is not the correct explanation of the assertion.
2. If assertion is true but reason is false.
3. If both assertion and reason are true and the reason is the correct explanation of the assertion.
4. If the assertion and reason both are false.

Q. The electromagnetic waves of frequency 2 MHz to 30 MHz are

1. In sky wave propagation
2. In microwave propagation
3. In ground wave propagation
4. In satellite communication

Q. Consider telecommunication through optical fibres. Which of the following statements is not true.

1. Optical fibres may have homogeneous core with a suitable cladding.
2. Optical fibres are subject to electromagnetic interference from outside.
3. Optical fibres can be of graded refractive index
4. Optical fibres have extremely low transmission loss

Q. A photodetector is made from a semiconductor In ${}_{0.53}Ga$${}_{0.47}As$ with $E_g$ = 0.73 eV. What is the maximum wavelength, which it can detect

1. 1000 nm
2. 1703 nm
3. 500 nm
4. 173 nm

Q. Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?(a) 10 kHz(b) 10 MHz(c) 1 GHz(d) 1000 GHz

Q. List any three applications of laser.

Q. For sky wave propagation of a 10 MHz signal, what should be the minimum electron density in ionosphere

1. ~ 1.2 ´ 10¹² m^–3
2. ~ 10⁶ m^–3
3. ~ 10¹⁴ m^–3
4. ~ 10²² m^–3

Q. Long distance short-wave radio broadcasting uses

1. Ionospheric wave
2. Ground wave
3. Direct wave
4. Sky wave

Q. A wave has velocity u in medium P and velocity 2u in medium Q. If the wave is incident in medium P at an angle of ${30}^{\circ }$ then the angle of refraction will be

1. 30°
2. 60°
3. 90°
4. 45°

Q.

Antenna is

1. inductive

2. capacitive

3. resistive above its resonant frequency

4. resistive at resonant frequency

Q. Two waves of light in air have the same wavelength and are initially in the same phase. They then travel through plastic layers with thicknesses of $L_1=3.5\text{mm}$ and $L_2=5.0\text{mm}$ and indices of refraction $n_1=1.7$ and $n_2=1.25$ as shown in the figure. The rays later arrive at a common point. The longest wavelength of light for which constructive interference occurs at the point is

1. $0.8\text{mm}$
2. $1.2\text{mm}$
3. $1.7\text{mm}$
4. $2.9\text{mm}$

Q. Mean optical power launched into an 8 km fiber is 120 $\mu$W and mean output power is 4 $\mu$W, then the overall attenuation is (Given log 30 = 1.477)

1. 14.77 dB
2. 16.77 dB
3. 3.01 dB
4. None of these