A.Light Alteration by Stimulated Emission of Radiation
B.Light Amplification by Stimulated Emission of Radiation
C.Light Amplification by Spontaneous Emission of Radiation
D.Light Absorption by Stimulated Emission of Radiation
Correct Answer: Light Amplification by Stimulated Emission of Radiation
Explanation:
LASER stands for Light Amplification by Stimulated Emission of Radiation. It describes the physical process responsible for generating the light.
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2In the context of laser physics, what is population inversion?
A.A state where the number of atoms in the ground state is greater than in the excited state ()
B.A state where all atoms are ionized
C.A state where the number of atoms in the excited state is greater than in the lower energy state ()
D.A state where the number of atoms in both states is equal ()
Correct Answer: A state where the number of atoms in the excited state is greater than in the lower energy state ()
Explanation:
Population inversion is a non-equilibrium state where more atoms exist in a higher energy state () than in a lower energy state (). This is a necessary condition for stimulated emission to dominate over absorption.
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3Which of the following processes ensures that the emitted photons are in phase, have the same polarization, and travel in the same direction as the incident photon?
A.Scattering
B.Stimulated Emission
C.Absorption
D.Spontaneous Emission
Correct Answer: Stimulated Emission
Explanation:
In stimulated emission, an incident photon triggers an excited atom to drop to a lower level, emitting a second photon that is coherent (same phase, frequency, direction, and polarization) with the incident photon.
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4The average lifetime of an atom in a metastable state is approximately:
A. seconds
B. seconds
C. nanoseconds
D. seconds
Correct Answer: seconds
Explanation:
Metastable states have a relatively long lifetime ( s) compared to ordinary excited states ( s), allowing population inversion to be achieved.
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5According to Einstein's theory of radiation, the probability of stimulated absorption () is equal to the probability of:
A.Stimulated emission ()
B.Zero
C.Non-radiative transition
D.Spontaneous emission ()
Correct Answer: Stimulated emission ()
Explanation:
Einstein showed that the coefficient for stimulated absorption equals the coefficient for stimulated emission, i.e., .
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6The ratio of the rate of spontaneous emission to the rate of stimulated emission is proportional to:
A.
B.
C.
D.
Correct Answer:
Explanation:
The ratio of Einstein's coefficients is given by . Thus, the ratio is proportional to the cube of the frequency .
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7Which component of a laser system is responsible for providing the necessary positive feedback to sustain oscillations?
A.Active medium
B.Cooling system
C.Optical resonator (Cavity)
D.Pumping source
Correct Answer: Optical resonator (Cavity)
Explanation:
The optical resonator, consisting of a pair of mirrors (one fully reflecting, one partially reflecting), reflects photons back and forth through the active medium to amplify the light and sustain lasing action.
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8In a Ruby laser, the active medium is:
A.Aluminum Oxide doped with Chromium ions ()
B.Aluminum Oxide doped with Neodymium ions ()
C.Ruby crystal doped with Helium
D.Pure Aluminum Oxide ()
Correct Answer: Aluminum Oxide doped with Chromium ions ()
Explanation:
Ruby is a crystal of Aluminum Oxide () where some Aluminum ions are replaced by Chromium ions (). The ions are the active centers responsible for lasing.
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9What type of pumping method is used in a Ruby laser?
A.Direct conversion
B.Chemical pumping
C.Electrical discharge
D.Optical pumping
Correct Answer: Optical pumping
Explanation:
Ruby lasers typically use a Xenon flash lamp to provide light energy (photons) to excite the atoms. This process is called optical pumping.
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10The Ruby laser is a three-level laser system. What is a major drawback of three-level systems?
A.More than 50% of ground state atoms must be pumped to the excited state to achieve population inversion.
B.They cannot produce visible light.
C.They do not require a resonant cavity.
D.The output is always continuous wave.
Correct Answer: More than 50% of ground state atoms must be pumped to the excited state to achieve population inversion.
Explanation:
In a three-level system, the lower lasing level is the ground state. Therefore, more than half of the total atoms must be pumped to the upper level to achieve , requiring high pump power.
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11The wavelength of the light emitted by a Ruby laser is approximately:
A.632.8 nm
B.550 nm
C.1064 nm
D.694.3 nm
Correct Answer: 694.3 nm
Explanation:
The Ruby laser emits red light at a wavelength of 694.3 nm (6943 ).
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12Which of the following is a characteristic of the output of a standard Ruby laser?
A.Incoherent output
B.Polychromatic output
C.Pulsed output
D.Continuous Wave (CW)
Correct Answer: Pulsed output
Explanation:
Standard Ruby lasers operate in a pulsed mode because the xenon flash lamp operates in pulses, and the high energy required for the 3-level inversion is difficult to maintain continuously.
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13In the Nd:YAG laser, the acronym YAG stands for:
A.Yellow Aluminum Garnet
B.Yttrium Aluminum Gallium
C.Yttrium Aluminum Garnet
D.Ytterbium Aluminum Garnet
Correct Answer: Yttrium Aluminum Garnet
Explanation:
YAG stands for Yttrium Aluminum Garnet (), which acts as the host crystal for Neodymium ions.
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14The Nd:YAG laser is an example of a ____ laser system.
A.Two-level
B.Three-level
C.Four-level
D.Five-level
Correct Answer: Four-level
Explanation:
Nd:YAG is a four-level laser. The lower lasing level is above the ground state, making it much easier to achieve population inversion compared to a three-level system.
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15The primary emission wavelength of an Nd:YAG laser is in which region of the spectrum?
A.Infrared (IR)
B.Visible (Red)
C.Visible (Green)
D.Ultraviolet (UV)
Correct Answer: Infrared (IR)
Explanation:
The standard emission wavelength of Nd:YAG is 1.064 m (1064 nm), which falls in the Near-Infrared region.
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16In a He-Ne laser, what is the function of the Helium atoms?
A.They act as the active centers for lasing.
B.They cool the discharge tube.
C.They absorb the laser light.
D.They transfer energy to Neon atoms via resonant collisions.
Correct Answer: They transfer energy to Neon atoms via resonant collisions.
Explanation:
Helium atoms are excited by electron impact and then transfer their energy to Neon atoms through collisions because the metastable states of He match the excited energy levels of Ne very closely.
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17The typical ratio of Helium to Neon in a He-Ne laser mixture is approximately:
A.100:1
B.10:1
C.1:1
D.1:10
Correct Answer: 10:1
Explanation:
The mixture usually contains about 10 parts Helium to 1 part Neon. The high concentration of Helium ensures efficient pumping of the Neon atoms.
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18What type of pumping is utilized in a He-Ne laser?
A.Injection current
B.Electrical discharge
C.Optical pumping
D.Chemical reaction
Correct Answer: Electrical discharge
Explanation:
He-Ne lasers use a high-voltage electrical discharge to accelerate electrons, which collide with Helium atoms to excite them.
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19The output of a He-Ne laser is typically:
A.Random spikes
B.Continuous Wave (CW)
C.Pulsed high energy
D.Unpolarized
Correct Answer: Continuous Wave (CW)
Explanation:
He-Ne lasers provide a highly stable, Continuous Wave (CW) output, making them ideal for alignment, interferometry, and barcode scanning.
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20Which material is commonly used for Semiconductor (Diode) Lasers?
A.Cadmium Sulfide (CdS)
B.Silicon (Si)
C.Gallium Arsenide (GaAs)
D.Germanium (Ge)
Correct Answer: Gallium Arsenide (GaAs)
Explanation:
Direct bandgap semiconductors are required for efficient light emission. Gallium Arsenide (GaAs) is a direct bandgap material commonly used, whereas Si and Ge are indirect bandgap materials.
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21In a semiconductor laser, the 'active region' where recombination occurs is usually located at:
A.The P-region
B.The metal contacts
C.The junction (depletion) region
D.The N-region
Correct Answer: The junction (depletion) region
Explanation:
Lasing action occurs at the junction of the p-n diode where electrons from the conduction band recombine with holes in the valence band.
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22What is the pumping mechanism in a semiconductor laser diode?
A.Reverse bias voltage
B.Thermal heating
C.Flash lamp
D.Forward bias current injection
Correct Answer: Forward bias current injection
Explanation:
A heavy forward bias current injects charge carriers (electrons and holes) into the junction region, creating the population inversion necessary for lasing.
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23Which of the following properties refers to the fixed phase relationship between the electric field vectors of the light wave at different points in time or space?
A.Intensity
B.Monochromaticity
C.Coherence
D.Divergence
Correct Answer: Coherence
Explanation:
Coherence refers to the correlation between the phases of the light wave. Temporal coherence relates to phase correlation over time, and spatial coherence relates to phase correlation across the beam profile.
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24If and are populations of lower and upper energy levels respectively, and and are their energies, Boltzmann's law states that at thermal equilibrium:
A.
B.
C.
D.
Correct Answer:
Explanation:
Boltzmann's distribution law describes the population ratio at thermal equilibrium: .
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25The basic principle behind Holography is:
A.Total Internal Reflection
B.Polarization and Dispersion
C.Reflection and Refraction
D.Interference and Diffraction
Correct Answer: Interference and Diffraction
Explanation:
Holography involves the recording of an image via Interference (forming the hologram) and the reconstruction of the image via Diffraction.
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26Unlike a standard photograph which records only intensity, a hologram records:
A.Only the polarization of light
B.Only the wavelength of light
C.Both amplitude (intensity) and phase of light
D.Only the phase of light
Correct Answer: Both amplitude (intensity) and phase of light
Explanation:
A hologram records the interference pattern between a reference beam and an object beam, preserving both the amplitude and the phase information, which allows for 3D reconstruction.
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27Which scientist is known as the father of Holography?
A.Dennis Gabor
B.Theodore Maiman
C.Albert Einstein
D.Arthur Schawlow
Correct Answer: Dennis Gabor
Explanation:
Dennis Gabor invented the technique of holography in 1947 and was awarded the Nobel Prize for it in 1971.
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28Which laser is most suitable for cutting and welding thick metal plates in industry?
A.Argon Ion Laser
B.CO Laser or High-power Nd:YAG
C.Semiconductor Laser
D.He-Ne Laser
Correct Answer: CO Laser or High-power Nd:YAG
Explanation:
CO lasers and high-power Nd:YAG lasers generate sufficient infrared thermal energy to melt and vaporize metals, making them suitable for industrial cutting and welding.
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29The extreme directionality of a laser beam is a consequence of:
A.The pumping source intensity
B.The active medium's density
C.The wavelength of light
D.The resonant cavity alignment
Correct Answer: The resonant cavity alignment
Explanation:
The mirrors of the resonant cavity are perfectly aligned so that only light traveling exactly along the axis is amplified, resulting in a highly directional beam with low divergence.
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30For a laser action to occur, the medium must have at least how many energy levels?
A.Three
B.Four
C.Two
D.One
Correct Answer: Three
Explanation:
A stable population inversion cannot be maintained in a two-level system (absorption and emission probabilities are equal). At least three levels (including a metastable state) are required.
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31The Einstein coefficient represents:
A.Probability of stimulated absorption per unit energy density
B.Probability of stimulated emission per unit energy density
C.Total energy of the atom
D.Probability of spontaneous emission per unit time
Correct Answer: Probability of spontaneous emission per unit time
Explanation:
is the Einstein coefficient for spontaneous emission, representing the probability per unit time that an atom in an excited state drops to a lower state spontaneously.
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32Which type of laser is widely used in barcode scanners and laboratory demonstrations due to its visible red beam?
A.Excimer Laser
B.CO Laser
C.He-Ne Laser
D.Nd:YAG Laser
Correct Answer: He-Ne Laser
Explanation:
The He-Ne laser (632.8 nm) emits a visible red beam, is low cost, and reliable, making it standard for scanners and lab demos.
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33In a heterojunction semiconductor laser, the active region is sandwiched between layers of:
A.Different materials with wider bandgaps
B.Same material with same bandgap
C.Metals
D.Insulators
Correct Answer: Different materials with wider bandgaps
Explanation:
In a heterojunction laser, the active layer has a narrower bandgap and is sandwiched by layers with wider bandgaps. This confines both carriers and light (optical guiding), improving efficiency.
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34The term LIDAR, a laser application, stands for:
A.Light Direction And Reading
B.Light Detection And Ranging
C.Laser Infrared Detection And Ranging
D.Long Interferometry Distance And Ranging
Correct Answer: Light Detection And Ranging
Explanation:
LIDAR stands for Light Detection And Ranging. It uses laser pulses to measure distances to the Earth or other targets.
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35Which of the following is NOT a property of Laser light?
A.Monochromaticity
B.High Divergence
C.Coherence
D.High Intensity
Correct Answer: High Divergence
Explanation:
Lasers are characterized by low divergence (high directionality). High divergence is a property of ordinary light sources.
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36In the Einstein relations, if the energy density of radiation is , the rate of stimulated emission is given by:
A.
B.
C.
D.
Correct Answer:
Explanation:
The rate of stimulated emission depends on the population of the upper level (), the energy density of the field (), and the Einstein coefficient .
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37The wavelength of the He-Ne laser (632.8 nm) is determined by the transition between energy levels of:
A.Helium atoms
B.Hydrogen atoms
C.The glass tube
D.Neon atoms
Correct Answer: Neon atoms
Explanation:
The actual light emission comes from the Neon transitions (specifically from the to levels in Paschen notation). Helium is only used for pumping.
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38Why is the Nd:YAG laser more efficient than the Ruby laser?
A.It uses a gas active medium.
B.It does not require cooling.
C.It emits visible light.
D.It is a four-level system requiring less energy to sustain inversion.
Correct Answer: It is a four-level system requiring less energy to sustain inversion.
Explanation:
As a four-level system, the lower lasing level of Nd:YAG is essentially empty at room temperature. Thus, any atoms pumped to the upper level contribute to inversion (), making it highly efficient compared to the 3-level Ruby laser.
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39The granular pattern observed when laser light reflects off a rough surface is called:
A.Dispersion
B.Laser Speckle
C.Diffraction grating
D.Newton's rings
Correct Answer: Laser Speckle
Explanation:
Laser Speckle is an interference pattern produced by the coherent light reflecting off the microscopic irregularities of a rough surface.
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40In the reconstruction step of holography, what is used to illuminate the hologram?
A.Ordinary white light (for transmission holograms)
B.An X-ray beam
C.A laser beam identical to the reference beam
D.The object beam
Correct Answer: A laser beam identical to the reference beam
Explanation:
To view the stored image (reconstruction), the hologram is illuminated by a light source identical to the reference beam used during recording.
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41Which laser is commonly used as a source for fiber optic communication systems?
A.Semiconductor Laser (Laser Diode)
B.Excimer Laser
C.CO Laser
D.Ruby Laser
Correct Answer: Semiconductor Laser (Laser Diode)
Explanation:
Semiconductor lasers are compact, efficient, can be modulated at high speeds, and emit wavelengths compatible with optical fibers, making them ideal for telecommunications.
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42What is the physical unit of the Einstein Coefficient ?
A.
B.
C.Dimensionless
D.
Correct Answer:
Explanation:
multiplies energy density ( or similar depending on definition) to get a probability rate (). Based on energy density in , the unit is or equivalent volume per energy-time.
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43The sharp monochromatic line width of a laser is due to:
A.Spontaneous emission
B.Collision broadening
C.Temporal Coherence
D.Doppler broadening
Correct Answer: Temporal Coherence
Explanation:
High temporal coherence implies the light wave remains sinusoidal with a specific frequency for a long time, which corresponds to a very narrow frequency bandwidth (monochromaticity).
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44Which medical application utilizes the precise tissue ablation properties of UV Excimer lasers?
A.Bone fracture repair
B.Thermal therapy
C.MRI imaging
D.LASIK eye surgery
Correct Answer: LASIK eye surgery
Explanation:
Excimer lasers (UV) are used in LASIK because they can ablate corneal tissue precisely without heating the surrounding area (photo-ablation).
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45In a laser cavity, if the gain of the medium is and the loss factor is , the condition for oscillation is:
A.
B.
C.
D.
Correct Answer:
Explanation:
For lasing to start and sustain, the round-trip gain provided by the active medium must be greater than or equal to the total round-trip losses (absorption, transmission through mirrors, diffraction).
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46In the notation , what does represent?
A.Planck's constant
B.Wave vector
C.Thermal conductivity
D.Boltzmann's constant
Correct Answer: Boltzmann's constant
Explanation:
In statistical mechanics and the Boltzmann distribution, (or ) represents Boltzmann's constant.
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47Which of the following describes the output of a Q-switched laser?
A.Extremely high power, short duration pulses
B.Frequency modulated wave
C.Incoherent radiation
D.Low power continuous beam
Correct Answer: Extremely high power, short duration pulses
Explanation:
Q-switching is a technique to produce giant pulses of light by preventing lasing until a high population inversion is achieved, then releasing the energy suddenly.
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48The active centers in a Ruby laser are:
A.Aluminium atoms
B.Chromium ions ()
C.Xenon atoms
D.Oxygen atoms
Correct Answer: Chromium ions ()
Explanation:
The lasing action in Ruby () arises from the transitions of the Chromium ions embedded in the crystal lattice.
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49What is the typical beam divergence of a gas laser?
A.10 degrees
B.1 radian
C.45 degrees
D.0.5 to 1 milliradian
Correct Answer: 0.5 to 1 milliradian
Explanation:
Gas lasers have very low divergence, typically in the range of milliradians, meaning the beam spreads very little over distance.
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50The main difference between photography and holography regarding the recording medium is:
A.There is no difference.
B.Photography requires coherent light, Holography does not.
C.Holography requires a lens, photography does not.
D.Photography records a 2D projection, Holography records a 3D wavefield.
Correct Answer: Photography records a 2D projection, Holography records a 3D wavefield.
Explanation:
Photography captures a flat 2D intensity map. Holography captures the interference pattern of the wavefield, encoding depth (3D) information via phase differences.