Dawn Of Modern Physics MCQs | Physics MCQs

This is the first postulate of special relativity. It applies to inertial (non-accelerating) frames of reference, stating that there is no absolute or preferred state of rest.

This radical postulate states that the speed of light, 'c', is a universal constant, independent of the motion of the source or the observer.

Time dilation (moving clocks run slower), length contraction, and mass-energy equivalence are all key consequences derived from the two postulates of special relativity.

This famous equation shows that mass is a form of energy and that a small amount of mass can be converted into a large amount of energy, and vice versa.

The ability to resolve fine details is limited by the wavelength of the probe. High-energy electrons have a very short de Broglie wavelength, allowing for much higher resolution than is possible with longer-wavelength visible light.

The energy released is found by multiplying the mass defect by 931.5 MeV/amu. Energy = 0.05 amu * 931.5 MeV/amu = 46.575 MeV.

The classical Rayleigh-Jeans law incorrectly predicted that the energy radiated by a blackbody would increase infinitely as the wavelength decreased into the UV region, a clear contradiction of experimental data.

Photoemission depends on the energy of individual photons (frequency), not the number of photons (intensity). If the photon energy is below the work function, no amount of intensity will cause emission.

The number of half-lives is 60 hours / 15 hours = 4. After 1st half-life: 400g. After 2nd: 200g. After 3rd: 100g. After 4th: 50g.

Since de Broglie wavelength is inversely proportional to momentum (λ = h/mv), the particle with the smallest mass (the electron) will have the longest wavelength for a given speed.

Photons are massless particles. They possess energy and momentum due to their motion at speed 'c', but they have no rest mass.

Initial mass = 2 * 2.0141 = 4.0282 amu. Mass defect = 4.0282 - 4.0026 = 0.0256 amu. Energy = 0.0256 amu * 931.5 MeV/amu = 23.8 MeV.

The number of half-lives is 24 days / 8 days = 3. After 1st half-life: 10 mg. After 2nd: 5 mg. After 3rd: 2.5 mg.

The energy required is the binding energy. Energy = mass defect * 931.5 MeV/amu = 0.09894 * 931.5 = 92.16 MeV.

Producing two photons traveling in opposite directions allows for the conservation of both energy and momentum, as the initial momentum of the electron-positron system is often zero.

The stopping potential is the reverse voltage required to stop the most energetic electrons, so their maximum kinetic energy is equal to the work done by this potential (K.E.max = eVs).

Wave-particle duality is a fundamental principle of quantum mechanics, stating that everything exhibits both wave and particle properties depending on the experiment being performed.

The law (λ_max * T = constant) correctly states that as an object's temperature increases, the peak of its emission spectrum shifts to shorter wavelengths.

The formula for the shift is Δλ = (h/mₑc)(1 - cosθ). This value is maximized when cosθ = -1, which occurs at a backscattering angle of θ = 180°.

The particle theory states that a single photon must have enough energy (hf) to overcome the work function (φ). If hf < φ, no emission occurs, regardless of intensity, thus explaining the threshold.

GPS requires corrections from both: special relativity for the high speed of the satellites (time dilation) and general relativity for the weaker gravitational field they experience.

From the equation E=hf, we can see that h=E/f. The units are Joules / (1/second), which is Joule-seconds (J·s), a unit of angular momentum or action.

Max Planck's successful solution to the blackbody radiation problem required the revolutionary assumption that the energy of oscillators was quantized.

Mass is converted into energy according to E=mc², typically producing high-energy photons (gamma rays).

Using the formula f = E/h, we get (3.3 × 10⁻¹⁹ J) / (6.6 × 10⁻³⁴ J·s) = 0.5 × 10¹⁵ Hz, which is also 5.0 x 10¹⁴ Hz.

The relativistic factor gamma (γ) only becomes significantly greater than 1, causing a noticeable mass increase, as velocity approaches the speed of light, c.

The principle is a fundamental property of nature. The wavelike nature of a particle means its position and momentum are inherently 'fuzzy' and cannot be simultaneously defined with perfect precision.

Intensity is total energy per second. Since blue light photons have more energy individually than red light photons, more of the lower-energy red photons are needed to deliver the same total energy per second.

The regular atomic spacing in the nickel crystal acted as a natural diffraction grating for the electron waves, producing the observed interference pattern.

Since wavelength is inversely proportional to momentum (λ = h/p), doubling the momentum will halve the wavelength.

In the realm of everyday experience (slow speeds, large objects), the effects of relativity and quantum mechanics are negligible, and classical physics provides extremely accurate predictions.

Higher intensity means more photons arriving per second. If the frequency is above the threshold, this results in more electrons being ejected per second, leading to a larger current.

Max Planck introduced this fundamental constant of nature (h ≈ 6.626 x 10⁻³⁴ J·s) to explain the quantization of energy in blackbody radiation.

The wave model predicted a time delay for energy to accumulate. The instantaneous emission supports the particle model, where a single photon's energy is transferred to an electron in a single, immediate event.

This is a consequence of time dilation and the principle of relativity. From the spaceship's frame of reference, Earth is the moving clock, and it is therefore observed to be running slower.

This relation (often written with ħ = h/2π) indicates that a state that exists for a short time (small Δt) has a large uncertainty in its energy (large ΔE).

Phenomena like blackbody radiation, the photoelectric effect, and atomic spectra directly contradicted the predictions of classical theories, necessitating the new approaches of relativity and quantum mechanics.

This is the second postulate of special relativity: the speed of light in a vacuum, 'c', is constant for all inertial observers, regardless of the source's or observer's motion.

The law (P = σAT⁴) states that the total power radiated is directly proportional to the fourth power of the absolute temperature.

Let K₁ = hf - φ. The new energy is K₂ = h(2f) - φ = 2hf - φ. Substituting hf = K₁ + φ gives K₂ = 2(K₁ + φ) - φ = 2K₁ + φ. Since φ > 0, K₂ > 2K₁.

The number of half-lives is 60 hours / 15 hours = 4. After 1st half-life: 400g. After 2nd: 200g. After 3rd: 100g. After 4th: 50g.

Setting the photon energy equal to the electron's rest mass energy (E = hc/λ = mₑc²) gives the Compton wavelength, λ_c = h/(mₑc).

Due to their large mass, macroscopic objects have enormous momentum, resulting in a de Broglie wavelength (λ = h/p) so small that diffraction effects are impossible to detect.

A photon cannot decay into a pair in empty space as momentum cannot be conserved. The nearby nucleus absorbs some recoil momentum, satisfying the conservation law.

The relation ΔE Δt ≥ ħ/2 implies that if the lifetime Δt is small, the uncertainty in energy ΔE must be large, allowing for temporary 'borrowing' of energy.

Decay constant lambda = 0.693 / Half-life = 0.693 / 5730 years = 0.000121 per year, or 1.21 x 10⁻⁴ per year.

The number of half-lives is 12 minutes / 4 minutes = 3. After 1st half-life: 600. After 2nd: 300. After 3rd: 150 decays per minute.

Using E = mc². Energy = 0.001 kg * (3 x 10⁸ m/s)² = 0.001 * (9 x 10¹⁶) = 9 x 10¹³ Joules.

According to the principle of relativity, all motion is relative. From the astronaut's frame, Earth is the moving object, so its clock appears to run slower.

According to the Stefan-Boltzmann law, the total radiated power (energy per time) increases dramatically with the fourth power of the temperature.

The threshold wavelength is λ₀ = hc/φ. First convert φ to Joules: 2.0 eV * 1.6e-19 J/eV = 3.2e-19 J. Then λ₀ = (6.63e-34 * 3e8) / 3.2e-19 ≈ 6.2e-7 m = 620 nm.

This relationship is derived from the de Broglie relation (p = h/λ) and applies to photons as well as massive particles.

Both gain the same kinetic energy (K.E.). Since p = √(2m*K.E.), the more massive proton has greater momentum and therefore a shorter wavelength (λ = h/p).

Frame dragging is a prediction of General Relativity where a massive rotating object 'drags' the fabric of spacetime around with it.

Intensity is power per unit area. Since power is energy per time, intensity is proportional to the number of photons arriving per unit area per unit time.

The contracted length is L = L₀√(1 - v²/c²). So, L = 1 * √(1 - 0.8²) = √(1 - 0.64) = √0.36 = 0.6 m.

Mass-energy equivalence (E=mc²) is a profound consequence derived from the two postulates, but it is not a starting assumption (postulate) itself.

Since electrons are charged particles, their path can be bent and focused by carefully shaped magnetic fields, which act as magnetic lenses.

A decrease in wavelength would mean the photon gained energy. This is impossible as it would violate the conservation of energy, since the electron was initially at rest.

Due to the wave nature of matter and radiation, it is impossible to resolve details smaller than the wavelength of the probe being used to observe the system. This is a fundamental limit.

The uncertainty principle dictates an inverse relationship. A small uncertainty in position (Δx) necessarily leads to a large uncertainty in momentum (Δp).

The total energy is the sum of the energy a particle has due to its mass (m₀c²) and the additional energy it has due to its motion (K.E.).

Einstein's 1905 paper explained the features of the photoelectric effect by postulating that light itself is made of discrete energy quanta, which he called photons.

According to Wien's displacement law, doubling the temperature shifts the peak emission to a much shorter wavelength, into the blue or ultraviolet part of the spectrum.

Momentum, total energy (mass-energy), and charge are all fundamental quantities that are conserved in this process.

This is a key consequence of special relativity. Whether two spatially separated events occur at the same time depends on the observer's state of motion.

The relationship is given by Planck's fundamental equation, E = hf.

Using Einstein's equation E = mc². Energy = 2 kg * (3 x 10⁸ m/s)² = 2 * (9 x 10¹⁶) = 1.8 x 10¹⁷ Joules.

Half-life T = 0.693 / lambda. So, T = 0.693 / 0.0231 per day = 30 days.

Diffraction effects are prominent only when the wavelength of a wave is similar in size to the aperture or spacing of a grating, which in this case was the crystal lattice.

The experiment's 'null result' famously failed to detect the hypothetical ether wind, which was a crucial step towards the development of special relativity.

From the energy-momentum relation E² = (pc)² + (m₀c²)², if rest mass m₀=0, then E=pc. This relationship only holds for particles, like photons, that travel at speed c.

Relativistic momentum is p = γm₀v. At 0.99c, the Lorentz factor γ is approximately 7.1, so the momentum is over 7 times its classical value.

From ΔE Δt ≥ ħ/2, if ΔE = 0, then Δt must be infinitely large. This means a state with a perfectly defined energy must be stable and exist forever.

This is the definition of quantization. The quantity is restricted to specific, discrete levels rather than being able to take on any continuous value.

Visible light photons have energies of a few electron-volts, which is far below the minimum energy threshold of 1.02 MeV required to create an electron-positron pair.

Einstein's explanation of the photoelectric effect solidified the idea that light, long considered a wave, also has particle-like properties (photons).

The spacetime interval, defined as (Δs)² = (cΔt)² - (Δx)², is an invariant quantity in special relativity, meaning all inertial observers will calculate the same value for it.

The classical Rayleigh-Jeans law predicted infinite energy emission at short wavelengths, a failure famously dubbed the 'Ultraviolet Catastrophe'.

The maximum kinetic energy is directly related to the stopping potential by K.E.max = e*Vs. A stopping potential of 1.5 Volts corresponds to a kinetic energy of 1.5 electron-Volts.

This 'backscattering' angle of 180° corresponds to the maximum energy transfer to the electron and thus the maximum increase in the photon's wavelength.

Confining the particle to a smaller space decreases the uncertainty in its position (Δx). By the Heisenberg Uncertainty Principle, this must cause an increase in the uncertainty of its momentum (Δp).

The situation is not symmetrical. The traveling twin must accelerate to leave, turn around, and return. This acceleration places them in non-inertial frames, breaking the symmetry and causing a real age difference.

Using E = m₀c², the electron's rest mass energy is calculated to be approximately 0.511 Mega-electron-volts (MeV), a fundamental constant in particle physics.

A black body emits radiation at all wavelengths, forming a continuous spectrum whose shape and peak intensity depend only on the body's temperature.

The photon transfers some of its energy to the electron, so its final energy (E') must be less than its initial energy (E), unless the scattering angle is zero.

A photocell is a direct application where incident light ejects electrons from a surface, creating a measurable electric current, which is the definition of the photoelectric effect.

Helium-4 has 4 nucleons (2 protons, 2 neutrons). Binding energy per nucleon = Total binding energy / number of nucleons = 28.3 MeV / 4 = 7.075 MeV.

An activity of 25% (or 1/4th) means two half-lives have passed (100% -> 50% -> 25%). The age is 2 * 5730 years = 11460 years.

The radius R = R₀ * (A to the power of 1/3). R = (1.2 x 10⁻¹⁵ m) * 4.12 = 4.944 x 10⁻¹⁵ m.

In the modern quantum interpretation, the square of the amplitude of the matter wave at a point is proportional to the probability of finding the particle at that point.

This formula shows that the measured (relativistic) mass 'm' of an object increases with its velocity relative to the observer, from its rest mass 'm₀'.

The Davisson-Germer experiment showed that a beam of electrons could be diffracted by a crystal lattice, a behavior that is exclusively characteristic of waves.

The work function is an intrinsic, characteristic property of a specific material, representing how tightly its electrons are bound to the surface.

Number of half-lives = 500 / 100 = 5. Atoms remaining = (6.4 x 10²⁰) * (1/32) = 0.2 x 10²⁰. Atoms decayed = Initial - Remaining = (6.4 x 10²⁰) - (0.2 x 10²⁰) = 6.2 x 10²⁰ atoms.

The activity reduced from 8000 -> 4000 -> 2000 -> 1000. This is a reduction by a factor of 8 (2*2*2), which means 3 half-lives have passed in 6 hours. So, the half-life is 6 hours / 3 = 2 hours.

The total energy produced is the sum of the rest mass energies of both particles. Total Energy = 0.511 MeV + 0.511 MeV = 1.022 MeV.

The resolution of any imaging system is fundamentally limited by diffraction. To 'see' an object's features, the wavelength used must be smaller than those features.

The late 19th and early 20th centuries saw the emergence of problems (like blackbody radiation) that classical physics could not solve, heralding the new era of modern physics.