Physics "Explain" Objectives

Explain how the double slit experiment provides evidence for the wave model of light.

• Two coherent beams of light from a single source is passed through two narrow slits

• an interference pattern caused by constructive and destructive interference is observed on a screen opposite to the slits

• Only waves have the ability to diffract through the narrow slits and interfere to create the interference pattern

• Hence the experiment provides evidence for the wave model of light

 

Explain the concept of blackbody radiation and the significance of the evidence it provides

• Black body radiation is the electromagnetic radiation emitted by a blackbody

• When intensity is plotted against the wavelength of electromagnetic radiation emitted by the blackbody there is a characteristic curve

• as the temperature increases, the peak wavelength decreases

• As temperature increases, intensity increases

• This curve can only be explained by considering light as a particle

• Hence it provides evidence that light is a particle and its energy is quantised

• lambda max = b/T

Explain how the photoelectric effect provides evidence for the particle model of light

• When light is shone onto a metal surface, electrons are ejected from the surface

• As intensity of the light increases, the number of ejected photoelectrons increases rather than its kinetic energy

• The frequency of incident light must be above the threshold frequency for photoelectrons to be ejected, increasing the intensity alone will not eject photoelectrons

• As frequency of light increases, the maximum kinetic energy of photoelectrons increases

• E=hf

Compare the Rutherford and Bohr model of the atom, and state Bohr’s 3 postulates

Rutherford - small central positive nucleus with negatively charged electrons orbiting it
Bohr - refined version of Rutherford’s model, based on three postulates:

1. electrons exist in stationary states, stable orbits where electrons lose no energy

2. transition between stationary states occurs via the absorption or emission of a photon

3. the angular momentum of orbiting electrons is discrete or quantised

Rutherford model limitations

1. Since electrons orbiting the nucleus are constantly changing direction and hence accelerating, they’re continuously emitting EM radiation. This decreases their energy, then velocity, until they fall into the nucleus

2. Cannot explain atomic spectra as according to the model, the wavelength of EM radiation emitted would depend on the acceleration of the electron

Explain how the detection of muons at the Earth’s surface provides evidence for the Theory of Special Relativity

• Muons are very unstable particles with a short lifespan. According to classical physics, they should not be able to reach the earth’s surface before decaying.

• In the muon’s frame of reference, because it is travelling at a relativistic speed, the length from the atmosphere to the Earth’s surface is shorter than the rest distance due to length contraction.

• The contracted distance it has to travel allows the muon to reach the Earth’s surface

• Hence it provides evidence for the Theory of Special relativity

Explain the momentum of high speed particles in particle accelerators

Explain the concept of simultaneity

When two events occur at the same time, they are simultaneous. However if the events are simultaneous to one person in an reference frame, doesn’t mean they are to a different person in another frame of reference, even if they’re both inertial. All perspectives are valid.

Explain and resolve the twins paradox

• The twins paradox describes a scenario where one twin travels on a rocket around Earth at relativistic speeds, whereas the other twin stays on Earth

• When the travelling twin comes back, the Earth twin looks like they’ve aged more

• It can be argued that either twin can age more depending on whose reference frame in considered

• The paradox can be resolved with deciding who’s travelling with uniform motion

• Since the travelling twin must change velocity (not necessarily the magnitude, but the direction), they are in a non-inertial frame of reference when this occurs

• Earth twin measures proper time and travelling twin experiences time dilation

• Travelling twin measures shorter time to cover shorter length, and thus returns to Earth having aged less than the Earth twin

Explain and resolve the flashlights on a train paradox

• A scenario where there’s one person standing at the centre of a moving train, and another person standing at the train station

• The person on the train has a flashlight that sends a light towards both the front and rear doors of the train. When the light comes in contact with those doors, they automatically open

• To the person on the train, both doors open simultaneously, but to the person standing at the station, the rear door opens before the front door as the rear door “meets” the light closer to the centre of the train and the front door “moves away” from the light

• Both are correct, and it depends on your frame of reference

Explain and resolve the ladder in a barn paradox

• Ladder in a barn paradox describes a scenario where a ladder that’s too long to fit inside a barn at rest approaches it at a relativistic speed

• In the ladder’s frame of reference, the barn is coming towards it at relativistic speeds, so the barn’s length will contract and hence the ladder will not fit with both doors shut

• In the barn’s frame of reference, the ladder is coming towards it at relativistic speeds, so the ladder’s length will contract and it will fit inside the barn with both doors shut

• The paradox is resolved by simultaneity

• In the ladder’s frame of reference, the barn moves towards it, and once the front end is inside the barn, away from it, hence the barn’s doors do not open and close simultaneously

• The barn’s frame of reference is the same as the door’s frame of reference, so the ladder will fit inside