topic 3 random Qs

(3.1) Explain how a radio wave of a given frequency can be transmitted using a transmitting antenna

Electrons in the transmitting antenna are fored to oscillate along the length of the antenna by an alternating potential difference. This creates an oscillating E field and a corresponding magnetic field which perpendicular to the E field. Both fields remain perpendicular to each other and the direction of travel, and propagate outwards. 

(3.1) What is ‘plane polarisation’ of EM waves

When vibrations/oscillations are restricted to a single plane. The plane of polarisation for an EM wave is defined by the plane of the oscillating E field

(3.1) Why are transmitted TV waves plane polarised

TV waves are produced when electrons are forced to oscillate in the transmitting antenna by an alternating potential difference. Since electrons can only oscillate in the plane parallel to the antenna, emitted waves are plane polarised in the same direction as the vibrating charge.

(3.1) Explain how a radio signal is received

The transmitted wave with an oscillating E field exerts a force (F=Eq) on stationary electrons in the receiving antenna. Electrons oscillate at the same frequency as the E field, inducing an alternating potential difference across the receiving antenna. The frequency of this alternating potential difference is the same as the frequency of electron oscillation. Thus, signal received

(3.1) Why is light from an incandescent source neither coherent nor monochromatic

Light produced is of varying frequencies (not one colour/one frequency) due to random vibrations from heating. Random oscillation of electrons means that emitted light doesn’t maintain a constant phase relationship, hence not coherent

(3.1) Describe the role of diffraction in producing interference patterns (double slit)

Diffraction is the bending/spreading of waves as they pass by/through an obstacle. Light diffracts at both slits. This creates circular waves that overlap and undergo constuctive interference at points where waves are in-phase, creating bright fringes. The creation of bright/dark fringes (from destructive interference - out of phase and when PD=(m+1/2)lambda at the screen) allows an interference pattern to appear on the screen. 

(3.1) In a diffraction pattern produced by white light, why is the central maximum white?

White light contains a range of wavelengths from violet to red. A white line is observed in the centre as the path difference between light from each slit in the grating is zero for all wavelengths. The light is in-phase at the centre, all wavelengths undergo constructive interference and recombine to produce a white line. (?)

(3.1) In a diffraction pattern produced by white light, why does the 1st order max consist of a continuous spread of frequencies? (rainbow)

Since sinθ is directly proportional to 𝜆, violet appears first and red last. Since violet’s wavelength is the smallest, it produces a maximum (violet band/line) at a smaller angle. All other wavelengths produce maxima between violet & red, in ascending order of their wavelength (hence rainbow!)

(3.2) Describe how Einstein explained the ejection of electrons from such a metal surface

Einstein used the concept of photons (as discrete quanta/bundles of energy which is absorbed) and the Law of conservation of energy to explain the ejection of photons. 1 electron in the metal’s surface absorbs the entire energy of 1 photon. When an electron absorbs a photon’s energy, the transfer of energy is immediate (3rd postulate), and the electron is emitted instantaneously. Hence, using LOCOE, part of the incident photon’s energy is used to release the electron, and the rest is transformed into the electrons KE.

(3.2) When the intensity of light incident on a photocell/photosurface increases, more electrons are emitted. Describe how Einstein used concept of photons to explain this.

Increasing intensity of light increases the number of photons incident on the metal without changing their energy (i.e. frequency remains same). This means more electrons can absorb the energy of photons and be emitted (where absorbtion is done on an all-or nothing basis - one electron absorbs one photon). Hence, more electrons emitted.

(3.2) Describe the photoelectric effect

When electrons are emitted from a material’s surface due to being illuminated by light of sufficiently high frequency

(3.2) Using Einstein’s explanation, experimentally, why are electrons ejected with a range of kinetic energies up to a maximum value?

Using the law of conservation of energy, part of the energy of incident photons is used to release an electron ,and the rest is transformed into the electron’s KE. Since electrons deeper in the metal/that are more tightly bound require more energy to be released, the amount of energy available to transform into these electron’s KE varies. Thus, electrons closer to the surface are emitted with more KE, up to a max value. 

(3.2) Explain why xrays with a continuous range of frequencies up to a max frequency are produced by an xray tube

As electrons collide with the target metal, their path is deviated by the electrostatic force between the electrons and the nucleus of the target atoms. This slows the electrons down, causing their kinetic energy to decrease. Due to this change in energy, the difference in kinetic energy of the electron before and after the collision is transformed into an Xray photon. Hence, as the difference in KE of the electron varies depending on how close it collides with the nucleus, the energy transformed into an xray photon also varies. This results in a continuous range of frequencies.

When an electron collides head on with nucleus, all of its initial kinetic energy is transferred to a single xray photon- which has max energy, and hence frequency.

(3.2) Describe the Davisson-Germer experiment

Low energy electrons were fired towards a nickel crystel. Electrons were diffracted by the surface layers of the crystal in a similar way to light. The equation dsinθ=mλ was able to predict the angles at which the electrons were diffracted. The crystal spacing, d, was known and the angle of the diffracte deletrons was measured for a given order, m. Davisson & Germer compared the wavelength found experimentally to the theoretical wavelength calculated using the de Broglie relationship 𝜆=ℎ/𝑝. The results matched, confirming the de Broglie relationship, and the fact that particles act as waves and have a wavelength that depends on their momentum