Waves, Electrons & Photons (Edexcel A Level Physics): Exam Questions

In 1965, Richard Feynman proposed a double slit experiment to investigate the wave properties of electrons.

The experiment was later carried out using the arrangement shown.

A beam of electrons was directed at a barrier with two slits.

The detector recorded the positions where electrons arrived after passing through the slits.

The following pattern was obtained. Black dots represent points where electrons were detected. A band where electrons were not detected has been labelled X and a band where electrons were detected has been labelled Y.

The path difference for electrons arriving at band X from the separate slits was 2.5 × 10^-11 m.

For electrons arriving at band Y the path difference was 5.0 × 10^-11 m.

Explain why this pattern is observed when the electron energy is 9.6 × 10^-17 J.

The electrons are travelling at non-relativistic speeds.

The photograph shows a school spectrometer.  

The spectrometer allows parallel rays of light to be passed through a diffraction grating and the resulting angles of diffraction to be measured.

In the telescope, light from the grating is focused to make a real image 16.7 mm in front of the eyepiece lens. The eyepiece lens then uses this real image as an object to produce a magnified virtual image for the observer.

Calculate the magnification produced by the eyepiece lens.

focal length of eyepiece lens = 17.9 mm  

Magnification = ........................................

The spectrometer and diffraction grating are used to analyse the light from a sodium lamp. In the sodium lamp, sodium is heated until it becomes a vapour and an electric current is passed through it. The vapour then emits light.

After the light passes through the diffraction grating a line spectrum is observed.

i) Explain why only certain wavelengths are observed.  

(6)

ii) Diffraction gratings with the following spacings are available: 

Explain which would be the best spacing to use to measure the diffraction angle for the third order maximum for yellow light of wavelength 589 nm. 

(3)

The diagram shows some of the energy levels in a sodium atom. 

Add an arrow to the diagram to show the transition involved in the emission of yellow light of wavelength 589 nm.

Show your working below.

A student used an electron beam tube to accelerate electrons towards a thin polycrystalline gold film. A pattern consisting of concentric bright rings is observed on a fluorescent screen.

In Experiment 1, the electrons are accelerated from rest through a potential difference of $5.00\text{ kV}$.

In Experiment 2, the potential difference across the tube is increased.

Discuss the conclusions that can be made from these experiments about the nature of electrons and the structure of gold.

Calculate the de Broglie wavelength of the electrons in Experiment 1.

The student suggested that alpha particles accelerated through a potential difference of $5.00\text{ kV}$ would also be suitable for investigating the structure of the gold film.

Comment on the student's suggestion.

Ultrasound can be used to investigate the structure of organs of the human body using the pulse-echo technique. A transducer emits ultrasound waves through body tissue and detects the reflected waves.

Describe how an ultrasound wave travels through body tissue.

The diagram shows the transducer emitting a short pulse of ultrasound into a patient's body.

The oscilloscope trace below shows the transmitted and detected pulses. Each division on the oscilloscope trace represents $40\text{μs}$.

Calculate the diameter of the kidney.

speed of ultrasound in soft body tissue = $1540{\text{ m s}}^{-1}$

Give a reason why the ultrasound must be emitted in short pulses rather than as a continuous wave.

The transducer emits ultrasound at a frequency of $2.5\text{ MHz}$.

Explain why ultrasound waves of much lower frequency are unsuitable for medical imaging.

A student used a diffraction grating to check the wavelength of the light emitted by a laser labelled $630\pm 10\text{ nm}$. Light from the laser was directed through the diffraction grating and onto a screen a distance $D$ away.

The diffraction pattern consisted of the central maximum and the first and second order maxima, as shown.

The student measured the distance between the two first-order maxima as $1640\text{ mm}$. The diffraction grating had $600{\text{ lines mm}}^{-1}$.

$D=2.00\text{ m}$

Deduce whether the light from the laser was within the range $630\pm 10\text{ nm}$.

Explain one modification to this method that would produce a smaller percentage uncertainty in the value of wavelength.

A polarising filter is placed between the laser and the grating.

The polarising filter is rotated slowly through $360°$ in the plane perpendicular to the laser beam. The student makes the following observations.

• As the polarising filter is rotated, the intensity of the diffraction pattern on the screen varies.

• At two positions during the rotation, the diffraction pattern is not visible on the screen.

Explain what these observations show about the light emitted by the laser.