Diffraction is the spreading out of waves when they pass an obstruction, such as a narrow gap or the edge of an obstacle.

Define Huygens' construction .

A model in which every point on a wavefront is treated as a point source of secondary wavelets ; the new wavefront is the tangent to these wavelets.

Which property of a wave changes when it is diffracted, and why?

Its amplitude decreases, because some energy is dissipated as the wave is diffracted through the gap.

When are the effects of diffraction most pronounced?

When the gap or obstacle size is approximately the same as, or smaller than, the wavelength of the wave.

X-rays passing through a crystalline solid are strongly diffracted, but radio waves passing between strands of human hair are not. Explain why.

X-rays have a wavelength (10 -8 –10 -13 m) close to the spacing between atoms in the crystal , so diffraction is significant. Radio waves have a much longer wavelength (0.1–10 6 m) than the gaps between hairs, so they are not diffracted.

Diffraction is the .......... of waves when they pass an obstruction.

Diffraction is the spreading out of waves when they pass an obstruction.

True or False? Diffraction causes the wavelength of a wave to change.

False. Only the amplitude of a wave changes when it is diffracted; the wavelength remains constant .

Define a diffraction grating .

A plate containing a very large number of parallel, identical, closely-spaced slits , which produces a pattern of narrow bright fringes when monochromatic light is incident on it.

Define angular separation (of two maxima).

The difference between the angles of two orders of maxima, measured from the centre: θ 2 − θ 1 .

The maximum angle at which an order of maxima can be seen occurs when θ = .......... °, giving sin θ = 1.

The maximum angle at which an order of maxima can be seen occurs when θ = 90 °, giving sin θ = 1.

True or False? If n = d / λ is calculated as 3.8, the highest order visible is n = 4.

False. n must be an integer and cannot exceed the calculated maximum, so a decimal result must be rounded down — the highest order visible is n = 3.

Define the zero-order maximum .

The central beam of the diffraction pattern, undiffracted, found at θ = 0°.

What is the aim of Core Practical 6?

To determine the wavelength of light using a diffraction grating .

State the independent and dependent variables in this experiment.

Independent variable: distance between maxima, h Dependent variable: angle between the normal and each order, θ n

Why should the room be darkened and a set square used to align the laser and grating?

A darkened room makes the fringes clearer to see, and the set square ensures the beam meets the grating and screen at normal incidence, avoiding parallax error in measuring the fringe width.

Give two ways to reduce the percentage uncertainty in the measurement of h .

Any two from: Use a grating with more lines per mm (gives a greater h ) Use a Vernier calliper to measure h Measure across all visible fringes and divide by the number of fringes Increase the grating-to-screen distance, D

State two safety precautions for this experiment.

Any two from: Use a Class 2 laser with a maximum output of 1 mW Never allow the laser beam to shine into anyone's eyes Remove reflective surfaces from the room

The accepted wavelength of a standard school red laser is .......... nm.

The accepted wavelength of a standard school red laser is 635 nm.

True or False? Using a diffraction grating with fewer lines per mm reduces the percentage uncertainty in h .

False. A grating with more lines per mm gives a greater value of h , which lowers its percentage uncertainty.

Define electron diffraction .

The phenomenon in which electrons produce a diffraction pattern when directed through a thin film of graphite, providing the first clear evidence that matter can behave as a wave .

Why is a thin film of graphite used as the target in the electron diffraction tube?

Its crystalline lattice structure provides gaps between atomic planes similar in size to the electron's wavelength, so it acts like the slits in a diffraction grating .

What pattern is seen on the fluorescent screen, and what does it demonstrate?

A series of concentric rings , demonstrating that electrons show wave-particle duality by producing a diffraction pattern.

What effect does increasing the accelerating voltage have on the diffraction rings?

It reduces the diameter of the rings.

Why was the observation of electron diffraction significant?

It was the first clear evidence that matter, not just light, can behave as a wave .

If electrons behaved purely as particles, they would be distributed .......... across the screen rather than forming a diffraction pattern.

If electrons behaved purely as particles, they would be distributed uniformly across the screen rather than forming a diffraction pattern.

True or False? Increasing the accelerating voltage increases the diameter of the diffraction rings.

False. Increasing the accelerating voltage reduces the ring diameter; a lower voltage increases it.

Define matter waves .

The wave behaviour predicted by de Broglie for small, fast-moving particles (such as electrons), where a particle's momentum is linked to an associated wavelength.

Why is the de Broglie wavelength of an everyday moving object, such as a walking person, never observed?

It is roughly 10 20 times smaller than a nucleus, so it is negligible — the object behaves as a particle , not a wave.

The de Broglie equation shows that a particle's wavelength is inversely proportional to its .......... .

The de Broglie equation shows that a particle's wavelength is inversely proportional to its momentum .

True or False? All moving objects, including people, exhibit observable wave-like behaviour.

False. Large objects have de Broglie wavelengths that are far too small to observe (around 10 -36 m for a person), so they behave as particles , not waves.

Define transmission (of a wave).

A wave passes through a substance and emerges from the other side.

Define reflection (of a wave).

A wave hits a boundary between two media and does not pass through, instead staying in the original medium .

State the law of reflection.

The angle of incidence = the angle of reflection .

What determines whether a wave is mostly transmitted or mostly reflected at a boundary between two media?

Similar densities → wave is mostly transmitted Different densities → wave is mostly reflected

Why are rough surfaces less reflective than flat surfaces?

A rough surface scatters the light in all directions, so a strong, single reflected wave is not produced.

A transmitted wave may have a lower .......... than the incident wave, because of partial absorption as it passes through the material.

A transmitted wave may have a lower amplitude than the incident wave, because of partial absorption as it passes through the material.

True or False? Transmission is exactly the same process as refraction.

False. Transmission can involve refraction, but for transmission to occur the wave must pass through the material and emerge from the other side.

A transducer produces and detects a beam of ultrasound waves, converting electrical energy into sound waves and vice versa.

Define resolution (in pulse-echo imaging).

Resolution is the amount of detail that can be captured by a pulse-echo technique; it depends on the wavelength used.

Why is gel applied to the skin before an ultrasound scan?

The gel matches the density of the boundary between the transducer and the skin, allowing the ultrasound signal to be easily transmitted rather than reflected.

In sonar, why is the total distance travelled by an ultrasound pulse equal to twice the depth of the water?

The pulse must travel down from the transducer to the sea floor, then travel the same distance back up again as the echo.

Pulse duration is a consideration because ultrasound transducers cannot .......... and .......... pulses at the same time.

Pulse duration is a consideration because ultrasound transducers cannot transmit and receive pulses at the same time.

True or False? Using a longer wavelength of ultrasound gives a better (smaller) resolution.

False. Shorter wavelengths give a smaller (better) resolution, since they diffract (spread out) less and more detail can be resolved.

Define wave-particle duality .

Wave-particle duality is the phenomenon in which light behaves as both a wave and a particle: it interacts with matter (such as electrons) as a particle, but propagates through space as a wave.

A photon is a discrete quantum (packet) of electromagnetic energy, proposed by Einstein to explain how light behaves as a particle.

What experimental evidence supports light behaving as a wave?

The diffraction and interference of light, as demonstrated in Young's double-slit experiment.

Why does the wave theory of light fail to explain the existence of a threshold frequency in the photoelectric effect?

Wave theory suggests any frequency of light could cause photoelectric emission if the exposure time is long enough, since energy would build up gradually with each wave. This is not observed — only frequencies above the threshold frequency emit photoelectrons.

In the photoelectric effect, each electron can absorb only a .......... photon.

In the photoelectric effect, each electron can absorb only a single photon.

True or False? Increasing the intensity of light increases the kinetic energy of the emitted photoelectrons.

False. Increasing intensity increases the number of photoelectrons emitted per second, not their kinetic energy — each electron absorbs only a single photon, and photon energy depends on frequency , not intensity.

A photon is a massless packet (quantum) of electromagnetic energy; each photon transfers its energy in one discrete amount, rather than continuously.

International A Level (IAL)PhysicsEdexcelFlashcards2. Waves & ElectricityWaves, Electrons & Photons

Waves, Electrons & Photons (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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Cards in this collection (62)

Define diffraction.
Diffraction is the spreading out of waves when they pass an obstruction, such as a narrow gap or the edge of an obstacle.
Define Huygens' construction.
A model in which every point on a wavefront is treated as a point source of secondary wavelets; the new wavefront is the tangent to these wavelets.
Which property of a wave changes when it is diffracted, and why?
Its amplitude decreases, because some energy is dissipated as the wave is diffracted through the gap.
When are the effects of diffraction most pronounced?
When the gap or obstacle size is approximately the same as, or smaller than, the wavelength of the wave.
X-rays passing through a crystalline solid are strongly diffracted, but radio waves passing between strands of human hair are not. Explain why.
X-rays have a wavelength (10^-8–10^-13 m) close to the spacing between atoms in the crystal, so diffraction is significant. Radio waves have a much longer wavelength (0.1–10⁶ m) than the gaps between hairs, so they are not diffracted.
Diffraction is the .......... of waves when they pass an obstruction.
Diffraction is the spreading out of waves when they pass an obstruction.
True or False?
Diffraction causes the wavelength of a wave to change.
False.
Only the amplitude of a wave changes when it is diffracted; the wavelength remains constant.
Define a diffraction grating.
A plate containing a very large number of parallel, identical, closely-spaced slits, which produces a pattern of narrow bright fringes when monochromatic light is incident on it.
Define angular separation (of two maxima).
The difference between the angles of two orders of maxima, measured from the centre: θ₂ − θ₁.
State the diffraction grating equation and define each term.
$n λ = d \sin θ$
- n = order of the diffraction pattern
- λ = wavelength of light (m)
- d = slit separation (m)
- θ = angle between the normal and the maxima (°)
How is the slit separation, d, calculated from the number of lines per metre, N, on a grating?
$d = \frac{1}{N}$
How do you find the highest order of maxima visible for a given diffraction grating?
Set θ = 90° (so sin θ = 1) and calculate $n = \frac{d}{λ}$ . Since n must be an integer, round the result down.
The maximum angle at which an order of maxima can be seen occurs when θ = ..........°, giving sin θ = 1.
The maximum angle at which an order of maxima can be seen occurs when θ = 90°, giving sin θ = 1.
True or False?
If n = d/λ is calculated as 3.8, the highest order visible is n = 4.
False.
n must be an integer and cannot exceed the calculated maximum, so a decimal result must be rounded down — the highest order visible is n = 3.
Define the zero-order maximum.
The central beam of the diffraction pattern, undiffracted, found at θ = 0°.
What is the aim of Core Practical 6?
To determine the wavelength of light using a diffraction grating.
State the independent and dependent variables in this experiment.
- Independent variable: distance between maxima, h
- Dependent variable: angle between the normal and each order, θ_n
Why should the room be darkened and a set square used to align the laser and grating?
A darkened room makes the fringes clearer to see, and the set square ensures the beam meets the grating and screen at normal incidence, avoiding parallax error in measuring the fringe width.
Give two ways to reduce the percentage uncertainty in the measurement of h.
Any two from:
- Use a grating with more lines per mm (gives a greater h)
- Use a Vernier calliper to measure h
- Measure across all visible fringes and divide by the number of fringes
- Increase the grating-to-screen distance, D
State two safety precautions for this experiment.
Any two from:
- Use a Class 2 laser with a maximum output of 1 mW
- Never allow the laser beam to shine into anyone's eyes
- Remove reflective surfaces from the room
The accepted wavelength of a standard school red laser is .......... nm.
The accepted wavelength of a standard school red laser is 635 nm.
True or False?
Using a diffraction grating with fewer lines per mm reduces the percentage uncertainty in h.
False.
A grating with more lines per mm gives a greater value of h, which lowers its percentage uncertainty.
Define electron diffraction.
The phenomenon in which electrons produce a diffraction pattern when directed through a thin film of graphite, providing the first clear evidence that matter can behave as a wave.
Why is a thin film of graphite used as the target in the electron diffraction tube?
Its crystalline lattice structure provides gaps between atomic planes similar in size to the electron's wavelength, so it acts like the slits in a diffraction grating.
What pattern is seen on the fluorescent screen, and what does it demonstrate?
A series of concentric rings, demonstrating that electrons show wave-particle duality by producing a diffraction pattern.
What effect does increasing the accelerating voltage have on the diffraction rings?
It reduces the diameter of the rings.
Why was the observation of electron diffraction significant?
It was the first clear evidence that matter, not just light, can behave as a wave.
If electrons behaved purely as particles, they would be distributed .......... across the screen rather than forming a diffraction pattern.
If electrons behaved purely as particles, they would be distributed uniformly across the screen rather than forming a diffraction pattern.
True or False?
Increasing the accelerating voltage increases the diameter of the diffraction rings.
False.
Increasing the accelerating voltage reduces the ring diameter; a lower voltage increases it.
Define matter waves.
The wave behaviour predicted by de Broglie for small, fast-moving particles (such as electrons), where a particle's momentum is linked to an associated wavelength.
Define the de Broglie wavelength.
The wavelength associated with a moving particle, given by $λ = \frac{h}{m v} = \frac{h}{p}$
State the de Broglie equation and define each symbol.
$λ = \frac{h}{m v} = \frac{h}{p}$
- λ = de Broglie wavelength (m)
- h = Planck's constant (J s)
- m = mass (kg)
- v = velocity (m s^-1)
- p = momentum (kg m s^-1)
A person of mass 70 kg moves at 2 m s^-1. Calculate their de Broglie wavelength.
$λ = \frac{h}{m v} = \frac{6.63 \times 10^{- 34}}{70 \times 2} = 4.7 \times 10^{- 36} m$
Why is the de Broglie wavelength of an everyday moving object, such as a walking person, never observed?
It is roughly 10²⁰ times smaller than a nucleus, so it is negligible — the object behaves as a particle, not a wave.
The de Broglie equation shows that a particle's wavelength is inversely proportional to its ...........
The de Broglie equation shows that a particle's wavelength is inversely proportional to its momentum.
True or False?
All moving objects, including people, exhibit observable wave-like behaviour.
False.
Large objects have de Broglie wavelengths that are far too small to observe (around 10^-36 m for a person), so they behave as particles, not waves.
Define transmission (of a wave).
A wave passes through a substance and emerges from the other side.
Define reflection (of a wave).
A wave hits a boundary between two media and does not pass through, instead staying in the original medium.
State the law of reflection.
The angle of incidence = the angle of reflection.
What determines whether a wave is mostly transmitted or mostly reflected at a boundary between two media?
- Similar densities → wave is mostly transmitted
- Different densities → wave is mostly reflected
Why are rough surfaces less reflective than flat surfaces?
A rough surface scatters the light in all directions, so a strong, single reflected wave is not produced.
A transmitted wave may have a lower .......... than the incident wave, because of partial absorption as it passes through the material.
A transmitted wave may have a lower amplitude than the incident wave, because of partial absorption as it passes through the material.
True or False?
Transmission is exactly the same process as refraction.
False.
Transmission can involve refraction, but for transmission to occur the wave must pass through the material and emerge from the other side.
Define transducer.
A transducer produces and detects a beam of ultrasound waves, converting electrical energy into sound waves and vice versa.
Define resolution (in pulse-echo imaging).
Resolution is the amount of detail that can be captured by a pulse-echo technique; it depends on the wavelength used.
Why is gel applied to the skin before an ultrasound scan?
The gel matches the density of the boundary between the transducer and the skin, allowing the ultrasound signal to be easily transmitted rather than reflected.
In sonar, why is the total distance travelled by an ultrasound pulse equal to twice the depth of the water?
The pulse must travel down from the transducer to the sea floor, then travel the same distance back up again as the echo.
Pulse duration is a consideration because ultrasound transducers cannot .......... and .......... pulses at the same time.
Pulse duration is a consideration because ultrasound transducers cannot transmit and receive pulses at the same time.
True or False?
Using a longer wavelength of ultrasound gives a better (smaller) resolution.
False.
Shorter wavelengths give a smaller (better) resolution, since they diffract (spread out) less and more detail can be resolved.
Define wave-particle duality.
Wave-particle duality is the phenomenon in which light behaves as both a wave and a particle: it interacts with matter (such as electrons) as a particle, but propagates through space as a wave.
Define photon.
A photon is a discrete quantum (packet) of electromagnetic energy, proposed by Einstein to explain how light behaves as a particle.
What experimental evidence supports light behaving as a wave?
The diffraction and interference of light, as demonstrated in Young's double-slit experiment.
What experimental evidence supports light behaving as a particle?
The photoelectric effect.
Why does the wave theory of light fail to explain the existence of a threshold frequency in the photoelectric effect?
Wave theory suggests any frequency of light could cause photoelectric emission if the exposure time is long enough, since energy would build up gradually with each wave. This is not observed — only frequencies above the threshold frequency emit photoelectrons.
In the photoelectric effect, each electron can absorb only a .......... photon.
In the photoelectric effect, each electron can absorb only a single photon.
True or False?
Increasing the intensity of light increases the kinetic energy of the emitted photoelectrons.
False.
Increasing intensity increases the number of photoelectrons emitted per second, not their kinetic energy — each electron absorbs only a single photon, and photon energy depends on frequency, not intensity.
Define photon.
A photon is a massless packet (quantum) of electromagnetic energy; each photon transfers its energy in one discrete amount, rather than continuously.
State the two equations used to calculate the energy of a photon.
$E = h f$
$E = \frac{h c}{λ}$
How does the energy of a photon relate to its frequency and its wavelength?
- Photon energy is directly proportional to frequency
- Photon energy is inversely proportional to wavelength
A long-wavelength photon of light has a .......... energy than a shorter-wavelength photon.
A long-wavelength photon of light has a lower energy than a shorter-wavelength photon.
What are h and c in the equation $E = \frac{h c}{λ}$ ?
- h is Planck's constant (J s)
- c is the speed of light (m s^-1)
True or False?
A photon transfers its energy gradually over time, like a continuous wave.
False.
A photon transfers all of its energy in one go, as a discrete packet (quantum) of energy — it is not transferred continuously.

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