The spreading out of waves when they pass an obstruction, such as a narrow gap (a slit or aperture) or the edge of an obstacle.

Which property of a wave changes when it is diffracted?

Only the amplitude changes — the wavelength and frequency stay constant. The amplitude decreases because some energy is dissipated as the wave passes through the gap.

Diffraction effects are most pronounced when the gap or obstacle is approximately equal to or .......... the wavelength of the wave.

Diffraction effects are most pronounced when the gap or obstacle is approximately equal to or smaller than the wavelength of the wave.

True or False? Diffraction decreases the wavelength of a wave.

False. Only the amplitude changes when a wave is diffracted. The wavelength stays constant — take care to keep it constant when drawing diffracted wavefronts.

Define Huygens' construction .

A model in which every point on a wavefront acts as a point source of secondary waves ( wavelets ). The new wavefront is the tangent (envelope) to these secondary wavelets, producing the curved shape of the diffracted wave.

When a wave meets an obstacle, what forms around the edges and behind it?

A diffraction pattern forms around the edges of the obstacle. Behind it, a 'shadow' forms where no part of the wave reaches.

What happens to diffraction as the gap becomes much larger than the wavelength?

The effect becomes less pronounced . When the gap is much larger than the wavelength, the waves are no longer noticeably spread out.

Define diffraction grating .

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

A pattern of narrow bright fringes is produced when .......... light is incident on a diffraction grating.

A pattern of narrow bright fringes is produced when monochromatic light is incident on a diffraction grating.

True or False? In the grating equation, θ is the angle between two adjacent orders of maxima.

False. The angle θ is measured from the centre (the normal / zero-order beam) to the maximum, not between two orders.

What is the aim of Core Practical 8: Investigating Diffraction Gratings?

To find the wavelength of light using a diffraction grating.

Why is a set square used when setting up the diffraction grating?

To ensure the laser beam passes through the grating at normal incidence and meets the screen perpendicularly , avoiding parallax error in the fringe measurements.

The experiment is carried out in a .......... room so that the fringes appear clear on the screen.

The experiment is carried out in a darkened room so that the fringes appear clear on the screen.

Why does using a grating with more lines per mm reduce the percentage uncertainty?

More lines per mm produces greater values of *h (the distance between maxima), and a larger h has a lower percentage uncertainty *.

Why must the diffraction angle θ be found using trigonometry in this experiment?

Because the angle is not small , so the small-angle approximation cannot be used. It is calculated from the distance between maxima h and the grating-to-screen distance D .

State two safety precautions for this laser experiment.

Any two: use a Class 2 laser with a maximum output no more than 1 mW never let the beam shine into anyone's eyes remove reflective surfaces from the room

Define electron diffraction .

The spreading out of electrons as they pass through the gaps in an atomic lattice, producing a pattern that provides evidence for the wave nature of matter.

Why was electron diffraction such an important observation?

It was the first clear evidence that matter (such as electrons) can behave like light and has wave properties .

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

Its crystalline lattice acts like the slits of a diffraction grating. The gaps between the carbon atoms are similar in size to the electrons' wavelength, so the electron waves diffract and spread out.

In the electron diffraction tube, the pattern is observed as a series of concentric .......... on a fluorescent screen.

In the electron diffraction tube, the pattern is observed as a series of concentric rings on a fluorescent screen.

How does the observed pattern show that electrons behave as waves rather than particles?

A pattern of rings is produced. If the electrons acted purely as particles, they would be distributed uniformly across the screen with no pattern.

What happens to the diameter of a ring when the accelerating voltage is increased?

A larger accelerating voltage reduces the diameter of a given ring; a lower accelerating voltage increases it.

True or False? Electron diffraction can be observed by passing electrons through an ordinary optical slit.

False. The gap must be similar in size to the electrons' wavelength, so an atomic lattice (such as graphite) is needed — an ordinary slit is far too wide.

Define matter waves .

The wave-like behaviour associated with very small, fast-moving particles such as electrons, as theorised by de Broglie — not only EM waves behave as particles, but particles can also behave as waves.

According to the de Broglie equation, the wavelength of a particle is .......... proportional to its momentum.

According to the de Broglie equation, the wavelength of a particle is inversely proportional to its momentum.

Why can the de Broglie wavelength of everyday objects be ignored?

Their wavelength is extraordinarily small — for a walking person it is around 10 -36 m, about 10 20 times smaller than a nucleus. Such objects therefore behave like particles, not waves .

True or False? Only EM waves such as light can behave as both waves and particles.

False. de Broglie showed that matter — such as electrons — can also behave as waves (matter waves), not just electromagnetic waves.

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

Define transmission .

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

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

The relative densities of the two media: similar densities → energy mostly transmitted different densities → energy mostly reflected

The law of reflection states the angle of incidence equals the angle of ..........

The law of reflection states the angle of incidence equals the angle of reflection

True or False? Rough and smooth surfaces reflect waves equally well

False. Flat, smooth surfaces are the most reflective. Rough surfaces are the least reflective because they scatter the wave in all directions

Why is a transmitted wave often lower in amplitude than the incident wave?

Some of the wave's energy is absorbed as it passes through the material

State two uses of reflected waves .

Any two from: sonar ultrasound scans medical x-rays

Define transducer (in ultrasound).

The device that produces and detects the beam of ultrasound waves sent into the body

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

The gel has a similar density to skin, so the boundary transmits the signal into the body instead of reflecting it

In a pulse-echo survey, how does the total distance travelled by the pulse relate to the depth ?

The pulse travels to the boundary and back , so: total distance = twice the depth depth = ½ × speed × time

Shorter wavelengths diffract less and give a smaller (better) .......... , so more detail can be resolved

Shorter wavelengths diffract less and give a smaller (better) resolution , so more detail can be resolved

Why are ultrasound pulses made very short, with relatively long gaps between them?

the transducer cannot transmit and receive at the same time if outgoing and incoming pulses overlap , information is lost short pulses with large gaps produce the clearest images

True or False? Using a longer wavelength produces a more detailed ultrasound image

False. Shorter wavelengths give better resolution and more detail. The wavelength is chosen to be similar in size to the object being resolved

State two uses of sonar .

Any two from: locating fish detecting underwater vessels mapping the ocean floor

Define wave-particle duality .

Light can behave both as a particle (photons) and as a wave

What is the experimental evidence that light behaves as a particle , and as a wave ?

particle: the photoelectric effect wave: diffraction and interference (Young's double-slit experiment)

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

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

True or False? The wave theory of light predicts a threshold frequency for photoelectric emission

False. Wave theory does not predict a threshold frequency — it suggests any frequency could emit electrons given enough exposure time. Only the photon (particle) model explains the threshold frequency

Above the threshold frequency, what happens as the intensity of light increases?

More photoelectrons are emitted per second

A massless packet, or quantum , of electromagnetic energy — energy transferred in discrete packets rather than continuously

The energy of a photon is .......... to its wavelength

The energy of a photon is inversely proportional to its wavelength

True or False? A longer-wavelength photon carries more energy than a shorter-wavelength photon

False. Photon energy is inversely proportional to wavelength, so a longer -wavelength photon has lower energy

A LevelPhysicsEdexcelFlashcards5. Waves & Particle Nature of LightWaves, Electrons & Photons

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

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Define diffraction.
The spreading out of waves when they pass an obstruction, such as a narrow gap (a slit or aperture) or the edge of an obstacle.
Which property of a wave changes when it is diffracted?
Only the amplitude changes — the wavelength and frequency stay constant. The amplitude decreases because some energy is dissipated as the wave passes through the gap.
Diffraction effects are most pronounced when the gap or obstacle is approximately equal to or .......... the wavelength of the wave.
Diffraction effects are most pronounced when the gap or obstacle is approximately equal to or smaller than the wavelength of the wave.
True or False?
Diffraction decreases the wavelength of a wave.
False.
Only the amplitude changes when a wave is diffracted. The wavelength stays constant — take care to keep it constant when drawing diffracted wavefronts.
Define Huygens' construction.
A model in which every point on a wavefront acts as a point source of secondary waves (wavelets). The new wavefront is the tangent (envelope) to these secondary wavelets, producing the curved shape of the diffracted wave.
When a wave meets an obstacle, what forms around the edges and behind it?
A diffraction pattern forms around the edges of the obstacle. Behind it, a 'shadow' forms where no part of the wave reaches.
What happens to diffraction as the gap becomes much larger than the wavelength?
The effect becomes less pronounced. When the gap is much larger than the wavelength, the waves are no longer noticeably spread out.
Define diffraction grating.
A plate on which there is a very large number of parallel, identical, closely-spaced slits. Monochromatic light incident on it produces a pattern of narrow bright fringes.
State the diffraction grating equation.
$n λ = d \sin θ$
Where n = order of the maximum, d = slit separation, θ = angle from the centre (normal) to the maximum.
A pattern of narrow bright fringes is produced when .......... light is incident on a diffraction grating.
A pattern of narrow bright fringes is produced when monochromatic light is incident on a diffraction grating.
How is slit separation d found from the number of slits per metre N?
$d = \frac{1}{N}$
Where N is the number of slits (lines) per metre.
How do you calculate the highest order of maximum that is visible?
The maximum angle is θ = 90°, so sin θ = 1, giving:
$n = \frac{d}{λ}$
Since n must be an integer, round the result down (e.g. 2.7 gives a highest order of n = 2).
How is the angular separation between the first and second order maxima found?
Subtract the smaller angle from the larger one:
$θ_{2} - θ_{1}$
Higher orders sit at greater angles from the centre.
True or False?
In the grating equation, θ is the angle between two adjacent orders of maxima.
False.
The angle θ is measured from the centre (the normal / zero-order beam) to the maximum, not between two orders.
What is the aim of Core Practical 8: Investigating Diffraction Gratings?
To find the wavelength of light using a diffraction grating.
Why is a set square used when setting up the diffraction grating?
To ensure the laser beam passes through the grating at normal incidence and meets the screen perpendicularly, avoiding parallax error in the fringe measurements.
The experiment is carried out in a .......... room so that the fringes appear clear on the screen.
The experiment is carried out in a darkened room so that the fringes appear clear on the screen.
Why does using a grating with more lines per mm reduce the percentage uncertainty?
More lines per mm produces greater values of *h (the distance between maxima), and a larger h has a lower percentage uncertainty*.
Why must the diffraction angle θ be found using trigonometry in this experiment?
Because the angle is not small, so the small-angle approximation cannot be used. It is calculated from the distance between maxima h and the grating-to-screen distance D.
State two safety precautions for this laser experiment.
Any two:
- use a Class 2 laser with a maximum output no more than 1 mW
- never let the beam shine into anyone's eyes
- remove reflective surfaces from the room
True or False?
The slit separation d equals the number of slits per metre N.
False.
The slit separation is the reciprocal of the number of slits per metre: $d = \frac{1}{N}$
Define electron diffraction.
The spreading out of electrons as they pass through the gaps in an atomic lattice, producing a pattern that provides evidence for the wave nature of matter.
Why was electron diffraction such an important observation?
It was the first clear evidence that matter (such as electrons) can behave like light and has wave properties.
Why is a thin film of graphite used in the electron diffraction tube?
Its crystalline lattice acts like the slits of a diffraction grating. The gaps between the carbon atoms are similar in size to the electrons' wavelength, so the electron waves diffract and spread out.
In the electron diffraction tube, the pattern is observed as a series of concentric .......... on a fluorescent screen.
In the electron diffraction tube, the pattern is observed as a series of concentric rings on a fluorescent screen.
How does the observed pattern show that electrons behave as waves rather than particles?
A pattern of rings is produced. If the electrons acted purely as particles, they would be distributed uniformly across the screen with no pattern.
What happens to the diameter of a ring when the accelerating voltage is increased?
A larger accelerating voltage reduces the diameter of a given ring; a lower accelerating voltage increases it.
True or False?
Electron diffraction can be observed by passing electrons through an ordinary optical slit.
False.
The gap must be similar in size to the electrons' wavelength, so an atomic lattice (such as graphite) is needed — an ordinary slit is far too wide.
Define matter waves.
The wave-like behaviour associated with very small, fast-moving particles such as electrons, as theorised by de Broglie — not only EM waves behave as particles, but particles can also behave as waves.
State the de Broglie equation.
$λ = \frac{h}{m v} = \frac{h}{p}$
Where 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).
According to the de Broglie equation, the wavelength of a particle is .......... proportional to its momentum.
According to the de Broglie equation, the wavelength of a particle is inversely proportional to its momentum.
In the de Broglie equation, what quantity does the product of mass and velocity represent?
Momentum, p. This is why the equation can be written $λ = \frac{h}{p}$
Why can the de Broglie wavelength of everyday objects be ignored?
Their wavelength is extraordinarily small — for a walking person it is around 10^-36 m, about 10²⁰ times smaller than a nucleus. Such objects therefore behave like particles, not waves.
True or False?
Only EM waves such as light can behave as both waves and particles.
False.
de Broglie showed that matter — such as electrons — can also behave as waves (matter waves), not just electromagnetic waves.
Define reflection.
A wave hits a boundary between two media and does not pass through, but instead stays in the original medium
Define transmission.
A wave passes through a substance and emerges from the other side
What determines whether a wave is mostly transmitted or reflected at a boundary?
The relative densities of the two media:
- similar densities → energy mostly transmitted
- different densities → energy mostly reflected
The law of reflection states the angle of incidence equals the angle of ..........
The law of reflection states the angle of incidence equals the angle of reflection
True or False?
Rough and smooth surfaces reflect waves equally well
False.
Flat, smooth surfaces are the most reflective. Rough surfaces are the least reflective because they scatter the wave in all directions
Why is a transmitted wave often lower in amplitude than the incident wave?
Some of the wave's energy is absorbed as it passes through the material
State two uses of reflected waves.
Any two from:
- sonar
- ultrasound scans
- medical x-rays
Define transducer (in ultrasound).
The device that produces and detects the beam of ultrasound waves sent into the body
Why is gel applied to the skin during an ultrasound scan?
The gel has a similar density to skin, so the boundary transmits the signal into the body instead of reflecting it
In a pulse-echo survey, how does the total distance travelled by the pulse relate to the depth?
The pulse travels to the boundary and back, so:
- total distance = twice the depth
- depth = ½ × speed × time
Shorter wavelengths diffract less and give a smaller (better) .........., so more detail can be resolved
Shorter wavelengths diffract less and give a smaller (better) resolution, so more detail can be resolved
Why are ultrasound pulses made very short, with relatively long gaps between them?
- the transducer cannot transmit and receive at the same time
- if outgoing and incoming pulses overlap, information is lost
- short pulses with large gaps produce the clearest images
True or False?
Using a longer wavelength produces a more detailed ultrasound image
False.
Shorter wavelengths give better resolution and more detail. The wavelength is chosen to be similar in size to the object being resolved
State two uses of sonar.
Any two from:
- locating fish
- detecting underwater vessels
- mapping the ocean floor
Define wave-particle duality.
Light can behave both as a particle (photons) and as a wave
What is the experimental evidence that light behaves as a particle, and as a wave?
- particle: the photoelectric effect
- wave: diffraction and interference (Young's double-slit experiment)
In the photoelectric effect, each electron can absorb only a ..........
In the photoelectric effect, each electron can absorb only a single photon
True or False?
The wave theory of light predicts a threshold frequency for photoelectric emission
False.
Wave theory does not predict a threshold frequency — it suggests any frequency could emit electrons given enough exposure time. Only the photon (particle) model explains the threshold frequency
Above the threshold frequency, what happens as the intensity of light increases?
More photoelectrons are emitted per second
Why does the photon model explain the existence of a threshold frequency?
Each electron absorbs only one photon of energy $E = h f$ . Only light above the threshold frequency delivers a photon with enough energy to release an electron
According to Einstein, how does light carry energy?
In discrete packets called quanta, or photons, each carrying energy $E = h f$
Define photon.
A massless packet, or quantum, of electromagnetic energy — energy transferred in discrete packets rather than continuously
State the equation for the energy of a photon in terms of frequency.
$E = h f$
- E = energy of the photon (J)
- h = Planck's constant (J s)
- f = frequency (Hz)
Give the equation for photon energy in terms of wavelength.
$E = \frac{h c}{λ}$
- c = speed of light (m s^-1)
- λ = wavelength (m)
The energy of a photon is .......... to its wavelength
The energy of a photon is inversely proportional to its wavelength
True or False?
A longer-wavelength photon carries more energy than a shorter-wavelength photon
False.
Photon energy is inversely proportional to wavelength, so a longer-wavelength photon has lower energy
How does a photon's energy depend on its frequency?
The higher the frequency, the higher the photon energy — energy is directly proportional to frequency ( $E = h f$ )
How do you calculate the number of photons delivered by a light beam in a given time?
- total energy delivered = power × time
- number of photons = total energy ÷ energy per photon
- energy per photon = $h f$ or $\frac{h c}{λ}$

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