3.2.4

Diffraction

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Single Slit Diffraction

When light is shone through a single slit, it diffracts and produces a distinct pattern.

Laser light

Laser light

  • Laser light is monochromatic and coherent.
    • Monochromatic means that the light is all the same wavelength.
    • Coherent means the light is in phase and has the same frequency.
  • If the wavelength of the laser light is roughly the same as the width of the single slit, we see a diffraction pattern.
Laser diffraction pattern

Laser diffraction pattern

  • The image shows the pattern we expect to see in a successful single slit diffraction experiment.
    • We see a bright central fringe with alternating dark and bright fringes either side.
  • The bright fringes are caused by constructive interference.
  • The dark fringes are caused by destructive interference.
White light diffraction pattern

White light diffraction pattern

  • If we use white light instead of a laser, we see a different but similar diffraction pattern.
  • White light is made up of all the visible colours of light. It is not monochromatic.
  • Each colour is diffracted by different amounts.
    • Red has the longest wavelength, so is diffracted the most and appears on the outside of the fringes.
    • Blue has the shortest wavelength, so is diffracted the least and appears on the inside of the fringes.

Width of Central Diffraction Maximum

The width of the central diffraction maximum varies with slit width and wavelength.

Slit width

Slit width

  • If we increase the slit width, the width of the central maximum will decrease.
  • This is because the diffraction effects will decrease.
    • Imagine if the width becomes very large. The light will just pass straight through without being diffracted at all.
  • The intensity of the central maximum will increase because the photons are less spread out.
Wavelength

Wavelength

  • If we increase the wavelength of the incident light, the width of the central maximum will increase.
  • This is because diffraction effects will increase.
    • Think of longer wavelength light (red) being on the outside of white light fringes compared with shorter wavelength light (blue) being on the inside.
  • The intensity of the central maximum will decrease because the photons are more spread out.

Diffraction Gratings

When light is shone through a grating with slit widths comparable to its wavelength, the light is diffracted into a pattern of bright and dark lines.

Maxima and minima

Maxima and minima

  • The bright and dark lines correspond to where constructive or destructive interference has taken place.
  • The positions of maxima are called 'orders'.
  • This diffraction grating has many slits.
Maxima and minima 2

Maxima and minima 2

  • On a screen there will be a central point. This point is called the "zero order".
  • The zero order line has the largest brightness out of all the other lines.
  • Either side of the central line lies the first order lines. The zero order line lies in the middle of the two.
  • The further away from the central point, the dimmer the lines are and the more orders that are visible.
Number of slits

Number of slits

  • The more slits in the grating, the sharper the pattern of lines on the screen.
  • The pattern produced is the same as that of Young's double slit experiment, except that the lines are sharper and more easily measurable.
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1

Measurements & Errors

1.1

Measurements & Errors

1.1.1SI Base Units1.1.2Combining SI Units1.1.3Prefixes of Units1.1.4Standard Form1.1.5Converting Units1.1.6End of Topic Test - Units & Prefixes1.1.7Limitation of Physical Measurements1.1.8Uncertainty1.1.9Estimation1.1.10End of Topic Test - Measurements & Errors1.1.11Diagnostic Misconceptions - Converting between SI1.1.12Diagnostic Misconceptions - SI & Area/Volume
2

Particles & Radiation

2.1

Particles

2.1.1Atomic Model2.1.2Specific Charge, Protons & Neutron Numbers2.1.3End of Topic Test - Atomic Model2.1.4Isotopes2.1.5Stable & Unstable Nuclei2.1.6End of Topic Test - Isotopes & Nuclei2.1.7A-A* (AO3/4) - Stable & Unstable Nuclei2.1.8Particles, Antiparticles & Photons2.1.9Particle Interactions2.1.10Classification of Particles2.1.11End of Topic Test - Particles & Interactions2.1.12Quarks & Antiquarks2.1.13Application of Conservation Laws2.1.14End of Topic Test - Leptons & Quarks2.1.15Exam-Style Question - Radioactive Decay
2.2

Electromagnetic Radiation & Quantum Phenomena

2.2.1The Photoelectric Effect2.2.2The Photoelectric Effect Explanation2.2.3End of Topic Test - The Photoelectric Effect2.2.4Collisions of Electrons with Atoms2.2.5Energy Levels & Photon Emission2.2.6Wave-Particle Duality2.2.7End of Topic Test - Absorption & Emission2.2.8Diagnostic Misconceptions - Electron Volts2.2.9Diagnostic Misconceptions - Converting eV & Joules
3

Waves

3.1

Progressive & Stationary Waves

3.1.1Progressive Waves3.1.2Wave Speed & Phase Difference3.1.3Longitudinal & Transverse Waves3.1.4End of Topic Test - Progressive Waves3.1.5Polarisation3.1.6Stationary Waves3.1.7Stationary Waves 23.1.8End of Topic Test - Polarisation & Stationary Wave3.1.9A-A* (AO3/4) - Stationary Waves
3.2

Refraction, Diffraction & Interference

3.2.1Interference3.2.2Interference 23.2.3End of Topic Test - Interference3.2.4Diffraction3.2.5Diffraction Gratings3.2.6End of Topic Test - Diffraction3.2.7Refraction at a Plane Surface3.2.8Internal Reflection & Fibre Optics3.2.9End of Topic Test - Refraction3.2.10Exam-Style Question - Waves
4

Mechanics & Materials

4.1

Force, Energy & Momentum

4.1.1Scalars & Vectors4.1.2Vector Problems4.1.3End of Topic Test - Scalars & Vectors4.1.4Moments4.1.5Centre of Mass4.1.6End of Topic Test - Moments & Centre of Mass4.1.7Motion in a Straight Line4.1.8Graphs of Motion4.1.9Bouncing Ball Example4.1.10End of Topic Test - Motion in a Straight Line4.1.11Acceleration Due to Gravity4.1.12Projectile Motion4.1.13Friction4.1.14Terminal Speed4.1.15End of Topic Test - Acceleration Due to Gravity4.1.16Newton's Laws4.1.17Momentum4.1.18Momentum 24.1.19End of Topic Test - Newton's Laws & Momentum4.1.20A-A* (AO3/4) - Newton's Third Law4.1.21Work & Energy4.1.22Power & Efficiency4.1.23Conservation of Energy4.1.24End of Topic Test - Work, Energy & Power4.1.25Exam-Style Question - Forces4.1.26Diagnostic Misconceptions - Perpendicular Vectors4.1.27Diagnostic Misconceptions - Weight Acts Downwards4.1.28Diagnostic Misconceptions - Acceleration Direction4.1.29Diagnostic Misconceptions - Stationary Objects4.1.30Diagnostic Misconceptions - Action & Reaction4.1.31Diagnostic Misconceptions - RF Direction4.1.32Diagnostic Misconceptions - RF Acceleration4.1.33Diagnostic Misconceptions - Change in Momentum
4.2

Materials

4.2.1Density4.2.2Bulk Properties of Solids4.2.3Energy in Materials4.2.4Young Modulus4.2.5End of Topic Test - Materials
5

Electricity

5.1

Current Electricity

5.1.1Basics of Electricity5.1.2Current-Voltage Characteristics5.1.3End of Topic Test - Basics of Electricity5.1.4Resistivity5.1.5Superconductivity5.1.6A-A* (AO3/4) - Superconductivity5.1.7End of Topic Test - Resistivity & Superconductors5.1.8Circuits5.1.9Power and Conservation5.1.10Potential Divider5.1.11Emf & Internal Resistance5.1.12End of Topic Test - Power & Potential5.1.13Exam-Style Question - Resistance5.1.14Diagnostic Misconceptions - Constant Current5.1.15Diagnostic Misconceptions - Potential Difference
6

Further Mechanics & Thermal Physics (A2 only)

6.1

Periodic Motion (A2 only)

6.1.1Circular Motion6.1.2Circular Motion 26.1.3End of Topic Test - Circular Motion6.1.4Simple Harmonic Motion6.1.5Simple Harmonic Systems6.1.6Energy in Simple Harmonic Motion6.1.7Resonance6.1.8End of Topic Test - Simple Harmonic Motion6.1.9A-A* (AO3/4) - Simple Harmonic Motion
6.2

Thermal Physics (A2 only)

6.2.1Thermal Energy Transfer6.2.2Thermal Energy Transfer Experiments6.2.3Ideal Gases6.2.4Ideal Gases 26.2.5Boyle's Law & Charles' Law6.2.6Molecular Kinetic Theory Model6.2.7Molecular Kinetic Theory Model 26.2.8End of Topic Test - Thermal Energy & Ideal Gases6.2.9Exam-Style Question - Ideal Gases6.2.10Diagnostic Misconceptions - Material & Mass6.2.11Diagnostic Misconceptions - No Energy
7

Fields & Their Consequences (A2 only)

7.1

Fields (A2 only)

7.1.1Fields
7.2

Gravitational Fields (A2 only)

7.2.1Newton's Law7.2.2Gravitational Field Strength7.2.3Gravitational Potential7.2.4Orbits of Planets & Satellites7.2.5Escape Velocity & Synchronous Orbits7.2.6End of Topic Test - Gravitational Fields
7.3

Electric Fields (A2 only)

7.3.1Coulomb's Law7.3.2Electric Field Strength7.3.3Electric Field Strength 27.3.4Electric Potential7.3.5End of Topic Test - Electric Fields7.3.6A-A* (AO3/4) - Electric and Gravitational Field
7.4

Capacitance (A2 only)

7.4.1Capacitance7.4.2Parallel Plate Capacitor7.4.3Energy Stored by a Capacitor7.4.4Capacitor Discharge7.4.5Capacitor Charge
7.5

Magnetic Fields (A2 only)

7.5.1Magnetic Flux Density7.5.2End of Topic Test - Capacitance & Flux Density7.5.3Moving Charges in a Magnetic Field7.5.4Magnetic Flux & Flux Linkage7.5.5Electromagnetic Induction7.5.6Electromagnetic Induction 27.5.7Alternating Currents7.5.8Operation of a Transformer7.5.9Magnetic Flux Density7.5.10End of Topic Test - Electromagnetic Induction
8

Nuclear Physics (A2 only)

8.1

Radioactivity (A2 only)

8.1.1Rutherford Scattering8.1.2Alpha & Beta Radiation8.1.3Gamma Radiation8.1.4Radioactive Decay8.1.5Half Life8.1.6End of Topic Test - Radioactivity8.1.7Nuclear Instability8.1.8Nuclear Radius8.1.9Mass & Energy8.1.10Binding Energy8.1.11Induced Fission8.1.12Safety Aspects of Nuclear Reactors8.1.13End of Topic Test - Nuclear Physics8.1.14A-A* (AO3/4) - Nuclear Fusion
9

Option: Astrophysics (A2 only)

9.1

Telescopes (A2 only)

9.1.1Astronomical Telescopes9.1.2Reflecting Telescopes9.1.3Single Dish Radio Telescopes9.1.4Large Diameter Telescopes
9.2

Classification of Stars (A2 only)

9.2.1Classification by Luminosity9.2.2Absolute Magnitude9.2.3Black Body Radiation9.2.4Stellar Spectral Classes9.2.5Hertzsprung-Russell Diagrams9.2.6Astronomical Objects
9.3

Cosmology (A2 only)

9.3.1Doppler Effect9.3.2Hubble's Law9.3.3Quasars9.3.4Detecting Exoplanets
10

Option: Medical Physics (A2 only)

10.1

Physics of the Eye (A2 only)

10.1.1Physics of Vision10.1.2Defects of Vision10.1.3Lenses10.1.4Correcting Defects of Vision
10.2

Physics of the Ear (A2 only)

10.2.1Structure of the Ear10.2.2Sensitivity of the Ear10.2.3Hearing Defects
10.3

Biological Measurement (A2 only)

10.3.1Electrocardiography (ECG)
10.4

Non-Ionising Imaging (A2 only)

10.4.1Ultrasound Imaging10.4.2Ultrasound Imaging 210.4.3Fibre Optics & Endoscopy10.4.4Magnetic Resonance Scanning
10.5

X-Ray Imaging (A2 only)

10.5.1Diagnostic X-Rays10.5.2X-Ray Image Processing10.5.3Absorption of X-Rays10.5.4CT Scanners
10.6

Radionuclide Imaging & Therapy (A2 only)

10.6.1Imaging Techniques10.6.2Half Life10.6.3Gamma Camera10.6.4High Energy X-Rays10.6.5Radioactive Implants10.6.6Imaging Comparisons
11

Option: Engineering Physics (A2 only)

11.1

Rotational Dynamics (A2 only)

11.1.1Moment of Inertia11.1.2Rotational Kinetic Energy11.1.3Rotational Motion11.1.4Torque & Angular Acceleration11.1.5Angular Momentum11.1.6Angular Work & Power
11.2

Thermodynamics & Engines (A2 only)

11.2.1First Law of Thermodynamics11.2.2Non-Flow Processes11.2.3p-V Diagrams11.2.4Engine Cycles11.2.5Second Law & Engines11.2.6Reversed Heat Engines
12

Option: Turning Points in Physics (A2 only)

12.1

Discovery of the Electron (A2 only)

12.1.1Cathode Rays12.1.2Thermionic Electron Emission12.1.3Electron Specific Charge12.1.4Millikan's Experiment
12.2

Wave-Particle Duality (A2 only)

12.2.1Newton's & Huygen's Theories of Light12.2.2Electromagnetic Waves12.2.3Photoelectricity12.2.4Wave-Particle Duality12.2.5Electron Microscopes
12.3

Special Relativity (A2 only)

12.3.1Michelson-Morley Experiment12.3.2Einstein's Theory of Special Relativity12.3.3Time Dilation12.3.4Length Contraction12.3.5Mass & Energy

Practice questions on Diffraction

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What are the bright fringes of a diffraction pattern caused by?Multiple choice
  2. 2
    In the single slit diffraction experiment, what effect does increasing the wavelength of incoming light have on the width of the central maximum?Multiple choice
  3. 3
    In the single slit diffraction experiment, what effect does decreasing the wavelength of incoming light have on the width of the central maximum?Multiple choice
  4. 4
    Explain what effect increasing the wavelength of incoming light in the single slit diffraction experiment will have on the diffraction pattern produced.Flashcard
  5. 5
    What is the zeroth order maxima?Multiple choice
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End of Topic Test - Interference

Diffraction Gratings