Definitions (AQA A-Level Physics): Revision Notes

Definitions

1. A-Scan

• Definition: An ultrasound scanning method where a transducer placed on the body surface emits pulses, with reflections from tissues recorded to create a one-dimensional image.

2. Accommodation

• Definition: The process by which the eye adjusts its lens to focus on objects at various distances.
• Explanation: The ciliary muscles change the lens curvature to alter focal length, allowing clear vision of near or distant objects.

3. Acoustic Impedance

• Definition: The product of the speed of sound in a medium and the density of that medium.
• Formula: $Z = \rho v$, where $Z$ is acoustic impedance, $\rho$ is density, and $v$ is the speed of sound.
• Explanation: Differences in acoustic impedance between tissues influence ultrasound reflexion, essential for medical imaging.

4. Air-Soft Tissue Boundary

• Definition: A boundary with a large difference in acoustic impedance between air and soft tissue, causing nearly all ultrasound waves to reflect.

5. Astigmatism

• Definition: A vision defect caused by an irregularly shaped cornea that leads to distorted images.
• Correction: Astigmatism is corrected using cylindrical lenses that compensate for the uneven curvature of the cornea.

6. B-Scan

• Definition: A more complex ultrasound scanning method that uses echo brightness to create two-dimensional images.
• Application: B-Scans are used for imaging complex structures, such as locating the placenta during pregnancy.

7. Barium Meal

• Definition: A contrast medium containing barium, ingested by patients to improve the visibility of certain structures in X-ray imaging.
• Explanation: Barium's high atomic number increases contrast, allowing clear imaging of the digestive tract.

8. Bone-Soft Tissue Boundary

• Definition: A boundary with a large acoustic impedance difference, causing strong ultrasound reflexion at the interface between bone and soft tissue.

9. Ceruminous Glands

• Definition: Wax-producing glands in the ear canal that protect the eardrum by trapping dust and other particles.
• Explanation: These glands help maintain ear hygiene by producing earwax, which prevents infections.

10. Choroid

• Definition: A vascular layer of the eye that provides oxygen and nutrients to the retina and other eye structures.
• Explanation: The choroid absorbs stray light, reducing glare and enhancing visual clarity.

11. Ciliary Muscles

• Definition: Circular muscle fibres responsible for altering the lens shape during accommodation.
• Function: Contracting or relaxing these muscles adjusts the lens curvature to focus on objects at various distances.

12. Coherent Fibre Bundle

• Definition: A fibre optic bundle where each fibre is arranged in a precise, orderly pattern.
• Application: Coherent bundles maintain image structure, used in endoscopy for internal imaging.

13. Cones

• Definition: Light-sensitive cells in the retina that detect colour and detail under bright light conditions.
• Explanation: Each cone cell connects to a nerve fibre, contributing to high-resolution, colour vision.

14. CT Scans

• Definition: A cross-sectional imaging method using rotating X-ray beams and detectors to create detailed images of body structures.
• Advantages and Limitations: CT scans provide high-resolution images but involve high radiation exposure and are expensive.

15. Decibel (dB)

• Definition: A unit for measuring sound intensity, using a logarithmic scale.
• Explanation: Decibel levels indicate relative intensity, with each $10$ dB increase representing a tenfold increase in sound power.

16. Flat-Panel Detectors

• Definition: Light-sensitive panels used in medical imaging for faster, more sensitive image capture than traditional film.

17. Gamma Camera

• Definition: A device used in positron emission tomography (PET) and other nuclear medicine scans to detect gamma photons.
• Explanation: The camera converts gamma rays into electrical signals, creating images of internal structures.

18. Gelatinous Vitreous Humour

• Definition: The clear, gel-like substance filling the space between the lens and retina in the eye.
• Function: It helps maintain eye shape and allows light transmission to the retina.

19. Hypermetropia

• Definition: Also known as long-sightedness, a condition where distant objects are clear but near objects appear blurry.
• Correction: Convex lenses are used to adjust focal length, allowing clear vision of close objects.

20. Incoherent Fibre Bundle

• Definition: A fibre optic bundle without specific arrangement of fibres, suitable only for light transmission, not imaging.
• Application: Incoherent bundles are used in lighting applications rather than imaging.

21. Iris

• Definition: The coloured part of the eye that controls light entry by adjusting the pupil size.
• Function: It regulates the amount of light reaching the retina, aiding vision in varying light conditions.

22. Linear Attenuation Coefficient

• Definition: The fraction of X-ray intensity reduction per unit thickness of material.
• Explanation: This coefficient helps quantify how X-rays are absorbed in tissues, aiding in diagnostic imaging.

23. Magnetic Resonance Imaging (MRI)

• Definition: A scanning technique using strong magnetic fields and radio waves to produce detailed body images.
• Explanation: MRI is non-ionising and ideal for soft tissue imaging but can be costly and time-consuming.

24. Mass Attenuation Coefficient

• Definition: The linear attenuation coefficient divided by material density, providing attenuation data independent of material thickness.
• Application: This coefficient is used to compare the attenuation properties of different materials.

25. Myopia

• Definition: Also known as short-sightedness, a condition where close objects are clear, but distant objects appear blurry.
• Correction: Concave lenses help focus light correctly onto the retina for clearer distant vision.

26. Optic Papilla (Blind Spot)

• Definition: The part of the retina without light-sensitive cells, where the optic nerve exits the eye.
• Explanation: No image detection occurs here, creating a natural blind spot in vision.

27. Piezoelectric Effect

• Definition: A phenomenon in certain crystals, like quartz, where mechanical deformation produces an electrical charge and vice versa.
• Application: Utilised in ultrasound transducers to generate and receive sound waves for imaging.

28. Positron Emission Tomography (PET) Scans

• Definition: An imaging technique that produces detailed cross-sectional and 3D images of the body.
• Explanation: PET scans involve injecting a radionuclide that emits positrons. These positrons interact with electrons in the body, producing gamma photons, which are detected to create images of internal structures, commonly used to detect cancers and monitor brain function.

29. Retina

• Definition: The light-sensitive layer located at the back of the eye, which contains cells that detect light and send visual information to the brain.
• Explanation: The retina contains rods and cones, which are specialised for low-light and colour vision, respectively. It processes light into neural signals that the brain interprets as images.

30. Rods

• Definition: Light-sensitive cells in the retina that detect low light intensities and produce black-and-white images.
• Explanation: Rods are crucial for night vision but do not detect colour. They provide less detailed images than cones, focusing on grayscale images.

31. Scintillator

• Definition: A device that converts X-ray photons into visible light photons, allowing for easier detection and imaging.
• Application: Scintillators are used in medical imaging equipment, such as digital X-ray systems, to improve the efficiency of photon detection.

32. Sclera

• Definition: The white outer coating that covers the eye, providing protection and maintaining its shape.
• Explanation: The sclera is clear at the front, forming the cornea, but opaque around the rest of the eye, blocking light from entering the sides.

33. Suspensory Ligament

• Definition: Fibres that hold the eye's lens in place, attaching it to the ciliary muscles.
• Function: Suspensory ligaments work with ciliary muscles to adjust the lens's shape during accommodation, helping the eye focus on objects at different distances.

34. Transducer

• Definition: A device with a piezoelectric crystal that produces ultrasound waves and detects reflected echoes.
• Explanation: In medical ultrasound, transducers both emit and detect sound waves, creating images of internal structures by analysing reflected signals.

35. Ultrasound

• Definition: Sound waves with frequencies above $20$ kHz, beyond the range of human hearing.
• Application: Ultrasound is used in medical imaging because it can pass through soft tissues and reflect off boundaries, enabling non-invasive imaging of organs and foetuses.

36. Visual Purple (Rhodopsin)

• Definition: A light-sensitive pigment found in rod cells that breaks down in response to light and regenerates in darkness.
• Explanation: Visual purple is essential for night vision, as it helps rods detect low light. Its bleaching and regeneration process adapts vision to varying light levels.

37. Watery Aqueous Humour

• Definition: The clear fluid filling the space between the cornea and lens in the eye.
• Function: The aqueous humour maintains intraocular pressure, supplies nutrients to the cornea and lens, and removes waste products.