Definitions (AQA A-Level Physics): Revision Notes

Definitions

1. Absolute Magnitude (M)

• Definition: The apparent magnitude (brightness) an object would have if it were placed exactly 10 parsecs from Earth.
• Explanation: Absolute magnitude allows for comparison of intrinsic brightness of celestial objects, removing the effects of distance.

2. Achromatic Doublet

• Definition: A lens system composed of a convex crown glass lens and a concave flint glass lens that focuses light to the same point.
• Purpose: This design corrects both spherical and chromatic aberration, improving image quality by minimising colour fringing.

3. Apparent Magnitude (m)

• Definition: The brightness of a celestial object as seen from Earth, influenced by its actual luminosity and distance.
• Explanation: Apparent magnitude is used to rank objects' brightness; a lower magnitude number means a brighter object.

4. Arcsecond

• Definition: A unit for measuring small angles; one arcsecond equals $\frac{1}{3600}$ of a degree.
• Example: Used in astronomy to describe the apparent size of distant celestial objects.

5. Astronomical Unit (AU)

• Definition: The average distance between the Earth and the Sun, approximately 1.496 × 10⁸ km.
• Usage: A standard unit for measuring distances within our solar system.

6. Big Bang Theory

• Definition: The theory that the universe began as a small, hot, and dense state and has been expanding ever since.
• Explanation: Evidence supporting this theory includes the cosmic microwave background radiation (CMBR) and the redshift of galaxies.

7. Binary Star System

• Definition: A system of two stars orbiting a common centre of mass.
• Example: Observations of binary systems help determine star masses through their orbital mechanics.

8. Black Body Radiator

• Definition: An object that perfectly absorbs and emits all wavelengths of radiation.
• Explanation: Black body radiation is used to approximate the spectrum of stars, helping determine temperature based on emitted wavelengths.

9. Black Hole

• Definition: A region of space with gravitational pull so strong that nothing, not even light, can escape.
• Explanation: Black holes form when massive stars collapse under their own gravity, creating an event horizon from which escape is impossible.

10. Cassegrain Reflecting Telescope

• Definition: A type of reflecting telescope with a concave primary mirror and a convex secondary mirror.
• Explanation: Light reflects from the primary mirror to the secondary, focusing behind the primary mirror, allowing for compact telescope designs.

11. Charge-Coupled Device (CCD)

• Definition: A device with light-sensitive pixels that produce charge when exposed to light, utilising the photoelectric effect.
• Application: CCDs are used in modern telescopes and cameras to capture images of faint astronomical objects.

12. Chromatic Aberration

• Definition: An optical effect where different wavelengths focus at different points, causing colour fringing.
• Solution: Corrected with achromatic lenses or by using reflecting telescopes, which avoid chromatic aberration entirely.

13. Collecting Power

• Definition: The ability of a telescope's lens or mirror to gather light, proportional to the area of the objective lens or mirror.
• Explanation: Larger telescopes have higher collecting power, allowing observation of fainter objects.

14. Concave (Diverging) Lens

• Definition: A lens that spreads out light rays (causes them to diverge).
• Application: Used in correcting short-sightedness and in optical instruments.

15. Convex (Converging) Lens

• Definition: A lens that focuses light rays to a single point.
• Application: Used in magnifying glasses and as objective lenses in refracting telescopes.

16. Cosmic Microwave Background Radiation (CMBR)

• Definition: Radiation left over from the Big Bang, now redshifted to microwave wavelengths due to the universe's expansion.
• Explanation: CMBR provides evidence for the Big Bang, representing the early universe's residual heat.

17. Doppler Effect

• Definition: The change in wavelength of waves as a source moves relative to an observer.
• Example: Redshift indicates an object moving away (wavelengths stretch), while blueshift indicates an object moving closer (wavelengths compress).

18. Eclipsing Binaries

• Definition: A binary star system where the stars' orbits align with Earth's line of sight, causing them to pass in front of each other.
• Observation: Eclipsing binaries show characteristic brightness dips, used to study star sizes and orbits.

19. Event Horizon

• Definition: The boundary of a black hole beyond which nothing can escape, as the escape velocity equals the speed of light.
• Explanation: Inside the event horizon, all paths lead towards the black hole's centre, rendering escape impossible.

20. Exoplanet

• Definition: A planet that orbits a star outside our solar system.
• Detection: Exoplanets are often difficult to observe directly, but methods like the transit method and radial velocity detect their presence.

21. Eyepiece Lens

• Definition: The lens in a telescope that magnifies the image created by the objective lens.
• Explanation: Produces a virtual image at infinity, reducing eye strain and allowing for comfortable long-term viewing.

22. Focal Length (f)

• Definition: The distance from the centre of a lens to its focal point.
• Application: Longer focal lengths in telescopes allow for higher magnification of distant objects.

23. Hipparcos Scale

• Definition: A scale for classifying stars by apparent magnitude, from 1 (brightest) to 6 (faintest visible to the naked eye).
• Explanation: The scale is logarithmic; a magnitude 1 star is 100 times more intense than a magnitude 6 star.

24. Hubble's Law

• Definition: The recession speed of galaxies is proportional to their distance from Earth, with Hubble's constant as the proportionality factor.
• Formula: $v = H_0 d$
• Explanation: Hubble's Law supports the theory of an expanding universe.

25. Hydrogen Balmer Spectrum

• Definition: The spectrum produced when hydrogen electrons transition from higher energy levels to the n=2 level.
• Application: The Balmer lines are used to estimate a star's temperature based on the strength of specific spectral lines.

26. Intensity

• Definition: The power per unit area received from a star or other light source.
• Explanation: In astrophysics, intensity helps determine the energy output of stars and their apparent brightness.

27. Lens Power

• Definition: A measure of a lens's ability to focus parallel light beams, directly related to its focal length.
• Formula: $P = \frac{1}{f}$, where $f$ is the focal length in metres.
• Explanation: A lens with a shorter focal length has greater power, enabling it to bring light to focus more quickly.

28. Light Year ($ly$)

• Definition: The distance light travels in one year in a vacuum, approximately $9.46 \times 10^{12}$ km.
• Usage: Used to express distances to stars and galaxies, highlighting the vast scales involved in space.

29. Long-Lived Gamma Ray Burst

• Definition: Gamma-ray emissions lasting from 10 to 1000 seconds, thought to be linked to type II supernovae (massive star collapse).
• Explanation: These bursts provide insights into the life cycles of massive stars and the energy released in supernova events.

30. Luminosity

• Definition: The total energy per second (power) emitted by a star, often measured in watts.
• Explanation: Luminosity depends on a star's size and temperature, influencing its brightness when observed from Earth.

31. Magnifying Power / Angular Magnification (M)

• Definition: The ratio of the image angle to the object angle for a telescope.
• Formula: $M = \frac{f_{\text{objective}}}{f_{\text{eyepiece}}}$
• Explanation: Higher magnification allows for detailed viewing of distant objects but can reduce field of view and brightness.

32. Main Sequence Star

• Definition: A star in hydrostatic equilibrium, where gravitational forces balance the outward pressure from nuclear fusion.
• Explanation: During this stable phase, hydrogen fuses into helium in the star's core, defining most of a star's life.

33. Neutron Star

• Definition: A compact, dense star formed after a supernova when protons and electrons combine to form neutrons.
• Characteristics: Neutron stars have extremely high densities and can have strong magnetic fields, often observed as pulsars.

34. Normal Adjustment

• Definition: In a telescope, the condition where the distance between the objective and eyepiece equals the sum of their focal lengths.
• Explanation: This configuration ensures the image is at infinity, reducing eye strain and allowing for easier observation.

35. Objective Lens

• Definition: The primary lens in a telescope that gathers light to form a real image.
• Explanation: Larger objective lenses improve a telescope's light-gathering power, allowing it to observe faint objects.

36. Parallax

• Definition: The apparent shift in position of a nearby star against distant background stars due to Earth's orbit.
• Application: Parallax measurements help determine the distance to nearby stars, using the angle of shift.

37. Parsec (pc)

• Definition: The distance at which a star's parallax angle is 1 arcsecond, equal to approximately 3.26 light years.
• Usage: Parsecs are a standard unit for measuring astronomical distances.

38. Primary Mirror

• Definition: The main mirror in a reflecting telescope that collects and focuses light.
• Explanation: Primary mirrors are crucial for light gathering and are often large to maximise observing potential.

39. Principal Axis

• Definition: The central line passing through the middle of a lens or mirror, perpendicular to its surface.
• Explanation: The principal axis is used as a reference for positioning and directing light paths in optical devices.

40. Principal Focus ($F$)

• Definition: In a converging lens, the point where parallel light rays meet; in a diverging lens, the point where light rays appear to originate.
• Application: The focal point is essential in lens design for focusing and image formation.

41. Protostar

• Definition: A young star in the early stage of formation, where gas and dust collapse under gravity before fusion begins.
• Explanation: Protostars heat up as they contract; fusion initiates once the core reaches a sufficient temperature, marking the star's main sequence start.

42. Quantum Efficiency

• Definition: The percentage of photons incident on a CCD that release an electron.
• Explanation: Higher quantum efficiency improves image brightness and detail in telescopic and camera sensors.

43. Quasar

• Definition: A distant, highly energetic active galactic nucleus powered by a supermassive black hole.
• Explanation: Quasars have strong redshifts, indicating immense distance and velocity, and emit radiation equal to entire galaxies.

44. Radial Velocity Method

• Definition: A method for detecting exoplanets by observing shifts in a star's spectrum due to gravitational "wobble."
• Explanation: The redshift and blueshift of a star's light reveal its motion, suggesting the presence of an orbiting exoplanet.

45. Rayleigh Criterion

• Definition: A criterion for resolving two objects based on diffraction limits; two points are resolvable if the central maximum of one falls outside the first minimum of the other.
• Application: This criterion is fundamental in designing optical systems with high resolution.

46. Real Image

• Definition: An image formed when light rays converge after passing through a lens, which can be projected onto a screen.
• Example: Real images are created by cameras and telescopes, where light focuses on a detector or film.

47. Red Giant

• Definition: A star stage for stars with masses less than 3 solar masses, where hydrogen has been depleted and helium fusion begins.
• Explanation: Red giants expand and cool, developing a large, bright appearance as fusion of heavier elements starts.

48. Red Shift (z)

• Definition: The shift of spectral lines towards the red end of the spectrum, indicating an object moving away from Earth.
• Application: Redshift is evidence of the universe's expansion, with greater shifts indicating more distant galaxies.

49. Red Supergiant

• Definition: A later stage in massive stars (over 3 solar masses) where they expand significantly and fuse elements up to iron.
• Explanation: Red supergiants precede supernova explosions and can have sizes hundreds of times greater than the Sun.

50. Reflecting Telescope

• Definition: A telescope that uses mirrors to gather and focus light, rather than lenses.
• Advantages: Reflecting telescopes avoid chromatic aberration and can have larger apertures for observing faint objects.

51. Refracting Telescope

• Definition: A telescope that uses lenses to gather and focus light.
• Explanation: Traditional refracting telescopes were the first telescopes, but they are limited by chromatic aberration and lens weight.

52. Resolving Power

• Definition: The ability of a telescope to distinguish between two close objects.
• Explanation: Higher resolving power allows for sharper images and is critical for observing fine details in astronomical objects.

53. Schwarzschild Radius $( R_S )$

• Definition: The radius of the event horizon of a black hole, beyond which nothing can escape.
• Formula: $R_S = \frac{2GM}{c^2}$, where $G$ is the gravitational constant, $M$ is the mass of the black hole, and $c$ is the speed of light.

54. Short-Lived Gamma Ray Burst

• Definition: Gamma-ray bursts lasting from 0.01 to 1 second, thought to result from events like neutron star mergers or neutron stars falling into black holes.
• Explanation: These bursts provide valuable insights into extreme cosmic events and the formation of black holes.

55. Spectroscopic Binaries

• Definition: A binary star system where the stars are so close together that they cannot be visually resolved with a telescope.
• Detection: These systems are identified by observing Doppler shifts in the stars' spectral lines as they move towards or away from Earth.
• Explanation: Changes in wavelength (redshift and blueshift) provide information on the stars' motion, confirming the presence of a binary system.

56. Spherical Aberration

• Definition: An optical defect caused by the curvature of a lens or mirror, leading to different focal points for light rays passing through the edges versus the centre.
• Effect: Spherical aberration can make images appear blurry or distorted, as rays do not converge at a single point.
• Solution: This issue is minimised using parabolic mirrors or corrective lenses.

57. Stefan's Law

• Definition: States that the power output (luminosity) of a star is proportional to its surface area and the fourth power of its absolute temperature.
• Formula: $L = \sigma A T^4$, where $L$ is luminosity, $\sigma$ is Stefan-Boltzmann constant, $A$ is surface area, and $T$ is temperature.
• Explanation: Stefan's Law explains why larger, hotter stars are much more luminous than smaller, cooler stars.

58. Supernova

• Definition: A massive explosion occurring at the end of a large star's life cycle when it collapses inward and rebounds in a shockwave.
• Explanation: Supernovae create and distribute heavy elements like iron and nickel into space, enriching future star-forming regions. Stars above 1.4 solar masses (Chandrasekhar limit) are more likely to end as supernovae.

59. Transit Method

• Definition: A method for detecting exoplanets by monitoring dips in a star's light as a planet passes in front of it (transit).
• Explanation: When an exoplanet transits its star, it temporarily reduces the star's brightness. By measuring this dip, astronomers can deduce the planet's size, orbit, and sometimes its atmospheric composition.

60. Type I Supernova

• Definition: A supernova that occurs when a star in a binary system accumulates matter from its companion, leading to an explosion once it reaches a critical mass.
• Explanation: Type I supernovae do not contain hydrogen lines in their spectra, distinguishing them from Type II supernovae.

61. Type Ia Supernova

• Definition: A specific type of Type I supernova that occurs when a white dwarf explodes after accreting enough matter to reach the Chandrasekhar limit.
• Significance: Type Ia supernovae are used as standard candles for measuring astronomical distances because they consistently reach the same maximum brightness (absolute magnitude of $-19.3$).

62. Type II Supernova

• Definition: A supernova resulting from the collapse of a high-mass star (greater than 8 solar masses) once it exhausts its nuclear fuel.
• Characteristics: Type II supernovae exhibit hydrogen lines in their spectra, as the star's outer layers contain hydrogen.

63. Virtual Image

• Definition: An image formed by a lens on the same side as the object where the light rays do not actually converge, so the image cannot be projected onto a screen.
• Example: Virtual images are produced in magnifying glasses and eyepieces of telescopes, appearing larger than the object.

64. White Dwarf

• Definition: A dense, small remnant of a star with a mass less than 1.4 solar masses, left after it has exhausted its nuclear fuel and no longer supports fusion.
• Explanation: White dwarfs are supported by electron degeneracy pressure, preventing further collapse. They gradually cool and fade over billions of years.

65. Wien's Displacement Law

• Definition: A law stating that the peak wavelength of radiation emitted by a black body is inversely proportional to its absolute temperature.
• Formula: $\lambda_{\text{peak}} = \frac{b}{T}$, where $b$ is Wien's constant and $T$ is temperature.
• Explanation: Wien's Law helps determine the temperature of stars by analysing the peak wavelength of their emitted light; hotter stars emit light at shorter wavelengths (e.g., blue), while cooler stars emit at longer wavelengths (e.g., red).