AQA A LEVEL PHYSICS ASTROPHYSICS
SECTION A PAPER 3 STUDY GUIDE 2026
Definition of Principal focus (Focal Point) - answersThe point through which all light
parallel to the axis of the lens passes through
Definition of Focal length - answersThe distance between the centre of the lens and the
focal point.
Converging lenses - answersConvex (thicker in middle), bend light rays towards each
other
Ray diagram - horizontal axis - answersPrincipal axis
Ray diagram - vertical axis - answersLens axis
Rays parallel to principal axis - answersAxial rays
Each ray is from top of the object
Separate light rays all from same position of the object
Rays that aren't parallel to the principal axis - answersNon-axial rays
Ray diagram - focal plane - answersPlane perpendicular to the principal axis, contains
the principal focus
Formation of images by converging lenses - object beyond 2f - answersReal, inverted,
diminished image
Formation of images by converging lenses - object between 0f and 1f - answersVirtual,
upright, magnified
Formation of images by converging lenses - object at 1f - answersReal, inverted image,
same size
Formation of images by converging lenses - between 1f and 2f - answersReal, inverted,
magnified image
Ray diagram of a refracting telescope in normal adjustment - answersParallel non-axial
rays from top of object
Refracted through objective lens (thinner)
Rays meet at focal point, forming real, inverted, diminished image
Pass through eye piece lens (thicker)
Virtual, inverted, magnified image formed at infinity
,Objective lens - answersConverges ray from object to form a real, inverted, diminished
image inside the telescope
Eye piece lens - answersActs as a magnifying glass on real image to form a virtual,
inverted, magnified image at infinity
Must be within 1 focal length of the real image formed inside the telescope
Angular magnification in normal adjustment - answersM = B/a = fo/fe
M = angular magnification
B- angle to image (angle subtended by image at eye)
a- angle to object (angle subtended by object at unaided eye)
fo = focal length for objective lens
fe = focal length for eye piece lens
fo >> fe
Ray diagram of a reflecting telescope (Cassegrain arrangement) - answersAxial rays
are reflected off parabolic concave primary mirror (principal focus is behind the
secondary mirror)
Reflected onto convex secondary mirror and focused to the principal focus of the
secondary mirror through a hole in primary mirror (focus is behind primary mirror)
Refracted by eye piece lens, forming virtual, inverted, magnified image at infinity
Definition spherical aberration - lens - answersWhere parallel rays at different distances
from principal axis are brought to focus at different points (on principal axis).
Don't converge at the same point
How to solve spherical aberration - answersUse a parabolic mirror
Large mirrors of good quality are easier to make
Can be supported from underneath, so less distortion
Diagram of spherical aberration of a mirror - answersrays further from principal axis are
focused to a point closer to mirror
If mirror is not perfectly parabolic
Effect on image of spherical aberration - answersImage is blurred
Can happen at both primary and secondary mirrors
Secondary also blocks light and there is some diffraction around it
Definition chromatic aberration - answersWhere different wavelengths are refracted by
different amounts resulting in different focal lengths for different wavelengths (colours)
Blue refracted more than red
Building larger lenses of good quality is difficult and expensive
Very heavy and can only be supported by edges to not block light, causing distortion
, Effect on image of chromatic aberration - answersImages have multicoloured, blurred
edges
Diagram of chromatic aberration from a lens - answersblue light (shorter wavelengths)
are focused closer to lens that red light (longer wavelengths)
Minimum angular resolution
(Resolving power) - answersThe smallest angular separation at which the instrument
can distinguish two points
Smaller angle, better resolving power
Definition Rayleigh criteria - answerstwo objects can be just be resolved when the Airy
disc of one object lies at least as far away as the first minimum of the diffraction pattern
of the other.
θ = λ/D
θ = minimum angular resolution (radians)
λ = wavelength of EM radiation (m)
D = diameter of aperture (m)
- bigger diameter, smaller the angle, greater resolving power
Definition of Airy Disc - answersThe bright central maximum (spot) of the diffraction
pattern produced when light from a point source passes through a circular aperture.
Sketch of diffraction pattern from a circular aperture - answersnote that width of airy disc
(central bright spot) is twice the width of bright fringes
Sketch illustrating Rayleigh criteria - answers
Structure and operation of a CCD - answers• A CCD is silicon chip divided into picture
elements (pixels).
• Incident photons cause electrons to be released.
• The number of electrons liberated is proportional to the intensity of the light.
• These electrons are trapped in 'potential wells' in the CCD.
• An electron pattern is built up which is identical to the image formed on the CCD.
• When exposure is complete, the charge is processed to give an image.
Definition of quantum efficiency - answersQE = number of photons detected/number of
photons incident x 100
Proportion of the incident photons that are detected by a light detector (proportional of
incident photons that release electrons)
Quantum efficiency of a CCD - answersQuantum efficiency of CCD >80%
SECTION A PAPER 3 STUDY GUIDE 2026
Definition of Principal focus (Focal Point) - answersThe point through which all light
parallel to the axis of the lens passes through
Definition of Focal length - answersThe distance between the centre of the lens and the
focal point.
Converging lenses - answersConvex (thicker in middle), bend light rays towards each
other
Ray diagram - horizontal axis - answersPrincipal axis
Ray diagram - vertical axis - answersLens axis
Rays parallel to principal axis - answersAxial rays
Each ray is from top of the object
Separate light rays all from same position of the object
Rays that aren't parallel to the principal axis - answersNon-axial rays
Ray diagram - focal plane - answersPlane perpendicular to the principal axis, contains
the principal focus
Formation of images by converging lenses - object beyond 2f - answersReal, inverted,
diminished image
Formation of images by converging lenses - object between 0f and 1f - answersVirtual,
upright, magnified
Formation of images by converging lenses - object at 1f - answersReal, inverted image,
same size
Formation of images by converging lenses - between 1f and 2f - answersReal, inverted,
magnified image
Ray diagram of a refracting telescope in normal adjustment - answersParallel non-axial
rays from top of object
Refracted through objective lens (thinner)
Rays meet at focal point, forming real, inverted, diminished image
Pass through eye piece lens (thicker)
Virtual, inverted, magnified image formed at infinity
,Objective lens - answersConverges ray from object to form a real, inverted, diminished
image inside the telescope
Eye piece lens - answersActs as a magnifying glass on real image to form a virtual,
inverted, magnified image at infinity
Must be within 1 focal length of the real image formed inside the telescope
Angular magnification in normal adjustment - answersM = B/a = fo/fe
M = angular magnification
B- angle to image (angle subtended by image at eye)
a- angle to object (angle subtended by object at unaided eye)
fo = focal length for objective lens
fe = focal length for eye piece lens
fo >> fe
Ray diagram of a reflecting telescope (Cassegrain arrangement) - answersAxial rays
are reflected off parabolic concave primary mirror (principal focus is behind the
secondary mirror)
Reflected onto convex secondary mirror and focused to the principal focus of the
secondary mirror through a hole in primary mirror (focus is behind primary mirror)
Refracted by eye piece lens, forming virtual, inverted, magnified image at infinity
Definition spherical aberration - lens - answersWhere parallel rays at different distances
from principal axis are brought to focus at different points (on principal axis).
Don't converge at the same point
How to solve spherical aberration - answersUse a parabolic mirror
Large mirrors of good quality are easier to make
Can be supported from underneath, so less distortion
Diagram of spherical aberration of a mirror - answersrays further from principal axis are
focused to a point closer to mirror
If mirror is not perfectly parabolic
Effect on image of spherical aberration - answersImage is blurred
Can happen at both primary and secondary mirrors
Secondary also blocks light and there is some diffraction around it
Definition chromatic aberration - answersWhere different wavelengths are refracted by
different amounts resulting in different focal lengths for different wavelengths (colours)
Blue refracted more than red
Building larger lenses of good quality is difficult and expensive
Very heavy and can only be supported by edges to not block light, causing distortion
, Effect on image of chromatic aberration - answersImages have multicoloured, blurred
edges
Diagram of chromatic aberration from a lens - answersblue light (shorter wavelengths)
are focused closer to lens that red light (longer wavelengths)
Minimum angular resolution
(Resolving power) - answersThe smallest angular separation at which the instrument
can distinguish two points
Smaller angle, better resolving power
Definition Rayleigh criteria - answerstwo objects can be just be resolved when the Airy
disc of one object lies at least as far away as the first minimum of the diffraction pattern
of the other.
θ = λ/D
θ = minimum angular resolution (radians)
λ = wavelength of EM radiation (m)
D = diameter of aperture (m)
- bigger diameter, smaller the angle, greater resolving power
Definition of Airy Disc - answersThe bright central maximum (spot) of the diffraction
pattern produced when light from a point source passes through a circular aperture.
Sketch of diffraction pattern from a circular aperture - answersnote that width of airy disc
(central bright spot) is twice the width of bright fringes
Sketch illustrating Rayleigh criteria - answers
Structure and operation of a CCD - answers• A CCD is silicon chip divided into picture
elements (pixels).
• Incident photons cause electrons to be released.
• The number of electrons liberated is proportional to the intensity of the light.
• These electrons are trapped in 'potential wells' in the CCD.
• An electron pattern is built up which is identical to the image formed on the CCD.
• When exposure is complete, the charge is processed to give an image.
Definition of quantum efficiency - answersQE = number of photons detected/number of
photons incident x 100
Proportion of the incident photons that are detected by a light detector (proportional of
incident photons that release electrons)
Quantum efficiency of a CCD - answersQuantum efficiency of CCD >80%
