AQA Physics A-level: Section 9 - Astrophysics

Practice flashcards covering key terms and definitions from AQA Physics A-level Section 9: Astrophysics, including telescopes, stellar classification, cosmology, and exoplanet detection.

Convex / Converging lens

A lens that focuses incident light rays.

Concave / Diverging lens

A lens that spreads out incident light rays.

Principal Axis

The line passing through the centre of the lens at a $90^{\circ}$ angle to its surface.

Principal Focus ($F$)

In a converging lens, the point where incident beams passing parallel to the principal axis will converge; in a diverging lens, the point from which light rays appear to originate.

Focal length ($f$)

The distance between the centre of a lens and the principal focus ($F$).

Real image

An image formed when light rays cross after refraction; it can be projected onto a screen.

Virtual image

An image formed on the same side of the lens as the object where light rays do not cross; it cannot be formed on a screen.

Lens formula

The mathematical relationship between the distance of the object from the centre of the lens ($u$), the distance of the image from the centre of the lens ($v$), and the focal length ($f$), given by $\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$.

Power of a lens ($P$)

A measure of how closely a lens can focus a beam parallel to the principal axis, calculated as $P = \frac{1}{f}$ and measured in Dioptres $(D)$.

Objective lens

The lens in a refracting telescope that collects light and creates a real image of a very distant object; it should have a long focal length and a large diameter.

Eyepiece lens

The lens in a telescope that magnifies the image produced by the objective lens, producing a virtual image at infinity to reduce eye strain.

Normal adjustment

The configuration of a refracting telescope where the distance between the objective lens and the eyepiece lens is the sum of their focal lengths ($f_o + f_e$).

Angular magnification ($M$)

The ratio of the angle subtended by the image at the eye ($\beta$) to the angle subtended by the object at the unaided eye ($\alpha$).

Cassegrain reflecting telescope

A telescope configuration involving a concave primary mirror with a long focal length and a small convex secondary mirror in the centre.

Chromatic aberration

An effect where different wavelengths of light are focused at different points because blue light is refracted more than red light, resulting in coloured fringing around images.

Spherical aberration

A distortion where rays of light at the edge of a lens or mirror are focused in a different position than those near the centre, leading to blurring.

Achromatic doublet

A component used to minimise aberrations consisting of a convex lens made of crown glass and a concave lens made of flint glass cemented together.

Collecting power

A measure of a telescope's ability to collect incident EM radiation, which is directly proportional to the square of the objective diameter: $\text{collecting power} \propto (\text{objective diameter})^2$.

Rayleigh Criterion

A rule stating that two objects will not be resolved if any part of the central maximum of either image falls within the first minimum diffraction ring of the other.

Minimum angular resolution ($\theta$)

The minimum angle between two objects for them to be resolved as separate images, calculated as $\theta = \frac{\lambda}{D}$ where $\lambda$ is the wavelength and $D$ is the objective diameter.

Quantum efficiency

The percentage of incident photons that cause an electron to be released in a device like a CCD.

Luminosity ($P$)

The total power output of a star or the rate at which light energy is released.

Intensity ($I$)

The power received from a star per unit area, measured in $W\,m^{-2}$, following the inverse square law $I = \frac{P}{4\pi d^2}$.

Apparent magnitude ($m$)

A measure of how bright an object appears in the sky from Earth, which depends on the star's luminosity and distance.

Hipparcos scale

A logarithmic scale classifying astronomical objects by apparent magnitude from $1$ (brightest) to $6$ (faintest), where a magnitude change of $1$ corresponds to an intensity ratio of $2.51$.

Absolute magnitude ($M$)

The apparent magnitude an object would have if it were placed at a distance of $10\,parsecs$ from Earth.

Astronomical Unit ($AU$)

The average distance between the centre of the Earth and the centre of the Sun, approximately $1.50 \times 10^{11}\,m$.

Parsec ($pc$)

The distance at which the angle of parallax is $1\,arcsecond$, equivalent to approximately $3.08 \times 10^{16}\,m$.

Light year ($ly$)

The distance that electromagnetic waves travel in a vacuum in one year, approximately $9.46 \times 10^{15}\,m$.

Stefan’s law

The law stating that the power output ($P$) of a black body radiator is proportional to its surface area ($A$) and the fourth power of its absolute temperature ($T$): $P = \sigma AT^4$.

Wien’s displacement law

A law stating that the peak wavelength ($\lambda_{max}$) of emitted radiation is inversely proportional to the absolute temperature ($T$): $\lambda_{max}T = 2.9 \times 10^{-3}\,m\,K$.

Hydrogen Balmer lines

Absorption lines in the spectra of O, B, and A type stars caused by the excitation of hydrogen atoms from the $n = 2$ state.

Schwarzschild radius ($R_s$)

The radius of the event horizon of a black hole where the escape velocity equals the speed of light, calculated as $R_s = \frac{2GM}{c^2}$.

Type 1a supernova

The explosion of a white dwarf in a binary system that has reached a critical mass, characterized by a consistent peak absolute magnitude of approximately $-19.3$.

Dark energy

A constant energy throughout the universe that has a repulsive effect and is thought to be responsible for the accelerating expansion of the universe.

Red-shift ($z$)

The shift of spectral lines toward longer wavelengths when an object moves away from the observer, calculated as $z = \frac{v}{c} = \frac{\Delta f}{f} = -\frac{\Delta \lambda}{\lambda}$.

Hubble’s law

The observation that a galaxy's recessional velocity ($v$) is directly proportional to its distance ($d$) from Earth: $v = Hd$.

Quasar

An active galactic nucleus consisting of a supermassive black hole surrounded by a disc of matter that emits powerful jets of radiation.

Cosmological Microwave Background Radiation ($CMBR$)

Microwave radiation detected from all directions in space, representing the red-shifted remains of high-energy radiation from the Big Bang.

Radial velocity method

A method of detecting exoplanets by observing the Doppler shift in a star's spectrum as it 'wobbles' due to the gravitational pull of an orbiting planet.

Transit method

A method of detecting exoplanets by observing the regular dip in the intensity of light from a star as a planet crosses in front of it.