AQA AL Physics AstroPhysics

Convex/converging lens

focuses incident light.

Concave/diverging lens

spreads out incident light.

Principal axis

the line passing through the centre of the lens at 90º 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 the light rays appear to come from. This is the same distance either side of the lens.

Focal length (f)

the distance between the centre of a lens and the principal focus. The shorter the focal length, the stronger the lens.

Real image

formed when light rays cross after refraction. Real images can be formed on a screen.

Virtual image

formed on the same side of the lens. The light rays do not cross, so a virtual image cannot be formed on a screen.

Lens formula

1/u + 1/v = 1/f, where u is the distance of the object from the centre of the lens, v is the distance of the image from the centre of the lens, and f is the focal length of the lens.

Power of a lens

a measure of how closely a lens can focus a beam that is parallel to the principal axis. The shorter the focal length, the more powerful the lens. In converging lenses this value is positive and in diverging lenses this value is negative. Power is measured in Dioptres (D). P = 1/u + 1/v = 1/f.

Objective lens

The role of this lens is to collect light and create a real image of a very distant object. This lens should have a long focal length and be large so as to collect as much light as possible.

Eyepiece lens

This magnifies the image produced by the objective lens so that the observer can see it. This lens produces a virtual image at infinity since the light rays are parallel.

Normal adjustment for a refracting telescope

when the distance between the objective lens and the eyepiece lens is the sum of their focal lengths (fo + fe). This means the principal focus (F) for these two lenses is in the same place.

Resolving Power

the ability of a telescope to produce separate images of close - together objects

Collecting power of a telescope

is directly proportional to the square of the radius of the objective lens.

Luminosity (L)

Total power output of a star.

Intensity (I)

the amount of light received at a distance d / m^2

Apparent Magnitude (m)

the brightness as observed from the Earth (not taking into account of distance)

Scale of Apparent Magnitude

m = 6,5,4,3,2,1 ( getting higher; mag.1 ≈ 2.512; mag.4 star 2.512* mag.5)

Light Year

the distance light travels in a year (1 ly = 9*10^15m)

Parsec

The distance at which the angle of parallax is 1 arcsecond (1/3600th of a degree).

Absolute Magnitude (M)

the brightness of any star at a distance of 10pc.

Formula for Magnitudes

m - M = 5log10(d/10)

Stefan's Law

the total energy emitted by a black body per unit area per second

Optical Telescopes

Telescope which detects wavelengths of light from the visible part of the electromagnetic spectrum.

Non-optical Telescope

Telescopes that look at other parts of the electromagnetic spectrum.

Radio, Infrared, UV, X-ray

Rayleigh Criterion

states that two objects will not be resolved if the central maximum of one diffraction pattern falls within the first minimum of the other.

Features of increasing resolving power

Large aperture, short wavelength

Rayleigh criterion Formula

θ = λ/D for minimum angular resolution

For circular aperture

θ = 1.22 λ/D

Rayleigh Criterion Formula (θ in arcsecond)

θ(arcsecond) = 206265 * λ/D

Relationship between collecting power and diameter

Collecting Power is proportional to Diameter^2

Structure of Radio telescope (similarities with optical telescope)

Both use parabolic surfaces to reflect waves.

Structure of Radio telescope (differences with optical telescope)

Radio uses a single primary reflector, optical uses two mirrors

Radio dish does not need to be as smooth as optical mirrors

Positioning of Radio telescope (similarities with optical telescope)

Both can be ground-based as the atmosphere is transparent to most radio and optical wavelengths

Positioning of Radio telescope (differences with optical telescope)

Optical must be placed high up (to avoid atmospheric distortions) and away from cities (to avoid light pollution)

Radio must be located remotely

Uses of Radio telescope (similarities with optical telescope)

Both are used to detect hydrogen emission lines

Uses of Radio telescope (differences with optical telescope)

Radio waves are not absorbed by dust, whereas optical waves are, so radio telescopes are used to map the Milky Way

CCD (charge-coupled device)

A detector which is highly sensitive to photons making it ideal for use in the detection system of modern telescopes.

Quantum efficiency

The percentage of incident photons which cause an electrons to be released.

Quantum efficiency compariosn between CCD and human eye

CCD QE ~ (>70%)

Eye QE - (4%)

Resolution of CCD

Related to the total number of pixels per unit area, and their size

Resolution comparison between CCD and eye

CCD resolution = ~10µm (Resolution can be increase using smaller pixels)

Eye resolution = ~100µm

Convenience comparison between CCD and eye

CCD convenience = Remote viewing and images can be stored and analysed digitally

Eye convenience = no additional equipment needed

Doppler Effect

the compression or spreading out of waves that are emitted or reflected by a moving source.

Doppler Effect Formula

∆f/f= v/c - frequency shift

or

∆λ/λ = -v/c - wavelength shift

Eclipsing binary star system

Where two stars orbit around a common centre of mass with their orbital plane in the Earth's line of sight

> The primary minima (the larger dips) are caused by the cooler star passing in front of the hotter star

> The secondary minima (the smaller dips) are caused by the hotter star passing in front of the cooler star

Spectroscopic binaries

As the stars eclipse each other, they are travelling perpendicular to the line of sight form the observer so no Doppler effect in emitted radiation.

Universe is expanding because of ____________

Red shift

Hubble's Law

The recessional velocity of a galaxy is proportional to its distance from Earth.

Hubble's constant is the constant of proportionality H

Hubble's Law formula

v = Ho x d

Age of universe can be estimated using

time = 1/H0

Theories evident for supporting Big Bang Theory

1. Galactic redshift & Hubble's Law

2. CMBR

3. Relative abundance of hydrogen and helium

2. CMBR

3. Relative abundance of hydrogen and helium

Evidence for galactic redshift

1. Observation show distant galaxies moving away from us due to redshift on line spectra.

2. Hubble's Law show that further away the galaxy, the faster it is moving away.

Exoplanet

Planets that are not within our solar system.