Cambridge International A Levels A2 Physics Chapter 25 Astronomy and Cosmology
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Cambridge A Levels A2 Physics
Institution
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Chapter 25 Astronomy and Cosmology
25.1 Standard Candles
Luminosity, L
Total power of electromagnetic radiation emitted by an object (eg. Stars) /
Total energy emitted per unit time by an object.
Units: Watt, W/J s-1
Luminosity of the Sun = 3.83 × 1026 W
Radiant Flux Intensity, F
The radiant power passing normally through a surface per unit area. /
The observed amount of intensity measured on Earth.
Radiant flux intensity apparent brightness of a star seen from the surface of Earth
𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑠𝑡𝑎𝑟
𝑟𝑎𝑑𝑖𝑎𝑛𝑡 𝑓𝑙𝑢𝑥 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 =
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑠𝑝ℎ𝑒𝑟𝑒
𝐿
𝐹=
4𝜋𝑑 2 Unit: Wm-2
F: radiant flux intensity (observed intensity on Earth) / Wm-2
L: luminosity of the source / W
d: distance from the centre of the star / m
The power of the star spreads out uniformly through a spherical shell.
, Luminosity: The total radiation that leaves the star.
Radiant Flux Intensity: The amount of radiation from the star measured on Earth.
By the time the radiation reaches Earth, it will spread out a great deal, it will only be a fraction of the value
of luminosity.
Inverse Square Law and Radiant Flux Intensity
Inverse Square Law:
The energy emitted by the star will spread out in all directions
over the surface of an ever increasing sphere.
It is proportional to the square of the distance.
Light sources which are further away appear fainter because the light it emits is spread out over a greater
area.
As light leaves the surface of the star, it begins to spread out uniformly through a sphere.
For a given star, the luminosity, L is a constant.
𝐿
𝐹=
4𝜋𝑑 2
𝐿 1
𝐹= ( )
4𝜋 𝑑2
constant
1
𝐹∝
𝑑2
[Inverse – square law]
The greater the radiant flux intensity measured, the closer the star is to the Earth.
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