Cosmology for AQA A-level Physics
This page covers the following topics:
1. The Doppler effect
The Doppler effect is the change in frequency due to the relative motion between the wave source and the observer. When both the observer and the source are stationary, the waves are at the same frequency for both. When the source moves towards the observer, the wavelength becomes shorter, and therefore the wave that is received by the observer has higher frequency, and vice versa when the source is moved away. An example of this is the fact that when an ambulance moves away from the observer with its siren blaring, the siren will sound lower in pitch as it moves away from the observer.
Quasars are very bright compact objects which emit all wavelengths of electromagnetic radiation. The redshift that is observed from quasars is huge, meaning that they are at very great distances from Earth. However, they have bright apparent magnitudes despite this distance, and thus are very bright. After the discovery of quasars, it was discovered that quasars were at the centre of ancient galaxies and thus are supermassive black holes at the centre of young galaxies. As matter falls into them and loses its gravitational energy, quasars heat up, thus leading to their brightness. No local quasars exists, meaning that they only existed in the early universe. This supports the Big Bang theory, as it suggests that the Universe has changed over the years, but does not support the Steady State theory.
Exoplanets are planets which lie outside our solar system. There are difficulties in observing exoplanets, due to the fact that the light they emit is sometimes hidden by stars they orbit. They are also difficult to detect with telescopes, since they are often too close together. There are two methods used for their detection: observing variations in Doppler shift and the transit method. A star does not remain completely stationary when being orbited by a planet, as it responds to the gravitational pull of the planet. This is observed as very small variations in its light spectrum, ie. red and blue shifts, which acts as evidence for the existence of these exoplanets. The transit method relies on the fact that as the exoplanet orbits its star, the brightness of the star will at points appear to decrease slightly. Detecting this can be used to calculate the diameter of the planet.
State two methods of detecting exoplanets.
Using variation in Doppler shift and the transit method.
An ambulance is travelling at 30 m/s towards an observer. The frequency of the siren of the ambulance is 500 Hz. Given that the speed of sound is 340 m/s, calculate the frequency of the siren that the observer hears.
f = 500 × 340/(340 − 30) = 548 Hz (to 3 significant figures).
Explain what can be said about the redshift observed from quasars.
The redshift that is observed from quasars is huge, meaning that they are at very great distances from Earth.
Define the Doppler effect.
The Doppler effect is the change in frequency due to the relative motion between the wave source and the observer.
A police car is travelling away from an observer. Explain how the observer will hear the siren.
When a police car moves away from the observer with its siren blaring, the siren will sound lower in pitch as it moves away from the observer.
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