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Xaverian Physics > Module 5 Cosmology > Flashcards

Module 5 Cosmology Flashcards

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1
Q

What makes the line spectrum of elements different?

A

electron energy levels different for each chemical element

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2
Q

Define an energy level

A

the discrete energy with which electrons orbit the nucleus

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3
Q

Why are electron levels considered negative?

A

Energy is required to remove an electron from the atom

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4
Q

What is ionisation energy?

A

minimum energy required to remove electron that is closest to the nucleus (ground state)

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5
Q

What is the ground level?

A

n=1, first energy level in an atom (the most negative one)

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6
Q

What is excitation?

A

The process by which an electron moves from a lower to a higher energy level (by absorbing a photon)

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7
Q

Describe the process of excitation

A

Electron must receive the exact amount of energy

Usually by absorbing a photon

Electron gains this energy and moves to a higher energy level

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8
Q

What is de-excitation or relaxation?

A

the process by which an electron moves to a lower energy level by losing energy through emitting a photon

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9
Q

How do you find the number of possible wavelength that can be observed when firing a beam of electrons at a given KE at atoms using an energy level diagram?

A

Find the energy of electron in same units as energy diagram

Add this value to the value of the ground level (you should still get a negative number)

Take all the number of relaxations possible below this value (line below this value)

That is the number of wavelengths that can be observed ie the number of photons of different energies

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10
Q

What are the three main spectra?

A

Continuous

Emission line spectra

Absorption line spectra

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11
Q

What is continuous spectrum?

A

a spectrum containing all colours, wavelengths, and frequencies

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12
Q

How is a continuous spectrum produced?

A

viewing a white light through a diffraction grating

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13
Q

Describe an emission line spectrum?

A

a series of narrow, bright, coloured lines on a dark background

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14
Q

Explain how is an emission spectrum produced in terms of movement of electrons?

A

Each line is due to a photon of specific energy being emitted as electrons move to lower energy levels through relaxation

Gaps between levels have fixed energy therefor photons have fixed energy and wavelengths

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15
Q

Describe an absorption spectrum

A

A series of narrow dark lines on a continuous spectrum background

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16
Q

In simple terms, describe how an emission spectrum is produced

A

electron moves to a lower energy level by emitting a photon

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17
Q

How is an absorption spectrum produced?

A

When a continuous spectrum (from passing white light through a diffraction grating) passes through a cool gas

The gas absorbs certain photon energies producing excitation of electrons

Each photon absorbed (gap in spectra) corresponds to a gaps between energy levels

Producing a series of narrow dark bands on a continues background

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18
Q

How can an absorption line spectrum be used to identify elements

A

Each black line corresponds with a wavelength of photons being absorbed

Also corresponds to gaps between energy levels

Each element has a unique set of energy levels, thus can be compared to absorption spectrum

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19
Q

What is a diffraction grating?

A

Usually a piece of metal containing thousands of tiny slits of equal thickness which are equally spaced

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20
Q

State the diffraction grating equation

A

dsin0=nλ

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21
Q

Where are angles measured from when calculating diffraction gratings

A

from the central fringe (straight through position)

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22
Q

In the diffraction grating formula, what does d stand for?

A

distance between the gaps in the diffraction grating

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23
Q

When given a number of lines in a distance for a grating, how do you calculate d (use N=250 mm^-1)?

A

Take the unit of length over the number of divisions

1x10^-3/250

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24
Q

Explain how you calculate the largest number of orders that can be observed

A

Rearrange for n

sin90=1

n_max=d/λ

Truncate ie round down to nearest whole number

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25
Q

Explain why when white light is diffracted, does blue light appear on the centre and red on the outside of the diffraction patterns?

A

Blue light has a shorter wavelength

rearrange for sin0=nλ/d

If λ is smaller so is the angle

26
Q

Explain why a white light is seen in the centre of a diffraction grating

A

All wavelengths combine and interfere forming white light

because at centre, path difference is 0xwavelengths for all wavelengths

27
Q

How was redshift first observed?

A

Through comparing the absorption line spectra of hydrogen from distant galaxies to laboratory experiments

28
Q

What is the doppler effect?

A

the apparent change in wavelength due to relative motion between a source and the observer

29
Q

What is redshift?

A

the stretching of wavelengths when a source of EM radiation is moving away from the observer

30
Q

What is blueshift?

A

the compression of wavelengths when a source of EM radiation is moving towards an observer

31
Q

Explain how you would find the speed of a galaxy in relation to earth using the wavelengths of given spectra

A

Δλ/λ=Δf/f=v/c

Δλ is calculated by (observed λ-actual λ)

to find v=Δλ/λ*c

32
Q

Explain why longer wavelengths produce a larger Doppler shift Δλ?

A

speed of galaxy is the same

Δλ will be greater if λ is larger

33
Q

What is recessional speed/velocity?

A

the apparent movement of galaxies away from the observer due to the expansion of the universe

34
Q

State Hubble’s law

A

the speed of recession of distance galaxies is directly proportional to their distance from the observer

35
Q

Describe how the Hubble constant is measured?

A

plotting a graph of recessional speed against distance

straight line through origin.

Gradient is the Hubble contant

36
Q

How can the the change in wavelength from a distance galaxy and it’s distance can be used to find the Hubble Constant?

A

Δλ/λ=Δf/f=v/c

to find v=Δλ/λ*c

Speed of recession is proportional it’s distance

So Hubble constant H_0 = v/d

37
Q

What are the units for the Hubble constant?

A

s^-1
or
kms^-1*Mpc^-1

38
Q

Hubble constant - how to convert kms^-1*Mpc^-1 to s^-1?

A

Multiply by 10^3 to get from km to m

divide by 10^6x3.1x10^16 (to remove Mpc)

39
Q

Explain how Hubbles constant can be used to find the age of the universe

A

t=d/v

v=H_0d

v/d=H_0

d/v=1/H_0

therefore t = 1/H_0 (Hubbles constant must be in seconds) giving age of universe in seconds

40
Q

Explain how Hubble’s Law can be used as evidence for the big bang theory

A

galaxies moving away from each other, implying they initially were located at the same point

41
Q

What happens in the first instance of the the big bang?

A

high energy photons produce quarks and leptons from pair production

42
Q

What happens during the first 100s of the big bang

A

quarks combine to form hardons - universe contains leptons and photons

43
Q

What is the state of the universe during the first few thousand years?

A

protons and neutrons fuse to form simple nuclei (helium and hydrogen), universe is a plasma of nuclei, electrons and photons

44
Q

What is the last stage of the evolution of the universe (not how it ends)

A

Expansion of universe causes photons to loose energy

Allowing atoms form from nuclei and electrons

Photons no longer have enough energy to ionise so they are free to travel and form CMBR

45
Q

What initiates the formation of stars and galaxies in the early universe?

A

tiny variations in density in the universe lead to gravity forming stars and planets

46
Q

What is CMBR?

A

cosmic microwave background radiation

47
Q

What temperature does CMBR correspond with?

A

Background temperature of the universe 2.7K (now)

48
Q

CMBR are photons from the big bang, originally gamma radiation. Explain what would have caused this radiation to cool to the levels that it can be detected today

A

Decrease in energy = increase in wavelength

This is due to the expansion of the universe causing the stretching of wavelengths

49
Q

What causes variations in CMBR?

A

tiny variations in density in the universe lead to gravity forming stars and planets

50
Q

What is the cosmological principle?

A

on a large scale, the universe is homogenous and isotropic to universal laws of physics

51
Q

In terms of the universe, what does homogenous mean?

A

universe has uniform density on a large scale

52
Q

In terms of the universe, what does isotropic mean?

A

universe looks the same in all directions on a large scale

53
Q

What are hadrons

A

mainly protons and neutrons formed when quarks combine

54
Q

What are leptons?

A

mainly electrons

55
Q

What is the current composition of the universe?

A

5% ordinary matter

25% dark matter

70% dark energy

56
Q

What is dark matter?

A

the substance which provides the extra gravitational attraction needed to stop galaxies from falling apart

(what affects the rotational speed of galaxies leading to all stars rotating around the centre of the galaxy at the same speed))

57
Q

Why is dark matter so hard to observe and study?

A

does not absorb or emit electromagnetic radiation

58
Q

What is dark energy?

A

the force which is thought to cause the rate of expansion of the universe to increase

59
Q

What causes the rate of universe expansion to increase?

A

dark energy

60
Q

What makes scientists believe dark energy exist?

A

force of gravity should slow down the rate of expansion, whereas measurements show that the rate is increasing

61
Q

What are the different time stages to the evolution of the universe?

A

First instance
First 100 seconds
Thousand years
Present

Xaverian Physics (31 decks)

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