Module 5 Cosmology

Question 1

What makes the line spectrum of elements different?

Answer 1

electron energy levels different for each chemical element

Question 2

Define an energy level

Answer 2

the discrete energy with which electrons orbit the nucleus

Question 3

Why are electron levels considered negative?

Answer 3

Energy is required to remove an electron from the atom

Question 4

What is ionisation energy?

Answer 4

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

Question 5

What is the ground level?

Answer 5

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

Question 6

What is excitation?

Answer 6

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

Question 7

Describe the process of excitation

Answer 7

Electron must receive the exact amount of energy

Usually by absorbing a photon

Electron gains this energy and moves to a higher energy level

Question 8

What is de-excitation or relaxation?

Answer 8

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

Question 9

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?

Answer 9

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

Question 10

What are the three main spectra?

Answer 10

Continuous

Emission line spectra

Absorption line spectra

Question 11

What is continuous spectrum?

Answer 11

a spectrum containing all colours, wavelengths, and frequencies

Question 12

How is a continuous spectrum produced?

Answer 12

viewing a white light through a diffraction grating

Question 13

Describe an emission line spectrum?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

Describe an absorption spectrum

Answer 15

A series of narrow dark lines on a continuous spectrum background

Question 16

In simple terms, describe how an emission spectrum is produced

Answer 16

electron moves to a lower energy level by emitting a photon

Question 17

How is an absorption spectrum produced?

Answer 17

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

Question 18

How can an absorption line spectrum be used to identify elements

Answer 18

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

Question 19

What is a diffraction grating?

Answer 19

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

Question 20

State the diffraction grating equation

Answer 20

dsin0=nλ

Question 21

Where are angles measured from when calculating diffraction gratings

Answer 21

from the central fringe (straight through position)

Question 22

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

Answer 22

distance between the gaps in the diffraction grating

Question 23

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

Answer 23

Take the unit of length over the number of divisions

1x10^-3/250

Question 24

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

Answer 24

Rearrange for n

sin90=1

n_max=d/λ

Truncate ie round down to nearest whole number

Question 25

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

Answer 25

Blue light has a shorter wavelength rearrange for sin0=nλ/d If λ is smaller so is the angle

Question 26

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

Answer 26

All wavelengths combine and interfere forming white light because at centre, path difference is 0xwavelengths for all wavelengths

Question 27

How was redshift first observed?

Answer 27

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

Question 28

What is the doppler effect?

Answer 28

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

Question 29

What is redshift?

Answer 29

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

Question 30

What is blueshift?

Answer 30

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

Question 31

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

Answer 31

Δλ/λ=Δf/f=v/c Δλ is calculated by (observed λ-actual λ) to find v=Δλ/λ*c

Question 32

Explain why longer wavelengths produce a larger Doppler shift Δλ?

Answer 32

speed of galaxy is the same Δλ will be greater if λ is larger

Question 33

What is recessional speed/velocity?

Answer 33

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

Question 34

State Hubble's law

Answer 34

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

Question 35

Describe how the Hubble constant is measured?

Answer 35

plotting a graph of recessional speed against distance straight line through origin. Gradient is the Hubble contant

Question 36

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

Answer 36

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

Question 37

What are the units for the Hubble constant?

Answer 37

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

Question 38

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

Answer 38

Multiply by 10^3 to get from km to m divide by 10^6x3.1x10^16 (to remove Mpc)

Question 39

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

Answer 39

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

Question 40

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

Answer 40

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

Question 41

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

Answer 41

high energy photons produce quarks and leptons from pair production

Question 42

What happens during the first 100s of the big bang

Answer 42

quarks combine to form hardons - universe contains leptons and photons

Question 43

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

Answer 43

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

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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

Question 46

What is CMBR?

Answer 46

cosmic microwave background radiation

Question 47

What temperature does CMBR correspond with?

Answer 47

Background temperature of the universe 2.7K (now)

Question 48

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

Answer 48

Decrease in energy = increase in wavelength This is due to the expansion of the universe causing the stretching of wavelengths

Question 49

What causes variations in CMBR?

Answer 49

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

Question 50

What is the cosmological principle?

Answer 50

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

Question 51

In terms of the universe, what does homogenous mean?

Answer 51

universe has uniform density on a large scale

Question 52

In terms of the universe, what does isotropic mean?

Answer 52

universe looks the same in all directions on a large scale

Question 53

What are hadrons

Answer 53

mainly protons and neutrons formed when quarks combine

Question 54

What are leptons?

Answer 54

mainly electrons

Question 55

What is the current composition of the universe?

Answer 55

5% ordinary matter 25% dark matter 70% dark energy

Question 56

What is dark matter?

Answer 56

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))

Question 57

Why is dark matter so hard to observe and study?

Answer 57

does not absorb or emit electromagnetic radiation

Question 58

What is dark energy?

Answer 58

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

Question 59

What causes the rate of universe expansion to increase?

Answer 59

dark energy

Question 60

What makes scientists believe dark energy exist?

Answer 60

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

Question 61

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

Answer 61

First instance First 100 seconds Thousand years Present