Module 5 - 5.5.2 Electromagnetic Radiation From Stars

Question 1

Energy Levels (definition)

Answer 1

The specific energies that electrons can have when occupying specific orbits

Question 2

Electrons can move to a higher/lower energy level by

Answer 2

absorbing/emitting energy in the form of em radiation

Question 3

At low temperatures, most electrons will be in the — energy level

Answer 3

lowest (n=1)

Question 4

Excitation (definition)
De-Excitation (definition)

Answer 4

• when an electron moves from a lower to a higher energy level by absorbing energy
• when an electron moves from a higher to a lower energy level after releasing energy

Question 5

• The energy released in de-excitation is emitted as …
• emitted – can have …
• larger energy transition = …

Answer 5

• emitted as em radiation of a specific frequency depending on the difference in energy between the energy levels
• emitted photons can have a range of wavelengths spanning the whole em spectrum
• larger energy transition = longer wavelength of emitted photon

Question 6

When an electron is infinitely far from the nucleus, its energy is

Answer 6

0 (electron is said to be free from the atom)

Question 7

As an electron moves towards the nucleus, its energy (2)

Answer 7

• decreases below 0
• it becomes confined within the atom

Question 8

The value of an energy level is equal to

Answer 8

the amount of energy required to remove an electron from that energy level

Question 9

The value of the lowest energy level is equal to

Answer 9

the amount of energy required to remove an electron from the atom (ionisation)
most negative value at the ground state

Question 10

Chemical Composition of Stars (7)

Answer 10

• fusion reaction in star’s core produced photons of em radiation
• photons are constantly absorbed by atoms in gas
• then re-emitted with many different frequencies
• produces a continuous spectrum of wavelengths
• each gas produce a unique pattern of spectral lines
• due to the specific transitions between the element’s energy levels
• can be used to determine the presence of certain elements within the star

Question 11

A Continuous Spectrum (definition)

Answer 11

A spectrum that appears to contain all frequencies over a comparatively wide range

Question 12

Continuous Spectra are produced from

Answer 12

hot dense sources

Question 13

An Emission Line Spectrum (definition)

Answer 13

The spectrum of frequencies of em radiation emitted due to the electron transitions from a higher energy level to a lower one within an atom of that element

Question 14

Emission Line Spectra act as (3)

Answer 14

• a fingerprint for an element
• as each element produces a unique emission line spectrum
• due to its unique set of energy levels

Question 15

An emission line spectrum consists of (2)

Answer 15

• a series of bright lines against a dark background
• each line corresponds to a different wavelength of photon emitted by an electron transition

Question 16

Emission Line Spectra are produced by

Answer 16

hot low pressure gases

Question 17

An Absorption Line Spectrum (definition)

Answer 17

The pattern of dark lines in a continuous spectrum from a light source and is caused by light passing through an absorbing medium such as a gas
- the dark lines represent the frequencies absorbed by a medium

Question 18

The Sun’s atmosphere produces a — line spectrum (3)

Answer 18

• absorption
• since it is not hot enough to produce an emission one
• dark lines in the spectrum are caused by the presence of certain elements (e.g. H, He)

Question 19

Using a Transmission Diffraction Grating allows for (2)

Answer 19

• more accurate measurements
• since the angular dispersion of the colours will be much greater than using a prism

Question 20

Dispersion (definition)

Answer 20

The separation of visible white light into a spectrum of its colours

Question 21

Uses of Transmission Diffraction Gratings (2)

Answer 21

• analyse light from stars
• analyse the composition of a star

Question 22

The wavelength of em radiation emitted by an object depends on its

Answer 22

temperature

Question 23

The peak intensity of a star’s emission occurs at (3)

Answer 23

• a specific wavelength (λmax) related to the star’s temperature
• doesn’t mean it is the largest λ emitted by the star
• as surface temperature increases peak occurs at a lower λ

Question 24

For a graph of intensity against λ, the area under the curve — as – increases (3)

Answer 24

• increases drastically as temperature increases
• since the area represents the total energy emitted by the body
• which is strongly temperature dependent

Question 25

An Ideal Black Body (definition)

Answer 25

A body that absorbs and emits all wavelengths
(a theoretical object)

Question 26

Hotter objects tend to be (colour)
Cooler objects tend to be (colour)

Answer 26

• white/blue
• red/yellow

Question 27

Wien’s Law

Answer 27

λmax ∝ 1/T

Question 28

Stefan’s Law

Answer 28

The total energy emitted by a black body per unit area per second is proportional to the fourth power of the absolute temperature of the body

Question 29

If two stars have the same temperature, the star that appears brighter has the

Answer 29

larger diameter

Question 30

Difficulties in obtaining accurate results for Wien’s and Stefan’s Law (2)

Answer 30

• Earth’s atmosphere only allows certain wavelengths of em radiation through
• telescopes need to be placed at high altitudes to avoid dust and human light pollution