Energy levels and line spectra

21* Analysis of stellar light can identify the elements that are present in the outer layers of a star.

Absorption spectra of visible light collected from two main sequence stars, Vega and Tau Ceti, are shown below. The spectra were collected by a telescope orbiting the Earth. The dark lines on the diagram show wavelengths where the intensity was zero or close to zero.

• Explain the cause of the dark lines.

• Describe and explain at least one difference between the spectra.

• Analyse the spectra to determine which star is older.

Additional space if required

The Earth’s aurora is an atmospheric effect visible in the polar regions.

Electrons in the upper atmosphere initially move vertically downwards along one of the Earth’s magnetic field lines. They are accelerated by an electric field.

These electrons collide with oxygen atoms causing atomic electrons to be excited to higher energy levels. The atomic electrons then return to lower energy levels, releasing photons of green light, red light and ultraviolet.

The accelerated electrons are accelerated by electric fields parallel to the magnetic field lines.

The length of the acceleration region is 20 000 km and the electric field value is 0.2 mV / m. The electric field can be assumed to be uniform.

Calculate the kinetic energy gained by the electrons as they pass through the electric field, in joules.

State and explain the direction of the electric field required to accelerate the electrons downwards.

A simplified energy level diagram for atomic oxygen is shown below.

Determine the wavelength of the green light emitted.

Fully justify your answer.

The horizontal current closest to the Earth produces a magnetic field which can be detected at ground level.

With reference to Fig. 1.1, state and explain the direction of this magnetic field.

Suggest two ways that the horizontal current may affect the atmosphere.

Part of the emission spectrum for hydrogen in a laboratory is shown. All wavelengths are given

Which diagram shows the corresponding part of the absorption spectrum observed from Earth emitted from a galaxy moving away with a velocity of 0.031 c?

Your answer

The diagram below shows two energy levels for the electron in the hydrogen atom.

The electron makes the transition shown by the arrow.

What is the wavelength of the photon emitted?

A 293 nm

B 366 nm

C 488 nm

D 1460 nm

Your answer

Hydrogen atoms excited in a discharge tube only emit four different discrete wavelengths of visible photons.

Show that the energy of a photon of wavelength 486 nm is 4.09 × 10^–19 J. (b) (i)

Fig. 5.4 shows some of the energy levels of an electron in a hydrogen atom.

Draw an arrow on Fig. 5.4 to show an electron transition which would cause the emission

of a photon of wavelength 486 nm.

Explain why the energy levels are negative.

An electron makes a transition (jump) from level C to level A.

1 Calculate the energy gained by this electron.

2 Calculate the wavelength in nm of the photon absorbed by this electron.

State how an emission line is produced.

State an adjustment that could be made to the experimental arrangement that would space the emission lines more widely.

In the laboratory, the wavelength of the hydrogen-alpha emission line is 656.3 nm. Use Fig. 21.2 to determine the recession velocity of the galaxy.

recession velocity =........ m s⁻¹

Suggest why hydrogen spectral lines play an important role in determining red shift of galaxies.

Light from a similar star is viewed in a galaxy further away. The star is part of a pair of stars which orbit a common centre of mass. Describe and explain how the equivalent spectrum might appear.