Waves, Electrons & Photons (Edexcel International A Level Physics)

A laser, a diffraction grating and a screen are set up as shown. The laser emits monochromatic light.

When the laser is switched on, a series of bright dots is seen on the screen.

The diagram below shows the position of the central dot at O. The next bright dot appears at position X.

The diffraction grating has 450 lines per mm.

Determine the wavelength of the light from the laser.

Wavelength = ..................................

Explain why a series of bright dots is seen on the screen.

The laser is replaced by a source producing a parallel beam of bright white light.

Suggest what would now be observed on the screen.

Sirius A is the brightest star in the night sky and is mostly composed of hydrogen.

When light from Sirius A passes through the hydrogen in the outer layers of the star, some light is absorbed. This causes electrons in the hydrogen to be excited.

The diagram shows an electron being excited from the –3.40 eV level to the –1.50 eV level.

The wavelengths of the different colours of visible light are shown in the table below.

Deduce the colour of the visible light that caused the electron transition shown in the diagram.

A light year is the distance travelled by light in one year.

Sirius A is 8.60 light years from Earth. The intensity of radiation from Sirius A received on Earth is 1.17 × 10^–7 W m^–2.

Calculate the power of Sirius A.

Power of Sirius A = ...........................

When hydrogen gas is excited in the laboratory, only certain wavelengths of light are emitted.

Explain why.

ICESat-2 is a satellite launched into space by NASA in 2018. One purpose of the satellite is to measure the thickness of ice on the Earth’s surface. The satellite is powered using solar panels. A laser in the satellite produces a beam of photons, which travel to the Earth and back.

Calculate the intensity of solar radiation as it reaches ICESat-2.

distance from the Sun to ICESat-2 = 1.50 × 10¹¹ m    

power of the Sun = 3.83 × 10²⁶ W

Intensity of solar radiation = ........................................

The laser emits light with a wavelength of 532 nm. Calculate the energy, in J, of each photon.

Energy of photon = ...................................... J

The photons released by the laser are directed towards the Earth. The mean time for these photons to return to the satellite is 3.20 ms.

i) Calculate the height that ICESat-2 orbits above the surface of the Earth.

Height above Earth = .......................................(2)

ii) When photons arriving at the satellite are detected, only those with a wavelength of exactly 532 nm are used in the analysis of the results.

Suggest why.

(1)

At one point, ICESat-2 passes over a flat ice sheet. The ice sheet is 1000 m above sea level.

Explain how the measurements taken by ICESat-2 can be used to show that the ice sheet has a flat surface and is higher than sea level.