P waves travel through the Earth: ––they are
refracted by the core ––the paths taken by P
waves mean that scientists can work out the
size of the Earth’s core.
S waves are not detected on the opposite side of Earth to
an earthquake: ––they will not travel through liquid ––this
tells scientists that Earth’s core is liquid.
Tan or Burn
A tan is caused by the action of
ultraviolet light on the skin
Cells in the skin produce melanin, a pigment that
produces a tan.
People with darker skin do not tan as easily
because ultraviolet radiation is filtered out.
Use a sun screen with a high SPF, or sun protection factor,
to reduce risks.
maximum length of time to spend in the sun =
published normal burn time × SPF
People are becoming more aware of the dangers of
exposure to ultraviolet radiation, including the use of
sun beds.
Ozone Depletion
Ozone is found in the stratosphere.
Ozone helps to filter out ultraviolet radiation.
CFC gases from aerosols and fridges
destroy ozone and reduce the thickness
of the ozone layer. ––This increases the
potential danger to humans.
The ozone layer is at its thinnest above
the South Pole because ozone depleting
chemicals work best in cold conditions.
At first scientists did not
believe there was thinning
of the ozone layer – they
thought their instruments
were faulty but other
scientists confirmed the
results and increased
confidence in the findings.
Scientists monitor the thickness of
the ozone layer using satellites. There
is international agreement to reduce
CFC emissions.
Earthquake Waves
A seismograph shows the
different types of
earthquake wave.
L waves travel round the surface very
slowly.
P waves are longitudinal pressure waves:
––P waves travel through the Earth at
between 5 km/s and 8 km/s ––P waves can
pass through solids and liquids.
S waves are transverse waves:
––S waves travel through the
Earth at between 3 km/s and
5.5 km/s ––S waves can only
pass through solids.