Page 180 - Physics_Form_2

Physics for Secondary Schools

Mirages can commonly be observed creates a non-uniform optical medium.
when you walk down a tarmac roadway Consequently, as light travels from the
on a sunny day. As you walk down the sun, it passes through the cooler air layers
roadway, there appears to be a puddle of towards the hot air layers. This means
water on the road several metres (about light travels from an optically denser
100 m) ahead of you. Yet, when you arrive medium to a less dense medium. Thus,
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at the perceived location of the puddle, light will be refracted upon striking the
you recognise that the puddle is not there. boundary between the two layers of air.
Instead, the puddle of water appears to be Therefore, light bends upwards. As the
another one hundred metres before you. light continues to traverse different air
The appearance of the puddle of water is layers, it is bent more upwards to the
simply an illusion because of the mirage. extent that it reaches your eyes instead of
hitting the road. Your brain traces the light
Mirages occur on sunny days because straight back along the direction from
the sun heats the roadway to high which it comes to your eyes. Figure 5.10
temperatures. The heated roadway, in (a) shows an image of the sky, which is
turn, heats the surrounding air, keeping refracted by air with non-uniform optical
the air just above the roadway at a higher density, appearing as a puddle of water
temperature than the air above. Hot air just above the roadway. Figure 5.10 (b)
tends to be optically less dense than cooler illustrates the formation of an atmospheric
air. Thus, the layers of air just above the mirage. The mirage phenomenon can also
roadway are optically less dense than occur in bodies of water such as seas,
the air further above the roadway. This lakes, ponds, and deserts.

Light from
the sky

Total internal
Cool air reflection occurs here
Warm air
Hot air

Pool of water (image of the sky)
(a) Mirage observed (b) Formation of an atmospheric mirage
above the roadway
Figure 5.10: Road mirage

Task 5.2

Organise a walk with your friends down the tarmac roadway (if any) on a sunny day
and try to observe the occurrence of the mirage. Discuss the conditions necessary
for the occurrence of a mirage.

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Student’s Book Form Two

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