TOPIC 3 LIGHT Inferior mirage


In an inferior mirage, the image appears to the observer below its actual point of origin (Fig. 14.18). This is the type most commonly seen.


An inferior mirage is usually seen as a puddle of water in the distance. For example, at the far end of a long road on a hot summer’s day, a shimmering puddle can be seen (Fig. 14.19).


inferior mirage cool air


direct sight to sky


from sky light ray hot air apparent image


FIG. 14.18 In an inferior mirage, the image appears to the observer below its actual point of origin


What is happening is that the air is roughly split into two layers: a dense cold pocket on top and a rare hot pocket of air near the ground. Since the reflection of light off the clouds is coming through the dense cold layer into the rare hot layer, it begins to refract away from the normal and bends so much it totally internally reflects back up from the ground. It is this reflection that you see looking down at the ground. In effect, you are looking at the image of the clouds but situated on the ground, in the distance.


Because the cold and hot air is constantly moving, the effect is not a clear sharp image, but a ‘shimmering’ bright image, which your brain interprets as a puddle of water.


Superior mirage


A superior mirage is the reverse effect in that it can occur on a cold day over snow or ice. A ship below the horizon may actually be visible over the surface of the sea, due to the light rays bending as they move from the lower cold region of air to the upper hot air (Fig. 14.20).


Another effect of refraction mirages is to lead you to believe you are looking at the sun setting when in fact by the time it appears to be on the horizon, it has actually set.


Snell’s window


Divers experience another phenomenon of total internal reflection. When underwater and looking up to the surface of the water, they are looking from a dense medium (water) to a rare medium (air). Up to the critical angle, they can see through the water into the air (although the position of images is slightly altered), but when they look at an angle greater than the critical angle, the light is reflected back and they are unable to see through the water. This has the effect of allowing a transparent circle of water (‘Snell’s window’) with a radius determined by the critical angle (Fig. 14.21).


The circular area can be calculated using trigonometry. If the vertical depth is taken as the adjacent side and the radius of the circle on the water surface is the opposite side, the tangent of the angle can be used for the critical angle. This is illustrated in Fig. 14.22.


150 FUSION


FIG. 14.21 Snell’s window is a transparent circle of water seen by divers looking up at an angle greater than the critical angle


cool air superior mirage warm air


FIG. 14.19 An inferior mirage usually appears as a puddle of water


FIG. 14.20 In a superior mirage, a ship below the horizon can be visible over the surface of the water


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