Subject: Science/ Physics: optics; Earth Sciences.
Ages:12+
Length:Snippet:One minute and five seconds, in two segments.
Lesson:Adds 20 minutes to a class in which the optical phenomenon of refraction is introduced.
Learner Outcomes/Objectives:
Students will learn about real life occurrences of the optical effect of refraction.
Rationale:
Refraction is an effect of geometrical optics that has many real-life examples. The famous green flashis one of them that at the same timehas gathered around itself a special fascination, because it is not too easily observed. It has been used in the movie <emPirates of the Caribbean – At World’s End</em as a magical moment that signals special events. Its appearance in the movie is a good starting point to introduce the concept of refraction and the physics related to it, as well as further examples and explanations that together constitute a motivating way to introduce this topic.
Description of the snippet:
The first segment shows us the characters of the movie trying to interpret a peculiar navigational chart that is supposed to lead them to World’s End. The chart mentions "a flash of green" and an experienced sailor tells about the green flash. The second segment corresponds to the departure of Will Turner (Orlando Bloom), who sails off leaving behind his beloved Elizabeth Swann (Keira Knightley) to wait for his return. A green flash at sunset marks his disappearance at the horizon.
Helpful Background:
Light travels in a straight line and at a given speed through any medium, be it in vacuum, air, water or glass. When light has to pass from one medium to another, however, light waves are deflected if they hit the limit between them obliquely, because the speed in one medium is different than in the other (see the "Reflection and Refraction" section of <a href=“ Faces of Light: As Newton Sawit, withSomeMagicTricks</a>). We see a straw or a spoon sticking out of a glass of water as if it were bent. The light rays coming from the part in the water are bent at the surface and we are fooled to "see" them as coming from where the spoon is not.
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Surprisingly, there are fish that have learned to avoid the effects of refraction when hunting from underwater for insects sitting on rocks or plants on the shore. The Archer Fish hunts by using a stream of water shot out of their mouths to knock down insectsrestingonmangroverootsorotherfoliage. Theireyes are belowthewater and theyhavelearnedthatifthey position themselvesdirectlybelowtheirprey, so thatthereis no angle in their line of site and no displacement of theimagebyrefraction, theywillhave a better chance of knowingexactlywheretheirpreyissitting and to be ableto hit itwiththestream of water. (See a href=” Fish</a>).
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Refraction does not occur only when rays of light cross a clear boundary between two mediums. When the physical properties of a medium change gradually, the speed of light within that medium changes gradually too, and refraction occurs little by little. Light rays do not bend at one point but end up describing a curved path. This explains the formation of mirages where air is very hot near the ground, like deserts or asphalt. Light rays from the sun reach the surface of the Earth after passing through the whole atmosphere, where the pressure and temperature conditions change gradually from the uppermost layers to the ground level. The path through the atmosphere is longer when we see the sun low on the sky, shortly after sunrise or before sunset. The curving of these light rays becomes therefore stronger and, as in the case of a mirage, we see the image of the sun displaced with respect to its true position. Due to this effect, we can still see the sun when its true position is below the horizon. See <a href=” Formation of a Mirage</a> and "AtmosphericRefraction" in <a href=“ View of the Sky</a>). A nice interactive application to play with the position of the sun over the horizon, the atmospheric pressure and its temperature, showing the true and apparent position of the sun can be accessed at <a href=“ ApparentFlattening of theSun</a>.
If we analyze this situation in closer detail, we would need to take into account that refraction also depends on the wavelength (color) of the light.
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Different colors are refracted differently: See a href=“ here</a> for a graphic representation of the different refractions of different colors. This results in there being different “solar discs” in our view, one of each color of the rainbow. The violet and blue discs are those refracted most, and therefore the last to set on the horizon. However, blue light is dispersed away in the atmosphere (this is why the sky is blue in daylight) and leaves the blue image of the solar disc very weak, if visible at all. The violet disc is usually very weak, too, and difficult to distinguish. This leaves us with the green disc being the last to set – the fleeing instant in which only the last part of the green disc is seen above the horizon is what is called the green flash. The term “flash” is not very appropriate, as it raises expectations for a bright flash as in a photographic camera. It also probably influenced the filmmakers of Pirates of the Caribbean into depicting it as such. A great collection of real images of the green flash in its various manifestations can be seen at: <a href=“ Flash</a> and the subsequent pages that can be accessed by clicking on the arrows in the upper right corner of the website.
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The gallery includes photographs taken with exceptional atmospheric and visibility conditions that favor the viewing of even the blue and violet flashes. Here are some more each web site entitled Green Flash<a href=“
a href=“ Explorations</a> and
a href=“ Science: Picture of the Day</a>. See also
ahref=“ Green, blue and violet flashes</a>.
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The displacement of the green (and blue and violet solar discs) is of course present well before the instant the sun disappears behind the horizon. That instant is a good moment to see them without the brighter yellow disc outshining them. However, a photographic or video recording of the setting sun can show, when viewed in close up and slowed down, that the green, blue and violet discs become visible amidst the turbulent image of the setting sun even while the sun is above the horizon. Watch it 21 seconds into this beautiful video on the sky of the island of Tenerife, Spain:ahref=“ Sky - Tenerife</a>.
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Using the snippet in class:
Preparation
Be familiar with the location of the segments
Segment 1: From minute 23:25 to 24:20 (55 seconds). A navigational chart is being interpreted and as it mentions a “flash of green” a sailor is asked to explain about it. He mentions that it occurs only at sunset.
Segment 2: From minute 55:00 to 55:05 (five seconds). A sunset ends with a bright flash of green. (The chart has been deciphered, and the transit to World’s End is marked by the green flash on sunset.)
Segment 3:From minute 2:28:00 to 2:28:10 (ten seconds) Turner’s boat disappears at the horizon of the sea just as the sun sets with a green flash
Segment 4: After the end credits. From minute: 2:40:54 to 2:41:06 (12 seconds). A green flash is not seen directly, but through the green color it illuminates Elizabeth Swann’s face with. (The fourth segment shows the return of Will Turner. Elizabeth Swann is watching the sunset with a boy who is probably her and Turner’s son, and a green flash marks the appearance of a boat on the sea. Elizabeth smiles knowingly that it is Turner who is about to arrive.)
Step by Step
- Show the first and second segments in class as an introduction to the topic.
- Point out the two main factual errors contained the two segments: the green flash is not only seen at sunset, it can also be observed just before sunrise; and the green flash does not light up the sky with a bright flash of green but is rather a faint green dot that can be seen just above the solar disc either before it rises or after it sets.
- Introduce atmospheric refraction as the explanation of the green flash, using the information provided in the Helpful Background section and the linked web pages, according to the level of the students.
- Wrap up the lesson on refraction using the green flashes shown in segments 3 and 4.
Supplemental Materials and Links
The explanation given here is really just the beginning of the story of green flashes appropriate for the recommended educational level. The atmospheric phenomena at play are far more complex, and related to the concept of mirages. A thorough and complete explanation, with simulations and examples can be found at: