Film title: History of the lighthouse
One of the biggest dangers a boat faces is crashing into land...
Or underwater rocks, especially at nighttime.
In ancient times people set fires along the shoreline to guide boats...
But firelight isn't very bright and it doesn't travel very far.
It was the ancient Egyptians who first came up with idea of a lighthouse to increase the visibility of firelight.
visibility
The Lighthouse of Alexandria was built around 280 BC.
The Lighthouse of Alexandria
Built around 280 BC
At the top of the high tower, a large curved mirror concentrated firelight into a beam that could be seen by ships several kilometres away.
Over time, lighthouses were built around the world using multiple mirrors, or reflectors.
reflectors
Reflectors were used to increase the brightness of light from oil lamps.
brightness
Then in 1822, the Frenchman Augustin Fresnel invented the first modern lighthouse lens.
Augustin Fresnel
1788–1827
A lens refracts light and concentrates it into a beam.
A glass lens big enough to use in a lighthouse would be very heavy and very expensive.
light source
glass lens
refracts
Fresnel's solution was to carve a series of grooves onto a lens.
Because each groove was at a slightly different angle, it acted as a separate lens.
The Fresnel lens was just as effective at focusing the light into a beam as a normal lens but because it was much thinner it was much lighter and cheaper to manufacture.
light source
Fresnel lens
Numerous lenses concentrated the light into a beam that was many times brighter than its source.
The lighthouse could now be seen by ships more than 30 kilometres away
In the nineteenth century lighthouses switched to electric light.
Today many modern lighthouses use a system of rotating lenses so that the light appears to be flashing on and off, making the lighthouse even more noticeable in the dark.
refraction
lens
reflectors
brightness
Or underwater rocks, especially at nighttime.
In ancient times people set fires along the shoreline to guide boats...
But firelight isn't very bright and it doesn't travel very far.
It was the ancient Egyptians who first came up with idea of a lighthouse to increase the visibility of firelight.
visibility
The Lighthouse of Alexandria was built around 280 BC.
The Lighthouse of Alexandria
Built around 280 BC
At the top of the high tower, a large curved mirror concentrated firelight into a beam that could be seen by ships several kilometres away.
Over time, lighthouses were built around the world using multiple mirrors, or reflectors.
reflectors
Reflectors were used to increase the brightness of light from oil lamps.
brightness
Then in 1822, the Frenchman Augustin Fresnel invented the first modern lighthouse lens.
Augustin Fresnel
1788–1827
A lens refracts light and concentrates it into a beam.
A glass lens big enough to use in a lighthouse would be very heavy and very expensive.
light source
glass lens
refracts
Fresnel's solution was to carve a series of grooves onto a lens.
Because each groove was at a slightly different angle, it acted as a separate lens.
The Fresnel lens was just as effective at focusing the light into a beam as a normal lens but because it was much thinner it was much lighter and cheaper to manufacture.
light source
Fresnel lens
Numerous lenses concentrated the light into a beam that was many times brighter than its source.
The lighthouse could now be seen by ships more than 30 kilometres away
In the nineteenth century lighthouses switched to electric light.
Today many modern lighthouses use a system of rotating lenses so that the light appears to be flashing on and off, making the lighthouse even more noticeable in the dark.
refraction
lens
reflectors
brightness
Explore the history of lighthouses.
Learning points
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