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The Lens That Changed Lighthouses

8th grade nonfiction passage about light and technology: 286 words, five questions, an answer key and vocabulary.

8th grade nonfiction light and technology 286 words

The Lens That Changed Lighthouses

For centuries, sailors depended on lighthouses to warn them of rocks and guide them safely into harbor. Early lighthouses burned wood or coal fires on top of towers, and later ones used oil lamps. These lights were dim and wasteful, because a lamp sends its glow out in every direction, and most of it shines out over empty sea or up into the sky.

In the 1820s, a French physicist named Augustin Fresnel solved the problem with a new kind of lens. A normal lens strong enough for a lighthouse would have been enormously thick and heavy, and it would have absorbed much of the light. Fresnel's idea was to build the lens from many separate rings of glass, each one cut at a carefully chosen angle. The rings bend the light that hits them and gather it into one narrow, powerful beam. The finished lens looked like a giant glass beehive, yet it was much thinner and lighter than a solid lens of equal strength.

The lens was usually set on a rotating platform, so the beam swept across the sea like the hand of a clock. A sailor saw a flash each time the beam passed. Each lighthouse was given its own pattern of flashes, such as two quick flashes followed by a pause, so that a captain could tell which one was in view and work out the ship's position.

Fresnel lenses made lights shine much farther, and the beams of the largest ones could be seen from more than twenty miles away. Today, ships rely mostly on satellite navigation, and many lighthouses now use electric bulbs or LEDs, but some old Fresnel lenses are still working, and others are preserved in museums.

Questions

  1. What did early lighthouses burn? in the text

  2. What did Fresnel build his lens from? in the text

  3. How could a captain tell which lighthouse was in view? in the text

  4. Why was an ordinary lamp wasteful? think about it

  5. Why would a solid lens strong enough for a lighthouse have been a problem? think about it

Words to know

lighthouse
a tall tower with a bright light that guides ships
harbor
a sheltered place where ships can stay safely
lens
a curved piece of glass that bends light
beam
a narrow stream of light
physicist
a scientist who studies matter, energy, and light
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 275 words · Flesch-Kincaid grade 3.9

For hundreds of years, sailors counted on lighthouses. They warned ships of rocks and guided them safely into harbor. Early lighthouses burned wood or coal fires on top of towers. Later ones used oil lamps. These lights were dim and wasteful. A lamp sends its glow out in every direction. Most of it shines out over empty sea or up into the sky.

In the 1820s, a French physicist named Augustin Fresnel fixed the problem with a new kind of lens. A normal lens strong enough for a lighthouse would have been very thick and heavy. It would have soaked up much of the light. Fresnel built his lens from many separate rings of glass. Each ring was cut at a careful angle. The rings bend the light that hits them. They gather it into one narrow, strong beam. The finished lens looked like a giant glass beehive. But it was much thinner and lighter than a solid lens of the same strength.

The lens usually sat on a turning platform. The beam swept across the sea like the hand of a clock. A sailor saw a flash each time the beam passed. Each lighthouse had its own pattern of flashes, such as two quick flashes and then a pause. A captain could tell which lighthouse was in view. Then he could work out the ship's position.

Fresnel lenses made lights shine much farther. The beams of the largest ones could be seen from more than twenty miles away. Today, ships mostly use satellite navigation. Many lighthouses now use electric bulbs or LEDs. But some old Fresnel lenses still work. Others are kept in museums.

▲ This passage · 286 words · Flesch-Kincaid grade 9.2

For centuries, sailors depended on lighthouses to warn them of rocks and guide them safely into harbor. Early lighthouses burned wood or coal fires on top of towers, and later ones used oil lamps. These lights were dim and wasteful, because a lamp sends its glow out in every direction, and most of it shines out over empty sea or up into the sky.

In the 1820s, a French physicist named Augustin Fresnel solved the problem with a new kind of lens. A normal lens strong enough for a lighthouse would have been enormously thick and heavy, and it would have absorbed much of the light. Fresnel's idea was to build the lens from many separate rings of glass, each one cut at a carefully chosen angle. The rings bend the light that hits them and gather it into one narrow, powerful beam. The finished lens looked like a giant glass beehive, yet it was much thinner and lighter than a solid lens of equal strength.

The lens was usually set on a rotating platform, so the beam swept across the sea like the hand of a clock. A sailor saw a flash each time the beam passed. Each lighthouse was given its own pattern of flashes, such as two quick flashes followed by a pause, so that a captain could tell which one was in view and work out the ship's position.

Fresnel lenses made lights shine much farther, and the beams of the largest ones could be seen from more than twenty miles away. Today, ships rely mostly on satellite navigation, and many lighthouses now use electric bulbs or LEDs, but some old Fresnel lenses are still working, and others are preserved in museums.

■ Harder · 300 words · Flesch-Kincaid grade 11

For centuries, sailors depended on lighthouses to warn them of hidden rocks and to guide them safely into harbor. The earliest lighthouses burned wood or coal fires on top of their towers, and later ones relied on oil lamps. These lights were dim and inefficient, because a lamp radiates its glow in every direction, so most of the light is wasted on empty sea or on the sky overhead.

In the 1820s, a French physicist named Augustin Fresnel solved this problem with an ingenious new kind of lens. An ordinary lens powerful enough for a lighthouse would have been enormously thick and heavy, and it would have absorbed a considerable share of the light passing through it. Fresnel's insight was to construct the lens from many separate rings of glass, each one cut at a carefully calculated angle. The rings bend the light that strikes them and concentrate it into a single narrow, powerful beam. The finished lens resembled a giant glass beehive, yet it was far thinner and lighter than a solid lens of equal strength.

The lens was typically mounted on a rotating platform, so that the beam swept across the sea like the hand of a clock. Sailors observed a flash each time the beam passed. Every lighthouse was assigned its own distinctive pattern of flashes, such as two quick flashes followed by a pause, which allowed a captain to identify the lighthouse in view and work out the ship's position.

Fresnel lenses made lights shine dramatically farther, and the beams of the largest ones could be seen from more than twenty miles away. Today, ships rely primarily on satellite navigation, and many lighthouses have switched to electric bulbs or LEDs, but some of the old Fresnel lenses are still operating, and others are preserved in museums.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put lighthouse, harbor, lens, beam and physicist on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 4 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "The Lens That Changed Lighthouses"?

It was written for 8th grade. It has 286 words and works out to a Flesch-Kincaid grade level of about 9.2. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are five. 3 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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