The Speed of Light

It was not known whether light travels instantaneously or with finite speed until the late 1600s. Galileo had tried to measure the time a light beam takes to travel to a distant mirror and back, but the time was so short that he couldn’t begin to measure it. Others tried the experiment at longer distances with lanterns they flashed on and off between distant mountain tops. All they succeeded in doing was measuring their own reaction times.

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Fig. 6.1. Light coming from Jupiter’s moon Io takes a longer time to reach Earth at position D than at position A. The extra distance that the light travels divided by the extra time it takes gives the speed of light.

le Roemer. The first demonstration that light travels at a finite speed was supplied by the Danish astronomer Ole Roemer about 1675. Roemer made very careful measurements of the periods of Jupiter's moons. The moon Io is visible through a small telescope and was measured to revolve around Jupiter in 42.5 hours. Io disappears periodically into Jupiter’s shadow (eclipses by Jupiters), so this period could be measured with great accuracy. He found that while Earth was moving away from Jupiter (from position B to C in Figure 6.1) the measured periods of Io were all somewhat longer than average. When Earth was moving toward Jupiter (from position E to F) the measured periods were shorter than average. Roemer estimated that the cumulative discrepancy between positions A and D amounted to about 22 minutes. That is, when Earth was at position D, Io would pass into Jupiter’s shadow 22 minutes late, compared with observations at position A.

Christian Huygens. Christian Huygens correctly interpreted this discrepancy. When Earth was farther away from Jupiter, it was the light that was late, not the moon. Io passed into Jupiter’s shadow at the predicted time, but the light carrying the message did not reach Roemer until it had traveled the extra distance across the diameter of Earth’s orbit. There is some doubt as to whether Huygens knew the value of this distance. In any event, this distance is now known to be 300,000,000 km. Using the correct travel time of 1000 s for light to move across Earth’s orbit makes the calculation of the speed of light quite simple. The speed of light is 300,000km/s.

Albert Michelson. The most famous experiment measuring the speed of light was performed by the American physicist Albert Michelson in 1880. Light from an intense source was directed by a lens to an octagonal mirror initially at rest. The mirror was adjusted so that a beam of light was reflected to a sta­tionary mirror located on a mountain 35 km away and then reflected back to the octagonal mirror and into the eye of an observer. The dis­tance the light had to travel to the distant mountain was known, so Michelson had to find only the time it took to make a round trip. When the mirror was spun, short bursts of light reached the mountain mirror and were reflected back to the spinning octagonal mirror. If the rotating mirror made exactly one eighth rotation in the time the light made the trip to the distant mountain and back, the mirror was in a position to reflect light to the observer. If the mirror was rotated too slowly or too quickly, it would not be in a position to reflect light. When the light entered the eyepiece, Michelson knew that the time for the light to make the round trip and the time for the octagonal mirror to make one eighth of a rotation was the same. He divided the 70-km round trip distance by this time. Michelson’s experimental value for the speed of light was 299,920 km/s, which is usually rounded to 300,000 km/s. Michelson received the 1907 Nobel Prize in physics for this experiment. He was the first American scientist to receive this prize.

We now know that the speed of light in a vacuum is a universal constant. Light is so fast that if a beam of light could travel around Earth, it would make 7.5 trips in one second. Light takes 8 minutes to travel from the sun to Earth and 4 years from the next nearest star, Alpha Centauri. The distance light travels in one year is called a light year (from ‘Conceptual Physics’).

Exercise 3. Match the scientist with his idea about the nature of light. An idea may be used more than once.

Scientist Idea

__ 1. Einstein a) Light is a wave.

__ 2. Empedocles b) Light consists of tiny particles.

__ 3. Euclid c) Vision results from sreamers or filaments

__ 4. Huygens emitted by the eye making contact with an object.

__ 5. Plato

__ 6. Socrates

Exercise 4. A) Look for the words in the text which have the following meanings.

1. ____________ – the thin wire inside a light bulb.

2. _____________ – the total or partial obscuring of one celestial body by another.

3. ____________ – any natural satellite of a planet.

4. _____________ – a particle representing a quantum of light or other electromagnetic radiation.

5. _____________ – an object with at least one reflective surface.

6. _____________ – a narrow projection of light energy radiating from a source into a beam.

7. _____________ – a multiple star in Centaurus whose three components represent the brightest object in the constellation, 4.4 light-years from Earth.

B) Complete the sentences using the correct form of the words above.

1. How many __________ has Jupiter got?

2. Albert Michelson used a ___________ arrangement to measure the speed of light.

3. Light takes 8 minutes to travel from the sun to Earth and 4 years from the next nearest star, __________.

4. Galileo tried to measure the time a __________ takes to travel to a distant mirror and back, but the time was so short that he couldn’t begin to measure it.

5. Plato thought that vision resulted from streamers or _________ emitted by the eye making contact with an object.

6. Roemer did find a variation in the speed of Io’s orbits, which caused Io’s _________ by Jupiter (the moments when Io passed in front of or behind Jupiter).

7. Light consists of particles called ________.

Exercise 5. Answer the questions to the text.

1. What were the first ideas of light among Greek philosophers?

2. Who was the first to tell that light traveled in wave?

3. What characteristic of light did Huygens provide evidence of?

4. What phenomena did Einstein explain in the theory he published in 1905?

5. What is a photon?

6. What is the nature of light according to modern theory?

7. Who was the first to demonstrate that light travels at a finite speed?

8. What did Roemer measure? Describe Roemer’s measurements.

9. How did Huygens interpret the discrepancy in Roemer’s experiment?

10. Describe A. Michelson’s experiment to measure the speed of light?

11. What was Michelson's experimental value for the speed of light?

12. How much time does it take to travel from the Sun to Earth?

Exercise 6. Are the following sentences true or false. Correct the false ones.

1. The idea that light consists of tiny particles was first proposed in the early 1900s.

2. Photons are massless bundles of concentrated electromagnetic energy.

3. Roemer’s measurement of discrepancies in the position of Jupiter’s moon Io was the first demonstration showing that light travels at a finite speed.

4. Albert Michelson received the the Nobel Prize for using a system of mirrors to measure the speed of light.

5. Christian Huygens suggested that light had a dual nature.

6. The speed of light is 300,000,000 m/s.

Speaking

Exiercise 7. In small groups discuss the following questions.

1. If you observe thunder and lightning, you can tell how far away the storm is. Do you need to know the speed of sound, of light, or of both?

2. Why did Roemer only need to know the radius of the earth’s orbit, not Jupiter’s, in order to find the speed of light?

Grammar. Participle I (Present Participle (-ING)).

Exercise 8. Underline the words with –ING forms in the text above. Are they the Gerund or Present Participle? How have you found it out? Translate the following sentences in written form.

Exercise 9. Rewrite the sentences replacing the words in italics with the Present Participle.

1. She was talking to her friend and forgot everything around her.

2. Since we watch the news every day we know what's going on in the world.

3. They are vegetarians and don't eat meat.

4. The dog wagged its tail and bit the postman.

5. While she was tidying up her room she found some old photos.

6. He was a good boy and helped his mother in the kitchen.

7. The man was sitting in the cafe. He was reading a paper.

8. Since I didn't feel well I didn't go to the cinema.

9. She walked home and met an old friend.

Exercise 10. Translate the following sentences using Participle I.

1. Чоловік, що продає газети, показав мені дорогу до станції. 2. Студенти, які живуть на лівому березі Дніпра, щодня їдуть до університету на метро. 3. Дівчина, яка зараз переходить вулицю, моя одногрупниця. 4. Він підійшов до старої жінки, яка сиділа на лавці. 5. Жінка покликала дітей, які грались на дитячому майданчику. 6. Він вказав на чоловіка, що читав газету. 7. Читаючи цю книгу, я знайшов багато незнайомих слів. 8. Навчаючись в університеті, він опублікував три наукові статті. 9. Чекаючи тебе, я зустрів старого приятеля. 10. Він увійшов до кімнати посміхаючись. 11. Хлопець пройшов повз нас слухаючи плеєр. 12. Обговорюючи відомий роман, студенти виражали власні думки та погляди. 13. Входячи до зали я побачив дітей, що танцювали навколо ялинки.

Reading and Listening

Exercise 11. Read and translate the text below.