# Speed of Light

This is a phrase that often appears in student responses in Physics examinations. Let's look at various versions of it and see if they are correct. We will start from the most basic.

1. Light is constant. This is incorrect as the property of light has not been identified.
2. The speed of light is always constant. This is incorrect. The speed of light in water is less than the speed of light in a vacuum.
3. The speed of light in a vacuum is a constant value. This is not fully correct as the frame of reference has not been included.
4. The speed of light in a vacuum is the same value in any inertial frame of reference. Correct. This was one of Einstein's postulates in 1905.
5. The speed of light in a vacuum is the same value in any frame of reference. Incorrect. In accelerating (non-inertial) frames of reference the speed of light in a vacuum is not always c.

Here are some other misconceptions about the speed of light.

1. The speed of light in a vacuum is given the symbol c because c is the first letter of constant. Incorrect. The c comes from the Latin word celeritas for speed.

2. The Michelson and Morley experiment showed that the speed of light was always constant. Incorrect. There is no mention of the constancy of the speed of light in their paper describing the experiment. Einstein proposed this in 1905. M and M were not able to detect the motion of the Earth relative to the aether (this is a null result)

3. Light waves are different to sound waves. The speed of sound in air at 20 degrees C is 343m/s. Imagine that you move towards a source of sound at 20 m/s. What is the speed of the sound waves relative to you? [363 m/s]
4. The Michelson-Morley experiment found that the light waves in their interferometer always arrived in phase. Incorrect. The rays of light produced an interference pattern after travelling on perpendicular paths. This pattern did not change when the apparatus was rotated through 90 degrees. This is called no fringe shift.
5. The first determination of the speed of light was made by the Dutch physicist Christian Huygens in 1677 using astronomical measurements made by Olaus Roemer. The value obtained was 2.3x108 m/s. The French physicist Hippolyte Fizeau using an Earth based method determined the speed of light as 3.1x108 m/s in 1849.

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