Topic 3 – Is light a Wave?

In the exam you are expected to:

 

Isaac Newton (1642 – 1727) did many of the early experiments on light in the Seventeenth Century.  His argument was that light was a stream of particles.  This was nothing new; the Ancient Greek philosopher Democritus had proposed that objects were visible because of the swarm of particles that they put into the air.

 

Newton’s evidence was that:

 

His assumption was that particles travelled at a constant velocity except when near the boundary between two substance when unbalanced forces act on them.

 

In reflection the velocity perpendicular to the surface was reduced to zero, then increased to the original value in the opposite direction.  The velocity component parallel to the surface was unchanged.

 

In refraction the velocity perpendicular to the surfaces was increased by an attraction force.  The velocity component parallel to the surface was unchanged.  The light ray was bent towards the normal.

 

 

Question 1 What happens to the resultant velocity as a result of the change that Newton proposed?  Is it consistent with what you know about the speed of light in a material?  ANSWER

 

The splitting of white light into the colours of the rainbow was explained by the difference in forces on the particles of different colours.

 

There are other limitations:

 

Light as a Wave

At the same the same time as Newton, a Dutch physicist Hans Christiaan Huygens (1629 – 1695) proposed that light was a wave.  His view was prompted by the observation that beam of light cross each other without scattering.  If they were particles, there would be collisions between the beams.

 

His assumption was that light waves spread in all directions at a constant speed in a material called ether.  Ether permeated everything including a vacuum.

 

Huygens (pronounced “Harkens”) proposed models that consisted of plane wavefronts which consisted of secondary wavelets.

 

The mechanism for reflection is like this:

 

 

 Question 2   When a wave is reflected, its phase changes by 180o .  How do you think this diagram is consistent with this observation?  ANSWER

 


Huygens explained refraction in a similar way

 

 

 

His theory showed that the wave speed in the material was less than the wave speed in air.

 

 

Question 3 Is this theory of refraction consistent with what you know already about refraction?  ANSWER

 

These observations are very similar to what we see when we study water waves.
Huygen’s theory did not discuss ideas of frequency and wavelength.  Light was though to emitted in pulses of energy.  There was no mathematical theory of continuous waves.  Nor had diffraction of light been noticed.

 

Although the wave behaviour of light was supported by many eminent scientists, it was rejected out of hand by Newton, who was the pre-eminent scientist with a considerable reputation.  Newton was not a pleasant man, cantankerous and desperately self-opinionated.  Anyone who stood up to him risked having his work and character ridiculed.  That happened to Huygens, whose work remained overlooked for many years.  Also waves could not pass through a vacuum, so it was thought.  However on the continent, Huygen’s theory carried more weight.

 

There was little evidence either way and the debate ran on for many decades until the Nineteenth Century, until Thomas Young (1773 – 1829) performed his Double Slit experiment in 1801.  The details and quantitative treatment are given in Module 4 Topic 6.

 

 Question 4   Why was the wave nature of light slow to catch on? ANSWER

 

Young’s Double Slits

In this experiment, Thomas Young demonstrated interference.  Part of the problem of demonstrating interference using two light sources is that getting coherent waves is impossible.  However if the ray from one source is split into two, by its nature, the ray consists of coherent waves.  Also getting pure monochromatic light was not easy, even with coloured filters.

 

 Question 5 What is meant by coherent waves? ANSWER

 

Nowadays it's very easy with a laser, which always gives out coherent monochromatic light.

 

 

In his day Young could only use a dim light source such as a candle or oil lamp. However the effect could be seen convincingly, and the experiment became a turning point in the debate between those who considered light as a wave.  There was no way that it could be explained by particles.  On the other hand it could be explained easily by wave theory.

 

Although this was clearly a wave phenomenon, there was some delay before it was whole-heartedly accepted, which happened when a good mathematical argument was worked out.  Even then not all were convinced.

 

In 1850 the speed of light was measured in air and in water.  The speed of light in water was found to be lower than that in air, which:

 

Then the particle theory was finally abandoned in mainstream Physics thinking.

 

Question 6  What made physicists believe that light was a wave rather than a particle?  ANSWER

 

  Electromagnetic Waves

In 1865 the theoretical physicist James Clerk-Maxwell (1831 – 1879) predicted that an oscillating electric field would cause a magnetic field to oscillate and vice versa.  By dint of rigorous mathematical analysis he predicted that the waves would propagate as a transverse wave and gave a formula for their speed.

 

 Question 7     Show that  = 3 × 108 m/s.  ANSWER

 

It had been discovered in 1831 that a changing magnetic field always induced a voltage.  It would be reasonable to suppose that there would be an electric field associated with this voltage.  Maxwell also considered that it would be reasonable to say that a changing electric field would induce a magnetic field, which in turn would produce a changing electric field and so on.  From this he concluded that the electromagnetic wave where the electric field and the magnetic fields are at right angles to each other.

The diagram above shows:

 

Electromagnetic waves can be polarised.  When the electric field vector is vertical, the wave is vertically polarised; when it is horizontal, the wave is horizontally polarised.

 

Radio Waves

The electromagnetic wave remained a theoretical concept until their existence was demonstrated by a German Physicist Heinrich Hertz (1857 – 1894).  He set up this lethal looking apparatus (there was no Health and Safety at Work Act):

 

 

Question 8  What would the charge time graph for the electrical oscillation look like? ( Hint: It’s like a spring)  ANSWER


The high frequency oscillations induce a voltage in the loop which was sufficient to cause a small spark to jump.  The spark could be made more powerful if the loop was put at the focal point of a concave mirror.  If he put the loop on its side, the effect was not seen at all, indicating that the waves were polarised.

 

Hertz went on to work out the frequency on the assumption that the effect was due to electrical resonance.  He also set up standing waves and found the wavelength between  nodes.  This allowed him to determine the speed of the waves, getting a very similar result to what Maxwell had predicted in his theoretical calculations.

 

This experiment formed the basis for experiments by Guillermo Marconi, the son of an Italian Count, into wireless telegraphy.

 

 

It had been clinched.  Light was a wave.

 

It is easy to dismiss the early ideas as ludicrous.  Modern physicists have the truths based on years of painstaking research by the world’s most eminent physicists, which are handed down from generation to generation.  Each generation of teachers has its set of books on which their material is based; the style will change, but the essential truths will not.  The early physicists had none of these; their work was based entirely on observation from pretty primitive technology.  It is easy to pick up misconceptions where there is no bedrock of underlying truth.  The theories produced were presented in total good faith.  The early physicists were men of integrity, even if they were at times head-strong.  Cantankerous though he was, Newton produced laws on which space flights are based today. 

 

Contrast that to some scientists today who for whatever reason have sacrificed integrity in order to fulfill an unrealistic target set by a manager, or worse still to make a quick buck. 

 

Summary

Newton thought that light was a particle

 

Huygens thought light was a wave.

 

The debate went on for many decades.

 

Young showed the wave properties of light.

 

Maxwell predicted the wave nature of light which was an electromagnetic wave.

 

The electromagnetic wave model was proved by Hertz

 

 

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