Light as a wave - Physics - Turning Points A2

In the exam you are expected to:

• Compare Newton’s Corpuscular Theory with Huygens’ Wave Theory in general terms;

• Explain why Newton’s Theory was preferred;

• Explain fringes in Young’s Double Slit Experiment in general terms;

• Explain why Huygen’s wave theory was slow in being accepted;

• Discuss nature of electromagnetic waves;

• Be aware of Maxwell’s formula for speed of electromagnetic waves in a vacuum

• Be aware of Hertz’s discovery of radio waves.

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 that he called corpuscles.  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:

• Objects cast sharp shadows; if they were a wave, the shadows would be fuzzy;

• Light passed through a vacuum; there was no material for a wave to propagate through.

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 particles collided with the surface as in standard particle collisions, 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.

His theory suggested light travelled faster in substances other than air due to the attractive force at the surface.

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:

• The theory could not explain partial reflection or refraction.

• It could not explain interference.

• It seemed that there was both attraction and repulsion at some boundaries.

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 proposed models that consisted of plane wavefronts which consisted of secondary wavelets.

The mechanism for reflection is like this:

• Huygen's theory demonstrated how waves might interfere to form a wavefront, propagating in a straight line.
• The diagram here illustrates the wavelets reflecting at both A and B forming a new wave front.
• Geometry can be used to show that the wavefronts make equal angles to the boundary, so that angle of incidence = angle of reflection.

Huygens explained refraction in a similar way

• Wavefront 1 reaches A.

• Wavefront from