 At the time theories regarding light were being developed, scientists knew that light:  Refracted  Travels in a straight line  Reflects

 This conclusion was made by comparing the behaviour of light to behaviour of mechanical waves that were more observable  Scientists didn’t originally believe that light was a wave because they could not see light as a wave – can you see the wave motion of light?

 Newton originally stated that light was a stream of particles ONLY– most of the behaviour of light could be explained by looking at light as a stream of particles that moved very fast and had a very small mass  The properties of light: rectilinear propagation, the law of reflection (imagine bouncing a ball or hitting a pool ball against the bumper) could be explained with particles

 Newton’s particle theory did try to explain dispersion, refraction, and diffraction (the bending of waves around barriers) as well – but the arguments were less convincing  He suggested that different colours of light were made up of particles with different masses that accelerated at different rates when they travelled through various solids

 During Newton’s time, one scientist named Poisson seemed to prove that light diffracted  He argued that if Newton’s theory was correct, light directed through a hole should show clear edges – not fuzzy ones that would be caused by diffraction  Although a diffraction pattern with fuzzy edges was produced, it was waved quickly away by Newton – and re-explained that light particles could bounce off barriers and thus appear to “bend” around corners

 Poisson’s spot, using better equipment than was available during Newton’s time shows the true pattern of the spot that is more difficult to explain using the Corpuscular theory – but this pattern was not really observable  Because of this, Poisson’s explanation was waved off

 One property of waves that was never observed with light waves would prove to be the defining factor that switched scientists towards making the relationship between waves and light  Waves can INTERFERE with each other – where the sum of the amplitude of the wave can either create a larger one or cancel it out  This occurs as long as the waves are IN PHASE with each other

 PHASE – the “timing” of the wave  If two waves are “IN PHASE”, the crests and troughs are appearing at the same time  If they are “OUT OF PHASE” they are shifted so that crests and troughs are appearing at different times IN PHASE OUT OF PHASE BY OUT OF PHASE BY 90 0

1 + 1 = 2 (-1) +(-1) = (-1) = 0

 When straight waves reach a small opening, they diffract forming circular waves  Circular waves are created by a single disturbance (imagine dropping a rock into water)  When a straight wave attempts to pass through a small opening, only a small portion of it makes it through, creating a small disturbance similar to a point disturbance that creates circular waves

 When circular waves interfere with one another they produce specific patterns of interference if the two sources that are producing are in phase with one another  What you see is an interference pattern that is set up in a ripple tank – it forces one straight wave through two small openings creating two point sources that are in phase

 If path difference is equal to a half a wavelength, there will be DESTRUCTIVE interference  If path difference is equal to a multiple of a FULL wavelength – CONSTRUCTIVE interference occurs

DESTRUCTIVE INTERFERENCE: Notice that these areas are dark – there are NO WAVES because waves have cancelled out CONSTRUCTIVE INTERFERENCE: Has occurred wherever you see the waves – in these points, waves have added together and are still visible

 Many scientists that opposed Newton’s particle theory of light tried to produce interference patterns with light  Some placed light bulbs next to each other in an attempt to reproduce point sources as seen in ripple tanks  However, this wouldn’t work since these light bulbs weren’t in phase

 Young, a lesser known scientist at the time, managed to solve the problem by forcing light through a single slit and then two double slits to create two point sources that were in phase  Through this, he was able to create interference patterns on screens

 With Young’s experiment, eventually the scientific community at the time, including Newton, accepted the description of light as a wave  This theory held for some time until the 20 th century when experiments and theories by scientists like Einstein and Plank began to suggest that light behaves as a particle as well!

 It is difficult to understand light completely as a wave  In fact, scientists suggest that light has a “particle-wave duality” – that is, light behaves as a wave as well as a stream of particles  These particles are known as PHOTONS

 What you have to imagine is a water wave – a wave can travel through water, yet water itself is composed of many tiny particles  The wave is the result of the movement of these particles  You can predict where a water molecule will be in space based on the repetitive motion of the wave  Light is similar to that; though it behaves as a wave, it can be seen as a stream of particles  Technically, the wave nature of light represents the statistical possibility of locating a given photon within a path taken by light

 Light is known as ELECTROMAGNETIC RADIATION because it is created by fluctuating magnetic and electric fields  When electrons move, they create an electric field that in turn creates a magnetic field  The reverse of this occurs as well; when magnets move they create a magnetic field that in turn causes electrons to move as well

 Therefore, light is a type of wave known as an ELECTROMAGNETIC wave  These waves all travel at the speed of light, and do not require a medium to travel through  Once a changing electric or magnetic field initiates an electromagnetic wave, it becomes “self-propogating”, and pushes itself along  ve.htm ve.htm

 They can be seen as two waves placed together  Remember: waves show repeat motion  As the electric field increases and decreases, so will the magnetic field  Change perpendicular to each other

ALL TRAVEL AT THE SPEED OF LIGHT, AND VARY IN THEIR FREQUENCIES

 So how do we apply the wave-particle thoery to describe physical phenomenon observed in light?  Wave-particle theory states that one or the other is best used in a given situation – but not both  It is easier to understand the behaviour and characteristics of light by using one of these theories to describe it

 This is a good example of how particles can describe a phenomenon better than waves:  Don’t confuse the BRIGHTNESS of a light with the ENERGY of the EM wave; BRIGHTNESS of visible light is dependent on its INTENSITY – as in how MANY photons or points of light are present

 Other characteristics of light are better explained by understanding wave nature and applying them to light

 The energy of the wave is determined by its frequency (as well as its colour)  The frequency of the wave is determined by its source – whatever generated the wave  Notice that when a light ray refracts, the colour doesn’t change – that is because the frequency or energy of the wave remains constant – and the wavelength changes to accommodate the change in speed as it enters a new medium

 Photons are massless and they all travel at the same speed  Therefore, red light is not functionally made up of different photons than blue light is  So the particle theory – as in the types of particles that make up blue vs. red light are not adequate to describe their differences

 BUT the photons in red light move in a wave that has a lower frequency than that of blue light  That is what differentiates them – and also differentiates the amount of energy the two different types of light possess  See how the particle theory doesn’t work to describe those characteristics of light? You have to understand the wave theory to describe it!

 Strangely enough, particles that are very small (smaller than atoms – ie. Electrons) exhibit wave behaviour too  This picture was taken by forcing a stream of electrons through a double slit  And we know electrons are particles for sure!