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 180 0 OUT OF PHASE BY 90 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 http://physics.usask.ca/~hirose/ep225/emwa ve.htm http://physics.usask.ca/~hirose/ep225/emwa 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!