Light is a wave – in many ways it behaves just like a wave. Light is also a particle – in many ways it behaves just like a particle. This week we study the wave properties of light. Next week we’ll study the particle nature of light.
Cause of refraction
Refraction is caused by the change in the speed of light from one transparent medium to another, and the wave nature of light.
Consider two cart wheels on an axle rolling from the sidewalk to the grass at an angle. The wheel that hits the grass first slows down while the other keeps going fast, turning the wheels toward the normal.
A light wave behaves the same way when it moves from air into water for example. The portion of the wave fronts that hit the water first slow down, while the rest keeps moving fast, changing the direction of the light ray (which is always perpendicular to the wave fronts).
This provides a causal explanation for mirages (and all other refraction effects).
The wave crests that form the concentric circles of a spreading wave are called wave fronts.
Huygens said that the wave fronts of light are the overlapped crests of tiny secondary waves – wave fronts are made up of tinier overlapping wave fronts.
Every point on a wave front is a source of new wavelets that combine to produce the next wave front, etc.
ANIM: Spherical wave fronts
FIGS. 29.3 and 29.4
Laws of Reflection and Refraction can be understood by looking at the wavelet makeup of an incoming plane wave.
When waves go through openings, what comes out depends on what size the opening is compared to the wavelength:
Opening wider than the wavelength: plane waves go through fairly unchanged except at the corners, where the waves bend into the shadow area.
Opening smaller than the wavelength: the opening acts as a point source of new waves that fan out on the other side of the opening. This fanning out of waves, rather than going straight through, is called diffraction.
BOOK: show Fig. 29.7
Light can bend by reflection and refraction – it can also be bent by diffraction.
Light passing through an opening larger than its wavelength casts a sharp shadow. There is not much spreading out – there is little diffraction.
When it passes through a thin slit, the sharp shadow disappears and the light spreads out like a fan to produce a pattern of lighter and darker areas.
FIG. 29.8: light through a big opening in poster, light through slit in poster.
DEMO: laser through single slit
The amount of diffraction depends on how large the wavelength of light is compared to the size of the obstruction casting the shadow.
Longer wavelengths diffract more - they fill in the shadows better.
Diffraction and microscopes and radio
Diffraction fringes of light and dark regions are caused by interference.
Interference is the adding, or superposition, of different waves. Constructive interference is crest-to-crest reinforcement; destructive interference is crest-to-trough cancellation.
DEMO: circle interference plates
In 1801, Thomas Young discovered that light shining through two close pinholes produced light and dark fringes on a screen through interference.
DEMO: Monochromatic light through two slits
This happens because the light diffracts (fans out) through the two pinholes. Then the light from the two different sources (pinholes) travel different distances to a particular point on the screen.
If a crest from the light through one hole and a crest from the light through the other hole get to a point at the same time, then there is constructive interference – a bright fringe.
If a crest from the light through one hole and a trough from the light through the other hole get to a point at the same time, then there is destructive interference – a dark