Chapter 24 – Light and its Phenomena
24.1 – Waves Vs. Particles; Huygens’ principle and Diffraction:
Light clearly carries energy, two ways this is done: particles (that is,
physically carrying mass) and waves (the medium doesn’t carry, but energy
is carried by the medium)
Newton favored “particle/corpuscular” theory of light, taking as evidence the
sharpness of shadows (as compared to the way sound waves bend around
But experiments, concerning interference seemed to indicate that light was
It is said to be an electromagnetic wave, therefore it does not require a
medium at all
In the 1900s, it became apparent that it had particle properties too, photos
as “particles of light”
Huygen’s Principle: a wave theory, which states that each point on a wave front
can be considered as a new source of wavelets (starting point of new waves). The
new wave front is the envelope of all the wavelets.
Useful for analyzing what happens when waves impinge an obstacle and
the wave front is interrupted, also predicts that waves bend in behind the
Diffraction: the bending of waves around obstacles (occurs for waves, but
The law of diffraction says that an “edge” the (angular) bending of the wave will be:
𝜃𝑑𝑖𝑓𝑓 = 𝜆/𝐷
This means a wave only casts a “shadow” if its size D is greater than the
wavelength of the wave.
Diffraction, like refraction is a wave phenomenon.
o It does occur for light, but the angles are so small that we do not
normally perceive it: shadows appear sharp unless we look very
o So Newton was wrong, and light appears to be a wave
o The wave model of light accounts for diffraction, how ray models do
When light travels from one medium to another, its frequency does not change, but
the wavelength does.
𝑛 Chapter 24 – Light and its Phenomena
24.3 – Interference: Young’s Double-Slit Experiment
1. Light from a single source falls on a screen containing two closely space
slits 1 and S2
2. If light consisted of particles, one would see two bring lines on the screen
behind the slit.
3. But instead a series of lines are observed
4. This is due to diffraction, the waves leaving the two small slits spread out
Constructive Interference: (bright area at the center of the screen) the
waves from the two slits travel the same distance, so they are in phase: a
crest of one wave arrives at the same time as a crest of the other wave.
Hence the amplitudes of the two waves add to form a larger amplitude
o Also occurs when the paths of the two rays differ by one wavelength
(or any whole number of wavelengths)
𝑑sin𝜃 = 𝑚𝜆
Destructive Interference: (screen is dark) if one ray travels and extra
distance of ½ wavelength, the two waves are exactly out of phase when
they reach the screen: the crests of one wave arrive at the same time as the
troughs of the other wave and so they add to produce a zero amplitude
𝑑sin𝜃 = 𝑚 + 𝜆
The bright fringes are beaks or maxima of light intensity, while the dark fringes are
minima. The intensity of the bright fringes is greatest for the central fringe (m=0)
and decreases for higher orders.
Fringe spacing depends on wavelength, with longer wavelengths having larger
spacing. So what happens if you put a mix of wavelengths onto the slit?
The zeroth order fringe (n=0) has no wavelength dependence, so it will be
But all higher order fringes (n>0) will be