Dispersion is the spreading of white light into a spectrum of wavelengths when light propagates through a medium. It occurs when the speed of light depends on its wavelength. We see colors like red, orange, yellow, green, blue, and violet associated with different light wavelengths. The sequence of colors in a rainbow is the same as the visible light spectrum. Huygens' principle describes how each point on a wavefront acts as a secondary source, with the new wavefront determined by the tangent to the secondary wavelets. Interference patterns form when waves interact constructively or destructively, such as with double slits. Thin film interference produces the bright colors seen in oil slicks and soap bubbles.

1. Dispersion is defined as the spreading of white light into its full spectrum of
wavelengths. More technically, dispersion occurs whenever the propagation of light
depends on wavelength

2. We see about six colors in a rainbow—red, orange, yellow, green,
blue, and violet; sometimes indigo is listed, too. These colors are
associated with different wavelengths of light, as shown in. When
our eye receives pure-wavelength light, we tend to see only one of
the six colors, depending on wavelength. The thousands of other
hues we can sense in other situations are our eye’s response to
various mixtures of wavelengths. White light, in particular, is a fairly
uniform mixture of all visible wavelengths. Sunlight, considered to
be white, actually appears to be a bit yellow, because of its mixture
of wavelengths, but it does contain all visible wavelengths. The
sequence of colors in rainbows is the same sequence as the colors

6. The Dutch scientist Christiaan Huygens (1629–1695) developed a useful technique for determining in detail
how and where waves propagate. Starting from some known position, Huygens’s principle states that
every point on a wave front is a source of wavelets that spread out in the forward direction at the same
speed as the wave itself. The new wave front is tangent to all of the wavelets.
Huygens’s principle applied to a straight wave
front. Each point on the wave front emits a
semicircular wavelet that moves a distance s = vt.
The new wave front is a line tangent to the wavelets.

10. The bending of a wave around the edges of an opening or an obstacle is called diffraction

11. The most certain indication of a wave is interference. This wave characteristic is most prominent when the
wave interacts with an object that is not large compared with the wavelength. Interference is observed for water
waves, sound waves, light waves, and, in fact, all types of waves.

13. Photograph of an interference pattern
produced by circular water waves in a
ripple tank. Two thin plungers are
vibrated up and down in phase at the
surface of the water. Circular water waves
are produced by and emanate from each
plunger. The points where the water is
calm (corresponding to destructive
interference) are clearly visible.

15. Since S0 is assumed to be a point source of monochromatic light, the
secondary Huygens wavelets leaving S1 and S2 always maintain a constant
phase difference (zero in this case because S1 and S2 are equidistant from
S0 ) and have the same frequency. The sources S1 and S2 are then said to
be coherent. By coherent waves, we mean the waves are in phase or have
a definite phase relationship. The term incoherent means the waves have
random phase relationships, which would be the case if S1 and S2 were
illuminated by two independent light sources, rather than a single source
S0 . Two independent light sources (which may be two separate areas
within the same lamp or the Sun) would generally not emit their light in
unison, that is, not coherently. Also, because S1 and S2 are the same
distance from S0 , the amplitudes of the two Huygens wavelets are equal.

19. The equations for double-slit interference imply that a series of bright and dark lines are
formed. For vertical slits, the light spreads out horizontally on either side of the incident
beam into a pattern called interference fringes. The closer the slits are, the more the bright
fringes spread apart. We can see this by examining the equation
d sin θ = mλ, for m = 0, ±1, ±2, ±3… . For fixed λ and m, the smaller d is, the larger θ must
be, since sin θ = mλ/d .
This is consistent with our contention that wave effects are most noticeable when the object
the wave encounters (here, slits a distance d apart) is small. Small d gives large θ , hence, a
large effect. Referring back to part (a) of the figure, θ is typically small enough that sin θ ≈
tan θ ≈ ym/D , where ym is the distance from the central maximum to the mth bright fringe
and D is the distance between the slit and the screen. may then be written as

22. The bright colors seen in an oil slick floating on water or in a sunlit soap
bubble are caused by interference. The brightest colors are those that
interfere constructively. This interference is between light reflected from
different surfaces of a thin film; thus, the effect is known as thin-film
interference.