INTERFERENCE (PHYSICAL OPTICS )Chapter 35

WHAT IS PHYSICAL OPTICSIt is the study of light as waves. • Geometric optics treats light as particles (or rays)

that travels in straight lines. • Physical optics (wave optics) deals with the wave

nature of light, such as the spreading of waves (diffraction) and the interference of waves.

ADDING WAVES

Combining two waves does not always give you a bigger wave!

Constructive interference Destructive interference

SHIFT BY HALF A WAVELENGTH

Constructive

Destructive

CONSTRUCTIVE INTERFERENCEConstructive interference occurs whenever the path difference is an integral multiple of λ:

λ

DESTRUCTIVE INTERFERENCEDestructive interference occurs whenever the path difference is an half-integral multiple of λ:

λ

THE SAME IS TRUE IN 2D (CONSTRUCTIVE)

THE SAME IS TRUE IN 2D (DESTRUCTIVE)

PHASE DIFFERENCE AND PATH DIFFERENCE

INTERFERENCE ON WATER

RIPPLES

IT IS ALL ABOUT PATH DIFFERENCE

YOUNG’S DOUBLE SLIT (1800)

WHAT ARE THE FRINGES?

The bright fringes represents regions of high intensity I

The dark fringes represents regions of low intensity.

THE MIDDLE

r1

r2

Light can reach this place via two paths, r1 and r2.

Path difference is:

Why is I high here?

Constructive Interference!

WHAT ABOUT THE OTHER FRINGES?

BRIGHT AND DARK FRINGES

CALCULATING THE ANGLE

WHAT YOU SEE

EXAMPLEGiven λ = 700nm and d = 3500nm, find the angles at which you can find the 0th, 1st and 2nd order maxima. What if you double d? (Notation: m = order )

d

RADIO BROADCASTThe frequency f =1.5×106Hz, from two antennas shown below. In what directions will you find the strongest signal?

POSITION ON THE SCREEN

POSITION ON THE SCREEN (APPROXIMATION)

POSITION ON THE SCREEN (APPROXIMATION)

FIND THE WAVELENGTH

FIND THE WAVELENGTH

FIND THE WAVELENGTH (ALTERNATIVE)

PHASE DIFFERENCE AND PATH DIFFERENCE

INTENSITY

WAVELENGTH IN DIFFERENT MEDIUM

WAVELENGTH IN DIFFERENT MATERIALS

PHASE SHIFT DURING REFLECTION

WATCH OUT FOR THE PHASE SHIFT

No relative phase shift ΔΦ =0

Relative phase shift ΔΦ =π

EXAMPLE

Assuming θ =0°, λ=550nm, nMgF = 1.38, what is the smallest L so that the two light rays shown will cancel out each other?

EXAMPLE: THIN-FILM INTERFERENCE 1Is the fringe at the line of contact bright or dark? Given l=10cm, h=0.02mm and λ=500nm, find the fringe spacing.

EXAMPLE: THIN-FILM INTERFERENCE 2What if we change the upper plate to a plastic with n =1.4, and fill the wedge with a grease of n =1.5, while the bottom is a glass with n =1.6?

OTHER EXAMPLES OF INTERFERENCE

SOAP BUBBLES

NEWTON’S RINGS

X-RAYS DIFFRACTION