Light interference occurs when two light waves meet and their amplitudes combine, either constructively or destructively. Constructive interference occurs when the peaks and troughs of the waves align, amplifying the total amplitude. Destructive interference happens when peaks and troughs are out of phase, canceling each other out. Thomas Young first demonstrated light interference using a double-slit experiment, where light passing through two slits produces a pattern of bright and dark fringes on a screen due to the interference of the waves. Practical applications of light interference include sunglasses, camera lenses, and visors, which use interference to filter out harmful or inconvenient light wavelengths.

1. INTERFERENCE OF LIGHT WAVES
By
Mrs.P.Kanmani M.Sc., M.Phil.,
Assistant Professor of Physics,
Department of Physics (self),
V.V.Vanniaperumal College for Women,
Virudhunagar.

2. WHAT IS INTERFERENCE ?
 To grasp interference of light waves we need to understand the
terms interference, light, wave and wave front.
 Interference is defined as the combination of two or more
waveforms to form a resultant wave in which the displacement is
either reinforced or cancelled.

3. WHAT IS LIGHT?
Light is defined as electromagnetic wave with wavelength
lies between 400 and 700 nm which is visible to the
human eye.

4. WHAT IS A WAVE?
•A wave is characterized by its amplitude ,wavelength and phase.
•Amplitude tells about its intensity.
•Wavelength (λ) tells about its energy – inversely proportional.
• Phase of the wave is defined as the distance between the first
zero-crossing and the point in space that is defined as the
origin.

5. WHAT IS A WAVE FRONT?
 A wave front is a line or plane on which vibrations at every points
on it are in phase and are at the same distance from the source of
the wave.

6. WAVE FRONT

7. INTERFERENCE OF LIGHT WAVES
 With this basic knowledge lets get into interference of light waves.
 In order to observe interference of light waves, the following
conditions must be met:
 The sources must be coherent- that is, they must maintain a
constant phase with respect to each other.
 The sources should be monochromatic - that is, of a single
wavelength.

8. YOUNG’S DOUBLE SLITEXPERIMENT
Interference of light waves from two sources was first demonstrated by
Thomas Young in 1801. The schematic of young’s demonstration is shown
in figure.

9. INTERFERENCE OF LIGHT WAVES
 Wave fronts from source arrive at a barrier that contains two
parallel slits.
 These two Slits serve as a pair of coherent light sources because
waves emerging from them originate from the same wave front
and therefore maintain a constant phase relationship.
 The light from two slits produces on a viewing screen a visible
pattern of bright and dark parallel bands called fringes.

10. CONSTRUCTIVE ANDDESTRUCTIVE INTERFERENCE
 The bright bands appearing on the screen are due to constructive
interference.
 Constructive interference occurs when the phase difference
between the waves is an even multiple of π (180°).
 The dark bands appearing on the screen are due to destructive
interference.
 Destructive interference occurs when the difference is an odd
multiple of π.

11. PRACTICAL APPLICATIONS OF INTERFERENCE OF LIGHT
WAVES
 The phenomenon of light-wave interference with oily or filmy
surfaces has the effect of filtering light.
 Thus, has a number of applications in areas relating to optics:
sunglasses, lenses for binoculars or cameras, and even visors for
astronauts.
 In each case, unfiltered light could be harmful or, at least,
inconvenient for the user, and the destructive interference
eliminates certain colours and unwanted reflections.