Interference and Diffraction

Certain phenomena require the light (the electromagnetic radiation) to be treated as waves. Two relevant examples are: interference and diffraction

Plane Electromagnetic Waves E(x, t) = EP sin (kx-t) B(x, t) = BP sin (kx-t) ˆ z j x Ey Bz c Waves are in phase, but fields oriented at 900 Speed of wave is c At all times E = c B

Plane Electromagnetic Waves E and B are perpendicular to each other, and to the direction of propagation of the wave. The direction of propagation is given by the right hand rule: Curl the fingers from E to B, then the thumb points in the direction of propagation. Electromagnetic waves propagate in vacuum with speed c, the speed of light.

Combination of Waves + = In general, when we combine two waves to form a composite wave, the composite wave is the algebraic sum of the two original waves, point by point in space [Superposition Principle]. When we add the two waves we need to take into account their: Direction Amplitude Phase + =

The combining of two waves to form a composite wave is called: Combination of Waves The combining of two waves to form a composite wave is called: Interference + = Constructive interference (Waves almost in phase) The interference is constructive if the waves reinforce each other.

The combining of two waves to form a composite wave is called: Combination of Waves The combining of two waves to form a composite wave is called: Interference + = (Close to p out of phase) (Waves almost cancel.) Destructive interference The interference is destructive if the waves tend to cancel each other.

Interference of Waves + + = Constructive interference = (Waves cancel) (In phase) + = ( p out of phase) (Waves cancel) Destructive interference 20 20 10

* Interference of Waves + = When light waves travel different paths, and are then recombined, they interfere. Mirror 1 2 * Each wave has an electric field whose amplitude goes like: E(s,t) = E0 sin(ks-t) î Here s measures the distance traveled along each wave’s path. + = Constructive interference results when light paths differ by an integer multiple of the wavelength: s = m  20 20 11

* Interference of Waves + = When light waves travel different paths, and are then recombined, they interfere. Mirror 1 2 * Each wave has an electric field whose amplitude goes like: E(s,t) = E0 sin(ks-t) î Here s measures the distance traveled along each wave’s path. + = Destructive interference results when light paths differ by an odd multiple of a half wavelength: s = (2m+1) /2

Interference of Waves Coherence: Most light will only have interference for small optical path differences (a few wavelengths), because the phase is not well defined over a long distance. That’s because most light comes in many short bursts strung together. Incoherent light: (light bulb) random phase “jumps” 20 20 12

Interference of Waves Coherence: Most light will only have interference for small optical path differences (a few wavelengths), because the phase is not well defined over a long distance. That’s because most light comes in many short bursts strung together. Incoherent light: (light bulb) Laser light is an exception: Coherent Light: (laser) random phase “jumps” 20 20 13

Thin Film Interference We have all seen the effect of colored reflections from thin oil films, or from soap bubbles. Film; e.g. oil on water 20 20 14

Thin Film Interference We have all seen the effect of colored reflections from thin oil films, or from soap bubbles. Rays reflected off the lower surface travel a longer optical path than rays reflected off upper surface. Film; e.g. oil on water

Thin Film Interference We have all seen the effect of colored reflections from thin oil films, or from soap bubbles. Rays reflected off the lower surface travel a longer optical path than rays reflected off upper surface. Film; e.g. oil on water If the optical paths differ by a multiple of , the reflected waves add. If the paths cause a phase difference , reflected waves cancel out. 20 15 20

Thin Film Interference Ray 1 has a phase change of phase of  upon reflection Ray 2 travels an extra distance 2t (normal incidence approximation) 1 t 2 oil on water optical film on glass soap bubble n = 1 n > 1 Constructive interference: rays 1 and 2 are in phase  2 t = m n + ½ n  2 n t = (m + ½)  [n = /n] Destructive interference: rays 1 and 2 are  out of phase  2 t = m n  2 n t = m  20 16 20

Thin Film Interference When ray 2 is in phase with ray 1, they add up constructively and we see a bright region. Different wavelengths will tend to add constructively at different angles, and we see bands of different colors. 1 t 2 oil on water optical film on glass soap bubble n = 1 n > 1 Thin films work with even low coherence light, as paths are short When ray 2 is  out of phase, the rays interfere destructively. This is how anti-reflection coatings work. 20 20 18

Diffraction What happens when a planar wavefront of light interacts with an aperture? If the aperture is large compared to the wavelength this would be expected.... …Light propagating in a straight path.

Diffraction If the aperture is small compared to the wavelength, would this be expected? Not really…

Diffraction If the aperture is small compared to the wavelength, would the same straight propagation be expected? … Not really In fact, what happens is that: a spherical wave propagates out from the aperture. All waves behave this way. This phenomenon of light spreading in a broad pattern, instead of following a straight path, is called: DIFFRACTION

q Diffraction I Angular Spread: q ~ l / a Slit width = a, wavelength =  q I

Huygen’s Principle Huygen first explained this in 1678 by proposing that all planar wavefronts are made up of lots of spherical wavefronts..

Huygen’s Principle Huygen first explained this in 1678 by proposing that all planar wavefronts are made up of lots of spherical wavefronts.. That is, you see how light propagates by breaking a wavefront into little bits

Huygen’s Principle Huygen first explained this in 1678 by proposing that all planar wavefronts are made up of lots of spherical wavefronts.. That is, you see how light propagates by breaking a wavefront into little bits, and then draw a spherical wave emanating outward from each little bit.

Huygen’s Principle Huygen first explained this in 1678 by proposing that all planar wavefronts are made up of lots of spherical wavefronts.. That is, you see how light propagates by breaking a wavefront into little bits, and then draw a spherical wave emanating outward from each little bit. You then can find the leading edge a little later simply by summing all these little “wavelets”

Huygen’s Principle Huygen first explained this in 1678 by proposing that all planar wavefronts are made up of lots of spherical wavefronts.. That is, you see how light propagates by breaking a wavefront into little bits, and then draw a spherical wave emanating outward from each little bit. You then can find the leading edge a little later simply by summing all these little “wavelets” It is possible to explain reflection and refraction this way too.

Diffraction at Edges what happens to the shape of the field at this point?

Diffraction at Edges Light gets diffracted at the edge of an opaque barrier  there is light in the region obstructed by the barrier I

Double-Slit Interference Because they spread, these waves will eventually interfere with one another and produce interference fringes

Double-Slit Interference

Double-Slit Interference

Double-Slit Interference

Double-Slit Interference Bright fringes

Double-Slit Interference Bright fringes screen

Double-Slit Interference screen Bright fringes Thomas Young (1802) used double-slit interference to prove the wave nature of light.

Double-Slit Interference Light from the two slits travels different distances to the screen. The difference r1 - r2 is very nearly d sinq. When the path difference is a multiple of the wavelength these add constructively, and when it’s a half-multiple they cancel. L d y r1 r2 q P

Double-Slit Interference Light from the two slits travels different distances to the screen. The difference r1 - r2 is very nearly d sin q. When the path difference is a multiple of the wavelength these add constructively, and when it’s a half-multiple they cancel. d sin q = m l  bright fringes d sin q = ( m+1/2) l  dark fringes L d r1 r2 q P Now use y = L tan q; and for small y  sin   tan  = y / L y y bright = mlL/d y dark = (m+ 1/2)lL/d

Multiple Slit Interference With more than two slits, things get a little more complicated P y d L 26 27

Multiple Slit Interference With more than two slits, things get a little more complicated Now to get a bright fringe, many paths must all be in phase. The brightest fringes become narrower but brighter; and extra lines show up between them. P y d L Such an array of slits is called a “Diffraction Grating” 26 28

Multiple Slit Interference The most intense diffraction lines appear when: d sin(q) = m l Note that each wavelength  is diffracted at a different angle  S q All of the lines (more intense and less intense) show up at the set of angles given by: d sinq = (n/N) l (N = number of slits). 26 31

Single Slit Diffraction Angular Spread: q ~ l / a I Intensity distribution Each point in the slit acts as a source of spherical wavelets Slit width a q For a particular direction q, wavelets will interfere, either constructively or destructively, resulting in the intensity distribution shown.

Two points are resolved when the minimum of the second The Diffraction Limit Diffraction imposes a fundamental limit on the resolution of optical systems: Suppose we want to image 2 distant points, S1 and S2, through an aperture of width a: Two points are resolved when the maximum of one is at the minimum of the second L The minima occurs for sin  =  / a S1 D q a = slit width S2 Using sin     min =  / a Dmin / L   / a

Example: Double Slit Interference Light of wavelength l = 500 nm is incident on a double slit spaced by d = 50 mm. What is the fringe spacing on the screen, 50 cm away? d 50 cm

Example: Double Slit Interference Light of wavelength l = 500 nm is incident on a double slit spaced by d = 50 mm. What is the fringe spacing on the screen, 50 cm away? d 50 cm

Example: Single Slit Diffraction Light of wavelength l = 500 nm is incident on a slit a=50 mm wide. How wide is the intensity distribution on the screen, 50 cm away? a 50 cm

Example: Single Slit Diffraction Light of wavelength l = 500 nm is incident on a slit a=50 mm wide. How wide is the intensity distribution on the screen, 50 cm away? a 50 cm What happens if the slit width is doubled? The spread gets cut in half.

sin   y/L for small angles, and m = 1 for first order Beams of green ( = 520 nm) and red ( = 650 nm) light impinge normally on a grating with 10000 lines per cm. What is the separation y of the first order diffracted beams on a screen, parallel to the grating, and located at a distance L = 50 cm away from it? Grating equation  d sin  = m  sin   y/L for small angles, and m = 1 for first order 10000 lines/cm  d = 0.0001 d y / L =   y =  L /d Y green = 520x10-7x50 / 0.0001 = 26 cm Y red = 650x10-7x50 / 0.0001 = 32.5 cm y = 6.5 cm