Optics Series Lecture, Lecture – XIV, XV, XVI.
“Color of thin films, Newton’s rings, Lloyd’s mirror and Phase changes during reflection” These lectures were delivered on 16th February, 21st February and on 17th March. The lecture sessions were of 1 and 1/2 hours. The lectures were delivered to both Physics honors as well as Physics elective students on different days.
We have previously discussed what is interference and what is wave-front splitting and amplitude splitting interference. We have also discussed in much details two wave-front splitting interference viz. Young’s double slit interference (Lecture – IX) and Fresnel’s bi-prism (Lecture – XI). Today we will discuss one more wave-front splitting interference namely Lloyd’s mirror interference before moving onto the amplitude splitting interference of the Newton’s Rings. Also we will discuss two interesting and related concepts; i. Phase change on reflection and ii. Color of thin films.
Interference in Lloyd’s Mirror.
The Lloyd’s mirror is a set-up for wave-front splitting interference. Here two rays one of which undergoes reflection from a reflecting surface like a mirror meet up at the point of observation and subsequently interfere. The interference pattern that is produced in a Lloyd’s mirror interferometer is complimentary to the pattern produced in a Young’s interferometer. That is because of an additional phase change of ± π during reflection. That means instead of a central bright fringe we obtain a central dark fringe. The positions where minima are obtained in Young’s DS set-up are the positions where maxima are obtained in Lloyd’s set-up. Similarly the positions where maxima are obtained in Young’s DS set-up are positions where minima are produced in Lloyd’s set-up. Only top half of the interference fringes in Lloyd’s set-up are visible as the bottom half is occluded by the mirror.
Lets look at the above diagram which depicts the Lloyd’s mirror set-up. Here a is the distance between the slit S1 and S1‘ where S1‘ is the mirror image of the slit S1. Therefore S1 and S1‘ behave like two coherent sources. Here we would have the same fringe spacing as that found in Young’s Double Slit interferometer, viz. Δ = (S/a) λ. For derivation of this relation between fringe-spacing and wavelength refer to the treatment of the Young’s double slit interference (Lecture – IX). All other derivation for YDSI and LMI are exactly same except there is an additional phase change of ± π so the location of minima and maxima will be reversed.
For glaring incidence that is for θi = π/2 the reflected ray would undergo 1800 phase shift. Rays incident from rarer to denser media undergo an arbitrary phase change of π radian. We will soon see this result in the concurrent lecture when we discuss phase change during reflection by Stoke’s treatment. The path difference is r1 – r2 where r1 and r2 denote the path traversed by the ray in case of direct path and reflected path respectively, as shown in the diagram. Corresponding to this path difference we obtain the phase difference by multiplying the path difference to the wave number k. There is an additional phase difference of ± π. So total incumbent phase difference is: δ = k(r1 – r2) ± π. As we have seen in our discussions in Young’s Double Slit discussion (Lecture – IX) and in Interference in two plane harmonic waves (Lecture – VII) the irradiance is given as a function of this phase difference δ.
Crystals reflect x-rays, mirrors reflect ordinary light and wire-mesh reflect microwaves. In all of these cases Lloyd’s like interference is formed. Earth’s ionosphere reflecting radio-waves is also an example of Lloyd’s interference. Before turning our attention to yet another example of interference, the Newton’s rings, which are formed via amplitude splitting interference lets discuss two important results that we will find useful many times during our discussions in optical phenomena. One of this is the phase change during reflection and the other is the phase difference in amplitude splitting phenomena and formation of color in thin films.
Phase change during reflection, Stoke’s treatment.
In order to study the phase change during reflection and excavate the underlying relations by Stoke’s treatment we need to state the Principle of Optical Reversibility first. According to this principle in the absence of absorption a light ray when reflected or refracted would retrace its original path if the direction of ray incidence is reversed. Let us apply this principle to the situation depicted in the diagram.
Let the first medium be represented by refractive index n1 and the second medium (bottom) be represented by refractive index n2. Let r and t be the fraction of amplitude that gets reflected or transmitted (refracted) respectively, for medium 1. These are also called as reflection coefficient of amplitude and transmission coefficient of amplitude. Similarly we need to define these parameters for the medium which are different in general. Let r‘ and t‘ be those parameters for the 2nd medium, that is the medium with refractive index n2. We are following a color code in the diagram to easily follow which ray gets reflected to which and which gets transmitted to which so as to write their amplitudes in the specified color for that ray as well as the same color for drawing the rays. eg on RHS of the diagram the ray denoted as ar is the incident ray drawn in black, so ar is also written in black. Its easy to see from the black color with ar that this ray gets reflected to a ray in medium 1 with color burnt orange. So its resulting amplitude is ar × r = ar2, where ar is written in black and × r is written in burnt orange. The color code is applied to all the other rays in this way.
Let a ray with amplitude a be incident from medium 1 on medium 2 and partly reflected and partly refracted. Thus according to our notations and color coding the amplitude of reflected ray would be: ar and amplitude of refracted ray would be: at. Let us reverse the directions of the rays. Ray of amplitude at would be incident from medium 2 on medium 1. Thus the amplitude of the reflected ray would be: atr‘ = atr‘. Similarly the amplitude of the transmitted or refracted ray from at would be: att‘ = att‘. Let us reverse the direction of ray with amplitude ar which is incident from medium 1 on medium 2. The reflected amplitude now would be: arr = ar2. Similarly refracted amplitude would be: art = art. According to principle of reversibility the two rays of amplitude ar2 and att‘ must combine to give the incident ray of amplitude a. So we have: ar2 + att‘ = a. This gives us: Also two rays of amplitude atr‘ and art must cancel each other. that is; atr‘ + art = 0. This gives:
We see from equation – 2 that there is a relative phase change of π between rays (π corresponds to – sign) in two different media which are necessarily incident and refracted rays in pair. Thus if phase change of π occurs for incidence from rarer to denser medium then there would be no phase change for the opposite situation, namely: for incidence from denser to rarer medium. Equations – 1 and – 2 are known as Stoke’s Relations.
Color of thin films.
When light falls on a thin film whose thickness is of comparable dimensions to the wavelength of light, due to interference between light reflected from the top surface of the film and light refracted into the film and reflected from the back surface color fringes are produced. We will consider parallel films for the time being. A rigorous discussion for non-parallel or wedge shaped films is to be expected in the near future. In some situations where we will find applications of the following results we will consider near normal incidence because this makes the visibility of the fringes more likely if we are focused on a slightly larger region of the film. Newton’s Rings that we will study next assumes near normal incidence of the result that we will just derive.
Let us calculate the optical path difference between the 1st reflected (top surface) and the 2nd reflected rays (bottom surface). OPL difference = Γ = n(AB+BC) – AD. Its easy to see from symmetry that AB = BC, which is given by d/cos (β) from triangle shown with angle β. If we draw a perpendicular from C to AD, it meets at D. So from triangle ADC, AD = 2 d tan (β) sin (α). Let us apply Snell’s law, which says sin (α) = n sin (β). (eg if n = n1 and the fact that first medium is air). This gives us OPL difference Γ as: . Its easy to see from here using only basic trigonometric identities that the term inside the  bracket is nothing but 2nd cos(β). So Γ = 2nd cos(β). Depending on various situations of refractive indices of the 3 media (the media on both sides of the thin film can in general be considered different) there is an additional phase change of ± π. As we have seen in last section only for reflection from denser medium there is an abrupt change of phase of π. So in general there would be 2 different situations, one where reflection introduces a phase shift of π and one where it does not. Let us write the conditions of maxima and minima accordingly. If δadditional = 0; and and if δadditional = π; and .
Now we are in a situation to study an interference pattern that arises due to amplitude splitting interference that we just studied in the last section, viz. the color of thin films. The Newton’s rings are a pattern of circular bright and dark fringes which are formed when an almost monochromatic light falls on a combination of two refracting and reflecting surfaces. There is an optical path length difference which is introduced to one of the rays, due to thickness of refracting medium, d, between the surfaces. The reflected and refracted rays eventually meet up and given to the mechanism of amplitude splitting interference the fringes are produced.
Let us consider a Plano-convex lens of radius of curvature R which is placed on an optically flat surface POQ. In last section we have derived the optical path length difference introduced due to such refracting and reflecting pair of rays (1st order reflected ray and 2nd order reflected ray). We saw that the OPL difference was given by: Γ = 2nd cos(β). We will consider only near normal incidence. In a later lecture we will see that this enhances our chances to see the fringes in a wider region. Accordingly the incident angle and refracted angle will be both small and cos (β) = 1. This means Γ = 2nd. Let us draw a suitable diagram for better grasping of the situation here.
The point of contact of the two glasses acts like a center (of the circular fringes) and the thickness of air film between AOB and POQ is constant on circles of radius rm from the point of contact. Total destructive interference or minima occur such that Γ = 2nd = mλ. They correspond to dark fringes for m = 0, 1, 2, … etc. Similarly total constructive interference or maxima occur such that Γ = 2nd = (m+1/2)λ, for m = 0, 1, 2, … etc. These latter are the conditions for bright fringes. Remember that the conditions of maxima and minima are reversed compared to ordinary situations because there is reflection from a denser medium which introduces additional phase shift of π. The circular fringes thus produced are known as Newton’s rings. For more details on total constructive and total destructive interference, check this lecture, Lecture – VII.
Let us look at the RHS of the diagram above. We apply Pythagoras theorem on the circular cross section as shown. We have: rm2 = R2 – (R – d)2 = d(2R – d). Typically R ∼ 100 cm and d ∼ 10-3 cm so we have d << R. This leads us to: rm2 ≈ 2Rd and 2d ≈ rm2/R. For dark fringes: rm2 ≈ mλR, for m = 0, 1, 2, … etc. We see that the radii of the fringes vary as the square root of natural numbers m, since λ and R are constants. Between two dark fringes we would see a bright fringe whose radius is proportional to [(m+1/2)λR]1/2. Typically for λ = 6 × 10-5 cm, R = 100 cm, rm = 0.0774 (m)1/2. So spacing between successive fringes reduce as we go farther from point of contact. But if we measure the fringes at intervals of p instead of 1, such that usually p = 10 then rm+p2 – rm2 = pλR. This is independent of m. Hence this quantity is constant instead of reducing, for higher order fringe separation. We can write more conveniently: where diameter of fringe D is used and this way we can measure wave-length of light, very precisely. If instead of air we use material of refractive index n the expression for dark fringes change accordingly to: rm = (mλR/n)1/2.