Demonstrations in Optics

[Home] [Demonstrations] [References] [About Us] [Equipment] [Documents]

Newton's Rings

image one

Figure 1: The two slides create ring and line shaped slides.


image twoimage three

image four

Figure 2: Two microscope slides are stacked to form an 'air wedge', which produces dark and light fringes (top). A convex lens is placed above a flat lens, which produces rings of light and dark fringes (bottom).


In the traditional version of Newton's Rings, a slightly convex lens is placed above a flat glass plate or optical flat.

concave lens over flat lens

Figure 3: A slightly convex lens is placed above an optical flat.

Monochromatic light is used to illuminate the lenses. This can be produced by any laser. Frosted plastic is placed in front of the laser to diffuse the light.

The light goes through the convex lens. There is a small pocket of air between the convex lens and the flat lens. Some of the light will reflect at this barrier. The rest of the light will refract in accordance to Snell's Law.


Variable Name
n index of refraction
θ angle between light and normal

The refracted light reflects off of the flat lens. These two beams of light will be at different points. Because the light goes through the lens at multiple places, the reflected light will interfere both constructively and deconstructively. This is what causes the dark and bright fringes.

For more information on Snell's Law, see the information here and interference here.

light refracts through a convex lens, and reflexts off a convex and flat lens

Figure 4: Light is bent by the convex lens, and is reflected by the flat lens. The light interferes and creates dark and bright fringes.

The radius of the rings can be found with the following equation:

rn=square root of R*lambda*(n-.5)
Variable Meaning
rn radius of ring from center
R radius of curvature of lense
N ring number observed
λ wavelength of light

Alternatively, the radius of curvature could be found:

R=(rn)^2 over lambda*(n-.5)

Microscope slides are a variation on this concept. Instead of a convex lens over a flat lens, there are two lenses at an angle to each other. This can be achieved by placing a piece of hair or a piece paper at the edge of the two slides. This will create an air wedge between the two lenses.

air refracts through straight lens, and reflects off two flat lenses of an air wedge

Figure 5: Two flat lenses are placed at an angle which forms bright and dark fringes.

As with the convex lens, the light will reflect at different angles. However, they will appear as partial rings since the compressed spot is at one edge instead of the center.

variables of an air wedge

Variable Meaning
n dark fringe looked at
x distance to fringe
l total distance
t height at n
h total height
d distance between dark fringes
λ wavelength of light
θ Angle

There are three main equations for the above diagram:

x=m*d; 2*t=m*lambda or m=2*t over lambda; tan(theta)=t/x

From this, a more easily solvable equation can be derived.

x=m*d; x=(2*t over lambda)*d; tan(theta)=t/x; tan(theta)=t* lambda over (2*t*d); tan(theta)=lambda over 2*d; theta = arctan (lambda over 2*d)

This allows for the angle to be found, using the wavelength of light, and the distance between fringes.

Microscope slides do not necessarily need hair or paper to form a wedge. This is because the slides are random, and they will not rest completely flat against the other, and this will create pockets of air. However, this leads to less predictable patterns, but the above equations can be used to approximate the angles created by the slides' imperfections.



  1. Place two microscope slides on top of each other with a piece of paper at the edge.

  2. Tape plastic to the laser so the beam of light is diffused. This plastic should be frosted. For example, fold a bag (such as a ziplock bag) several times.
  3. Shine the light onto the slides. You may need to adjust the angle, depending upon where you are standing. It is sometimes difficult to see in large groups.



Demonstrations in Optics

[Home] [Demonstrations] [References] [About Us] [Equipment] [Documents]