Demonstrations in Optics

Newton's Rings

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

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

Variable | Meaning |

r_{n} |
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:

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.

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.

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:

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

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.

- Microscope slides
- paper
- frosted plastic
- tape
- ruler with millimeters (optional for measuring)

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

- Try not to touch the slides too much. They will need time to settle and form patterns. Also, try to avoid finger prints and scratches.
- Diffraction patterns can be formed without using a piece of paper for an air wedge.
- Multiple slides can be used to create more complicated patterns.

**http://en.wikipedia.org/wiki/Newtons_rings**

Wikipedia's Entry on air wedges.**http://scienceworld.wolfram.com/physics/NewtonsRings.html**

Wolfram's entry on Newton's Rings. It includes a more in depth explanation of the behavior of the rings.**http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/NewtonsRings/NewtonsRings.html**

Harvard's lab on Newton's Rings. It includes several very clear pictures.**http://www.citycollegiate.com/newtons_rings.htm**

This web site shows how the math for Newton's Rings is derived.**http://physics.bgsu.edu/~stoner/P202/interfere/sld011.htm**

An online slide show of different types of diffraction, including air wedges.**http://www.physics.upenn.edu/courses/gladney/phys151/lectures/lecture_apr_18_2003.shtml**

This is a detailed explanation of thin film and air wedge interference.

Demonstrations in Optics