This month, we’re excited to feature a new series on our blog; Making Waves- All About Wavefronts! This series will take a deeper look at the basics, functions, and applications of wavefronts and why they matter.
As we wade through the shores of VT, we can see first hand what we mean by "wavefront". When you stand in a still body of water, it is easy to see your toes. When the wind is whipping up and the surface of the water is uneven, your feet look very strange - if you can even see them at all! The strange image you see is due to wavefront distortion. Read on to learn more about this property of light.
What is a wavefront? And why does it matter?
A wavefront describes the advancement of light perpendicular to the direction of light propagation. You can think of it as how the light would travel if you were following the first photons emitted as you switch the light on. The easiest way to visualize an optical wavefront is to use a ripple-tank model. A ripple-tank is a shallow tank of water with a wave generator. Waves in the water are a model for light waves. If you look at the ripples in the water from the side, they look like sine waves going up and down (just like light waves). There are many videos on youtube that demonstrate this – check the sidebar for a few examples!
Most of the time, in optics and optical systems, people are using plane waves or spherical wavefronts. Collimated light has a planar wavefront, while a point source has a spherical wavefront. A point source can essentially become a collimated light source if the distance from the measurement and the point source gets very long. The curvature of the wavefront at long distances is so small that it's effectively a planar wavefront. Sunlight shares this property. This is why you can make great shadow puppets with the sun as your light source!
The converse is also true. If you pass a collimated light source through a small pinhole, it comes out the other side with a spherical wavefront, acting like a point source, due to diffraction. The ripple-tank videos illustrate this very well.
What role do wavefronts play in optical filters?
Optical filters are typically designed to be placed in a collimated beam containing planar wavefronts. An ideal optical filter (or mirror or window) will not deflect any of the light going through it. The path will be unaltered. However, we live in the real world. There are a number of things that can lead to a deflection of the light from its original course and a perturbation in the wavefront, including refractive index variations in the glass, sides of the glass that are not parallel (wedge) and curving or twisting of the glass due to stress in the coatings or the mounting scheme. Even gravity can cause small changes in the wavefront if the optic is heavy and not well supported.
Most of these issues do not affect the customer unless they are doing sensitive imaging (astronomy or microscopy) or if the optical path is very long where the deviations become significant.
Want to learn more? Be sure to check our next month's blog post, where we will discuss how to specify wavefront error. Subscribe now to make sure you don’t miss it!