April teaches high school science and holds a master's degree in education.
Introduction to Diffuse Reflection
Every time I go to the beach, I am fascinated by all of the colored pebbles I find. Tiny stones of pink, yellow, blue, and violet flash under the shallow waves. I can't resist the urge to collect them, and I usually go home with a pocketful of my favorite pebbles. But once I get them washed and dried, I am often disappointed with how dull they all appear. Where did their vibrant colors go? What happened to their glossy sheens? I should have known from the beginning that the dry pebbles would look different because of the effects of diffuse reflection.
Specular vs. Diffuse Reflection
We've already learned that the reflection of waves involves a change in the direction of waves when they strike a surface. When talking about reflection, we think of waves as straight-line rays. The incident ray is the ray that initially strikes a surface. The reflected ray, obviously, is the one that reflects off a surface. The law of reflection tells us that the angle of reflection is equal to the angle of incidence. In other words, the angle at which the incident ray strikes the surface is going to be the same as the angle at which it reflects. Rays and angles are convenient ways to talk about single waves, but sources of sound and light typically send out many waves at once. How do we talk about the law of reflection when more than one wave is involved?
All we have to do is imagine a whole bunch of rays parallel to one another. This picture below could represent a concentrated beam of light waves, say, from a flashlight. Since each ray is hitting the surface at the same angle - the angle of incidence - then each ray reflects at the same angle - the angle of reflection. A person standing in the path of the reflected rays would see a perfect image of the flashlight's beam. It wouldn't be much different from staring into the flashlight itself! This type of reflection, in which a smooth surface causes reflected rays to travel in the same direction, is called specular reflection.
Specular reflection is best known as the type of reflection you get from a mirror. It also occurs off of other smooth surfaces like glossy tabletops, car windows, and very still water. Specular reflection from a calm lake occurs because the surface is so flat that all the reflected rays bounce off in the same direction.
But, if a gust of wind disturbed the water, the resulting ripples would cause a different phenomenon called diffuse reflection. Diffuse reflection occurs when a rough surface causes reflected rays to travel in different directions. Most everyday objects exhibit diffuse reflection because of the tiny imperfections on the surface of the material. A piece of paper may look smooth on the surface, but at the microscopic level, the tiny fibers make it rough. Upholstery and clothing exhibit diffuse reflection because of the minute roughness of the fabric. Even a leaf has an element of roughness to it because of the multidimensional nature of the cells on the surface.
In diffuse reflection, each individual ray strikes a part of the surface that is oriented in a different direction. The law of reflection still applies, but the normal is different for each ray. So, the reflected rays end up going in all directions. As you can see below, one incident ray reflects over in one direction, another incident ray reflects elsewhere, and another one reflects somewhere else. The effect of all these rays going everywhere is that all of the waves are spread out. Diffuse reflection is the reason why you don't see your image reflected in most everyday objects.
Diffuse Reflection in Sound Waves
People who design auditoriums and recording studios are big fans of diffuse reflection. Imagine you are sitting in an auditorium, listening to someone play a trumpet. If the auditorium walls are smooth, the sound waves from the trumpet will bounce off the walls with specular reflection. If you're sitting in the right spot, you'll get a blast of intense sound waves as the reflected rays all hit you at once. If you move a few rows back, you might perceive the trumpet as sounding very soft. Specular reflection in an auditorium is not a good thing because different people hear different things depending on where they sit.
In order to break up the focus of the sound, people often use acoustic diffusers. These are wall and ceiling treatments that spread out sound waves by causing diffuse reflection. By engineering different shapes and angles into the walls, we can break up what would normally be specular reflections. Spreading out the sound waves results in a fuller, more uniform sound over the entire acoustic space. You'll often see diffusers inside recording studios and church sanctuaries. This is because diffuse reflection of sound waves sounds much better.
Diffuse Reflection in Light Waves
We already saw how the specular reflection of light waves works. When a beam of light rays reflects off a smooth surface, the reflected rays are also seen as a beam of light. If the surface is rough, then it causes diffuse reflection of the light waves. But sometimes, a rough surface is made smooth because a layer of liquid covers up the imperfections.
Rainwater coating a road is one example. Normally, an asphalt road exhibits diffuse reflection because of the irregular materials making up its surface. During a rainstorm, water builds up in the cracks and crevices. If enough water collects on the road, then it will actually cover the asphalt with a thin, smooth layer. The result will be the specular reflection that you see when driving on rainy days. Rain coating causes nasty glares and reflections of distant headlamps and streetlights. This is why driving on wet roadways is especially challenging at night.
The appearance of colored seashells and pebbles also changes when they are wet. These natural materials are somewhat rough at the microscopic level, even if they have been worn smooth by the elements. When I find my favorite pebbles at the beach, they are always richly colored because they are sitting in the ocean waves. The water coats the pebbles' imperfections, making them appear smooth and vibrant. But, after my pebbles have dried off, they don't look nearly as good. Perhaps this is why we like to polish our most beautiful rock and mineral specimens. We want to appreciate the shapes and the colors of the rock, whether the surface is wet or dry. Polishing the surfaces of rocks and minerals reduces diffuse reflections, so that we can see their true colors.
The law of reflection states that for any wave striking a surface, the angle of reflection will be equal to the angle of incidence. This law remains true whether the surface is rough or smooth. If the surface is smooth, then all incident rays coming from the same direction will also reflect in the same direction. This is called specular reflection.
If the surface is rough, then the rays will reflect in many different directions, in what is called diffuse reflection. Diffuse reflection of sound waves is preferable in auditoriums and studios, where it provides more uniform acoustics than specular reflection. Diffuse reflection is also preferable when driving a car at night. Specular reflection caused by rainwater can create disorienting glare. On the other hand, specular reflection looks better on colored stones, which are usually made to look dull by the effects of diffuse reflection.
At the end of this lesson, you'll be able to:
- Differentiate between specular and diffuse reflection
- Explain why diffuse reflection is preferred in auditoriums and stadiums with acoustics, and explain how acoustic diffusers can help achieve this
- Summarize why rainwater makes asphalt - a surface that normally exhibits diffuse reflection - exhibit specular reflection
- Describe why pebbles appear vibrant and shiny underwater but dull when dry, using the effects of specular and diffuse reflection
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