Reflection refers to a change in direction of light when they bounce off a substrate. Refraction is the change in the direction of light as it passes through a substrate.
So why should you know these terms? If you work with light modifiers, having an understanding of how they work will help you understand what kind of results you can expect from them. All modifiers used in photography fall under one of these two categories.
In physical space, a light wave will travel in a straight path, from the source of illumination, until one of two things happen. An object in the light path either absorbs it or the object deflects it. Typically all object tend to do both. As light encounters an object, some of the light's color spectrum gets absorbed and the rest gets deflected. What gets deflected is what allows us to see color. How an object deflects light is dependant on the physical structure, or surface makeup, of the object.
Dean Collins, a well respected photographer and teacher, used the term efficiency to describe how effectively a particular object's surface deflects light. Something hard and shiny will have a high efficiency, meaning it will reflect more of the light while something soft and rough will have a low efficiency, meaning it will diffuse more of the light.
* Read the article, "Understanding Angle of Incidence" for more information.
Some examples of light modifiers that rely on the principle of reflection are the appropriately named 5in1 light reflector with its silver, gold and white reflective fabrics. These fabrics have varying degrees of surface efficiency and will reflect different amounts of light.
Bounce umbrellas also have similar silver, gold or white reflective surfaces. Bounce cards, used in many product shots, are nothing more than silver, gold or white pieces of card that reflect light as needed. Beauty dishes have several reflective surfaces. An internal mirror with high efficiency and a white reflective bowl with low efficiency.
Efficiency of the object surface plays a role in how effectively light passes through. A clear object, like glass or plastic, will allow a lot of light to pass through whereas something colored or frosted will allow less light to pass. Material density also has an effect on the light particles passing through it.
Diffusion fabrics used in such modifiers as shoot through umbrellas and softboxes are prime examples of substrates that cause refraction. If you look at the plastic lens on your camera's strobe you will see a variation of the surface texture. This is called a fresnel lens and it uses the principle of refraction to effectively spread light out. Water glass, frosted sheeting and the textured plastic panel over the ceiling fluorescent light are all examples of material that refract light.
Some material, like water glass, create unusual patterns on a wall when light passes through it and can be used to create interest in an image. For a simple experiment of this take a flashlight and shine it through a cut crystal glass and watch the patterns play across the surface of the wall. You will see the effect of refraction at work.
Hope you learned something new with this. Share your thoughts and observations in the comments section.