Optics deals with the phenomena associated with light. Optics has different realms in which light can have different models. To say this means that light can be treated as -
- A stream of fast moving classical particles, or
- a wave (electromagnetic wave), or
- A collection of chunks of energy called photons (which are quantum particles).
Which particular model is used depends roughly upon the dimensions of the object with which light is interacting, it depends upon the actual physical situation. These divisions are only for convenience of analysis. In the actual physical situation, a lot of phenomena are going on at the same time, both on a microscopic and a macroscopic level. We just choose the model which helps us to describe the situation well for us.
Ray Optics
Interaction of light with lenses is part of a branch known as
ray optics. Here since the size of the the equipment with which light interacts is large. When I say large, I mean large compared to the wavelength of light, which is rather smaller than ordinary lengths we encounter. Ray optics is used to analyze lens systems in cameras, telescopes, microscoped and similar optical devices. One can explain the formation of rainbows using ray optics, again because the rain drops responsible for a rainbow are much larger than the wavelength of light.
Ray optics basically treats light as rays emerging from a source, travelling in straight lines unless disturbed and obeying the laws of reflection and refraction. Phenomena like interference, diffraction aren't treated (these are dealt with in wave optics, and the photon model also explains these with laws of quantum physics). These are phenomena that involve two waves coming together to give either regions brighter than both of them put together, or resulting in regions of darkness.
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