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# 9.2.6: Review

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Before starting on lenses in the next section,  let us summarize and review the ideas of rays and images:

• Each point of an object (luminous or reflecting) can be taken as a source of diverging rays.

• Light rays within a particular medium travel in straight lines if there is no scattering. Light rays can bend due to medium changes by either reflection or refraction. We neglect scattering in our discussion.

• Light rays do not represent the amount of energy in a wave. We can choose which light rays to draw, depending on which ones are relevant for the problem we are dealing with. In the case of lenses and mirrors we typically choose to only draw the light rays that hit the lens or mirror.

• If the rays appear to be coming from a location, the place the rays track back to is called an image.

• To find the complete image of an object, we choose points on the object and see if they form an image anywhere. By doing this for enough points we can reconstruct the image of the entire object.

• Reflection and refraction are laws that apply to all waves, not just light. Likewise, “images” can be formed for all types of waves as well.

• When discussing extended objects (like the arrow in this section) in addition to asking where the image is, we can ask what size the image is and whether it is “right-way-up” or “upside-down”. We will illustrate these concepts more clearly with lenses in the next section.