Objective: To examine how images can be formed from the light of distant sources.
Equipment: Example lens (magnifying, reading glasses, or eyeglasses for a farsighted person), a small hand mirror, blank paper; straight edge (ruler), protractor.
Description: Quantitative interpretation of astronomical images requires a knowledge of the relationship between the various points on the image and the corresponding celestial objects. This relationship follows from simple optical principles, i.e., the behavior of light in interaction with reflecting and refracting materials. Two important principles are used:
Law of Reflection: When light reflects from a smooth surface, the angle of incidence of a light ray is the same as the angle of reflection.
Law of Refraction: When light passes between a transparent media across a smooth boundary, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is in the same ratio as the speed of light in the corresponding media. For lenses, this results in light rays initially parallel to each other bending toward or away from a focal point after passing through the lens.
For our purposes, there are several important consequences of the law of refraction:
Point 1) A lens bends ("refracts") all rays which pass through it with one exception: A ray passing through the center of the lens emerges from the lens in exactly the same direction as it was going when it entered. We say that a "central ray" is "undeviated" by its passage through the lens.
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