Module 10: Optics

The Law of Reflection states that the angle of reflection is equal to the angle of incidence.  This law can be applied to mirrors of all shapes to trace light rays and determine image characteristics. Plane mirrors always form images that are virtual, upright and the same size as the object. Concave, or converging, mirrors can form a variety of types and sizes of images depending on where the object is located relative to the focal point and center of curvature. When the object is closest to the concave mirror, the image is virtual, upright and larger than the object. Moving the object farther from the mirror, when the object is at the focal point there is no image. Moving the object farther still, when the object is between the focal point and the center of curvature, the image is real, inverted and larger than the object. As the object moves even farther away, when the object is at the focal point, the image is real, inverted and the same size as the object. Moving the object beyond the center of curvature, the image is still real and inverted, but is not smaller than the object. In contrast, the convex mirror can produce only one type of image. The images produced by a convex, or diverging, mirror are always virtual, upright and smaller than the object. Each type of mirror has three principle rays – parallel, focal and center rays – that can be used to locate the image.

Refraction is the change in direction of a light ray due to a change in the speed of light as it changes medium. The index of refraction describes the optical density of a material and is equal to the speed of light in vacuum divided by the speed of light in the material. Snell’s Law describes the relationship between the indices of refraction, the angle of incidence and the angle of refraction. When light travels to a less optically dense medium, it speeds up and bends away from the normal. When light travels to a more optically dense medium, it slows down and bends toward the normal. Total internal reflection can occur when light is traveling from a more to a less optically dense medium and the angle of incidence is greater than or equal to the critical angle. When total internal reflection occurs, no light leaves the incidence medium.

Lens shapes are designed to use refraction to correct for vision defects. A convex lens is used to correct for farsightedness. A convex, or converging lens, can form a variety of types and sizes of images depending on where the object is located relative to the focal point and center of curvature. When the object is closest to the convex lens, the image is virtual, upright and larger than the object. Moving the object farther from the lens when the object is at the focal point there is no image. Moving the object farther still, when the object is between the focal point and the center of curvature, the image is real, inverted and larger than the object. As the object moves even farther away, when the object is at the focal point, the image is real, inverted and the same size as the object. Moving the object beyond the center of curvature, the image is still real and inverted, but is not smaller than the object. A concave lens is used to correct for nearsightedness.  In contrast, the concave, or diverging lens can produce only one type of image. The images produced by a concave lens are always virtual, upright and smaller than the object. Each type of lens has three principle rays – parallel, focal and center rays – that can be used to locate the image.

The processes of reflection and refraction are used in a wide variety of real-world applications. 

Mirrors and lenses have similar image formation characteristics. Converging mirrors and converging lenses can both make real or virtual images that can be larger, smaller or the same size as the object depending on where the object is relative to the focal point. Diverging mirrors and lenses only make virtual, upright images that are smaller than the object. The relationships between focal length, object distance and image distance are the same for mirrors and lenses. The mirror and lens equation can be used to mathematically located images and determine their characteristics.

 

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