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**Created by Stephanie Ingle Kingwood High School**

Refraction & Lenses Created by Stephanie Ingle Kingwood High School

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Snell’s Law normal incident ray Air (n=1.0003) 1 n = index of refraction for medium (no units) Boundary reflected ray 2 Water (n= 1.33) Angles are always measured from the normal, never the surface

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**Index of Refraction Light changes speed (v) as it enters a new medium**

In a vacuum the speed of light (c) is 3.0 x 108m/s The index of refraction (n) of a material is the ratio of the speed of light in a vacuum to the speed of light in the material. Index of refraction has no units!

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Critical Angle The incident angle of light that causes refraction along the boundary between surfaces The angle of refraction will always be 90o Only possible when going from more optically dense (high index of refraction) to less optically dense medium (low index of refraction Only possible when light speeds up as it passes through the boundary n=1 r=900 c n=1.5

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**Total Internal Reflection**

When incident light strikes a boundary at an angle greater than the incident angle it does not cross the boundary into the new medium. Instead, all of the light is reflected from the boundary back into the original medium according to the Law of Reflection.

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**Concave Lenses Thicker at the edges than in the center**

Parallel rays of light from a far object will refract throught the lense and diverge as if they came from the focal point. Concave lenses also called “diverging lenses” Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

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**Convex Lenses Thicker in the center than at the edges**

Parallel rays of light from a far object will refract through the lens and converge at the focal point. Convex lenses also called “converging lenses” Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

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**Calculations f = focal length do = object distance di = image distance**

hi = image height ho = object height M = magnification

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**Interpreting Calculations**

Focal length (f) converging, then f = + diverging, then f = - Image distance (di) di=+ , then image is real do= -, then image is virtual Magnification (M) M = +, image is erect M = - , image is inverted

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**Ray Diagram Convex Lens Image is real, inverted, & reduced**

Draw 3 rays from tip of object: 1) parallel, then through f 2) through f, then parallel 3) through the lens at the principal axis Image is real, inverted, & reduced f f

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**Ray Diagram Convex Lens (Inside f)**

Draw 3 rays from tip of object: 1) parallel, then through f 2) from same side f, through tip of object, then parallel 3) through the lens at the principal axis image f f object Image is virtual, erect, & magnified

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**Ray Diagram Concave Lens Image is virtual, erect, & reduced**

Draw 3 rays from tip of object: 1) parallel, then refracted ray from f on same side of lens 2) to lens along a line that would pass through f on the other side of lens, then parallel 3) through the lens at the principal axis concave lens (axis) object image f f Image is virtual, erect, & reduced

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