Refraction The Bending of Light

What is Refraction? Light travels in straight lines in one material. When light travels from one material (medium) to another, it bends. This is called refraction. Our brain still thinks that light travels in straight lines and creates the image.

Why Does Light Bend? Light in a vacuum travels at the speed of light, C = 3.00 x 10 8 m/s In other substances, like water, it travels slower – 2.26 x 10 8 m/s Just like a car bends when it hits a rougher surface, light bends when it changes speeds

Terminology Normal

Rules of Refraction The incident ray, refracted ray, and normal all lie in the same plane Light bends towards the normal when travelling to a slower medium Light bends away from the normal when traveling to a faster medium

Partial Reflection and Refraction When dealing with refraction, usually not all light will enter the second medium Some light will reflect and some will refract. The reflection can be enhanced with coatings creating a 2-way mirror – Example: Some sunglasses, office building windows, police interrogation rooms

The Index of Refraction Since light travels fastest in a vacuum, the speed of light in other media are comparred to it. The index of refraction, n, is defined as: n = c/v n = Index of refraction for some material c = Speed of light in a vacuum (3.00 x 10 8 m/s) v = Speed of light in that medium

Index of Refraction Cont’d n is always greater than 1 It is a dimensionless number (no units) It indicates how much slower light is in the medium than in the vacuum. Examples: – Air/Vacuum = 1.00 – Water = 1.33 – Vegetable Oil = 1.47 – Glass = 1.52 – Diamond = 2.42

Example Problem The speed of light in acrylic plastic is 2.01 x 10 8 m/s What is the index of refraction for acrylic? v = 2.01 x 10 8 m/s c = 3.00 x 10 8 m/s n = ? n = c/v n = (3.00 x 10 8 m/s) / (2.01 x 10 8 m/s) n = 1.49 – The index of refraction for acrylic is 1.49

Willebrod Snellius 1580 – 1626 “Rediscovered” sine ratio is important Formulated that the sine of the angle of incidence is related to the sine of the angle of refraction.

Snell’s Law n 1 Sin θ 1 = n 2 Sin θ 2 Knowing the indices of refraction lets us predict the angles of incidence or refraction