11.1 Refraction of Light

Refraction Refraction - the bending of light as it travels, at an angle, from a material with one refractive index to a material with a different refractive index

Fermat’s Principle Fermat’s Principle – When light travels from one point to another, it follows the path that will take the least time. 1 medium = straight line More than 1 medium = bending Example: Example:

Marching Band Analogy Why does light refract? the marching band marches from an area of firm ground to mud when one end of the band slows down, the direction of travel of the entire band changes

Refraction and Rays refracted ray the ray that is bent upon entering a second medium angle of refraction (R) the angle between the normal and a refracted ray

Direction of the Refracted Ray when a light ray travels from a medium in which its speed is faster (air) to a medium in which its speed is slower (water) the refracted ray bends toward the normal fast to slow – towards the normal

Direction of the Refracted Ray when a light ray travels from a medium in which its speed is slower to a medium in which its speed is faster, the refracted ray bends away from the normal slow to fast – away from the normal

Summary: Direction of Refraction FAST to SLOW towards the normal Example: Air to Water SLOW to FAST away from the normal Example: Water to Air

Dispersion the process of separating colours by refraction white light (all wavelengths of visible light) refract twice: 1. when it enters the prism 2. when it leaves the prism (separates into a spectrum of colours)

Dispersion each colour of light travels at a slightly different speed only in a vacuum (space with no matter) do all colours travel at the same speed: 3.00 x 108 m/s e.g. blue light bends more than red light (blue light travels slower than red light) Roy G. Biv

How much of the light refracts? speed of light in a vacuum is the speed of light in any other medium is less than that e.g. the speed of light in water is 2.26 x 108 m/s 3.00 x 108 m/s

Index of Refraction the ratio of the speed of light in a vacuum to the speed of light in a given medium the index of refraction is always greater than 1 as the speed of light decreases due to the medium, the index of refraction increases

Formula Manipulation C n V n = C V C = n x V V = C n

Index of Refraction (n) Index of Refraction (n) Table 11.1 Indices Pg. 454 Substance Index of Refraction (n) Vacuum 1.000 00 Gases at 0 C and 101.3 kPa Hydrogen 1.000 14 Oxygen 1.000 27 Air 1.000 29 Carbon dioxide 1.000 45 Liquids at 20 C Water 1.333 Ethyl alcohol 1.362 Glycerol 1.470 Carbon disulfide 1.632 Substance Index of Refraction (n) Solids at 20 C Quartz (fused) 1.46 Plexiglas 1.51 Glass (crown) 1.52 Sodium chloride 1.54 Glass (crystal) Ruby Glass (flint) 1.65 Zircon 1.92 Diamond 2.42

Sample Problem Calculate the speed of light in fused quartz. The index of refraction for fused quartz is 1.46. Equation n = c v n = index of refraction c = speed of light in a vacuum v = speed of light in a medium Given n = 1.46 c = 3.00 x 108 m/s v = ? Rearrange the equation!!! v = c n v = 3.00 x 108 m/s 1.46 The speed of light in fused quartz is 2.05 x 108 m/s v = 2.05 x 108 m/s 16

Homework Read Section 11.1 on Pg. 449-455 Complete “Practice Problems” Pg. 455 # 1-4 Complete “Section 11.1 Review” Pg. 456 # 1-5

Things you need for this lesson … Demonstration # 1: Ray box 1 beaker + water (Fermat’s Principle) Demonstration # 2: 1 beaker + v. oil + water + pencil (n =C/V) (different n’s) Demonstration # 3: 1 beaker v. oil + 2 test tubes (same n = 1.47)