Chapter 7 Light and Geometric Optics

7.3 Refraction of Light

What is refraction? The bending of light as it travels from one medium to another the light ray changes direction when it meets the boundary If light travels from one transparent medium to another at any angle other than straight on (normal to the surface), the light ray changes direction when it meets the boundary

What is refraction? Occurs when the light’s velocity changes the light ray changes direction when it meets the boundary If light travels from one transparent medium to another at any angle other than straight on (normal to the surface), the light ray changes direction when it meets the boundary

What is refraction? As in the case of reflection, the angles of the incoming and refracted rays are measured with respect to the normal. The normal line is extended into the refracting medium The angle between the refracted ray and the normal = the angle of refraction, θ r The angle of incidence is designated as θ i

Refractive Index  An important property of transparent substances  It is the ratio of the speed of light in a vacuum to the speed of light in that substance n: 1.Always larger than 1 2.The larger the index of refraction is i.the slower light travels in that substance ii.the more a light ray will bend when it passes from a vacuum into that material.

Refractive Index a medium with smallera medium with larger  when light passes from a medium with smaller n to a medium with larger n : The light bends towards the normal

Refractive Index a medium with larger a medium with smaller  when light passes from a medium with larger n to a medium with smaller n : The light bends away from the normal

Refractive Index

Snell’s Law of Refraction There is an exact relationship between the angle of incidence, the angle of refraction for light travelling from one medium to another

A light ray of wavelength 589 nm (produced by a sodium lamp) traveling through air strikes a smooth, flat slab of crown glass at an angle of 30.0° to the normal. Find the angle of refraction, θ r.

1.53 Light enters the glass at 30 ˚ from the normal, then refracts within the glass at 20 ˚ from the normal. When it leaves the glass it goes back into air, it will be at 30 ˚ from the normal again ˚ 19.5 ˚ PRISM, VIOLET

Calculating the Critical Angle Using Snell’s Law What do you notice about the picture on the left?

Calculating the Critical Angle Using Snell’s Law Total Internal Reflection = occurs when light moves along a path from a medium with a higher index of refraction to one with a lower index of refraction.

Calculating the Critical Angle Using Snell’s Law At some particular angle of incidence, called the CRITICAL ANGLE, θ c, moves parallel to the boundarythe angle of refraction equal to 90° the refracted ray moves parallel to the boundary, making the angle of refraction equal to 90°

Calculating the Critical Angle Using Snell’s Law ray is entirely reflected For angles of incidence greater than the critical angle, the ray is entirely reflected at the boundary. This ray is reflected at the boundary as though it had struck a perfectly reflecting surface.

Calculating the Critical Angle Using Snell’s Law Use Snell’s Law to calculate the critical angle for water! HINT: What is the angle of refraction?

Use Snell’s Law to calculate the critical angle for water! HINT: What is the angle of refraction? 48.6 ˚

24.4 ˚ 41.8 ˚ n = 1.27

On Your Own...

Critical Angle Worksheet

Review Questions 7.3 (pg ) 4, 5, 6,