Refraction Changing of speed of a wave (light) when it travels from one medium to another. When light travels from one medium into another at an angle, the difference in speed causes a change in direction of the light. When light travels from air into this glass block, it bent towards the normal. What do you think will happen when light travels from the block back into air?

For media of different optical densities, Refraction Angle of Incidence Incident Ray For media of different optical densities, θ(r) ≠ θ(i); for θ(r) ≠ 0 º θi Medium 1 Medium 2 Angle of Refraction θr Refracted Beam Example: Water is optically denser than air. This means that light travels more slowly in water than in air. Light travelling from air into water with an incident angle of 56º will bend, towards the normal, with an angle of refraction of 38º.

Refraction How can you predict whether light will bend towards or away from the normal? Light traveling from a less dense medium to a more dense medium, light bends TOWARDS the normal Less Dense More Dense Light traveling from a more dense medium to a less dense medium, light bends AWAY from the normal More Dense Less Dense

Index of Refraction (n) Ratio of speed of light in a vacuum to the speed of light in a medium. The higher the index of refraction: The higher the medium’s optical density The slower light travels in the medium Speed of light in a vacuum (c) Speed of light in medium (v) Example: The speed of light in a solid is 1.24 × 108 m/s. Determine the solid’s index of refraction. Identify the material, using Table 1 on pg. 524 of your textbook.

Refraction Effects: Apparent Depth Objects that are under water appear to be closer to the surface than they actually are. This is due to the refraction of light that travels up through the water, and out into the air towards your eyes.

Refraction Effects: Lateral Displacement Refraction can also shift light rays over to one side. When light passes from air into water then back into air, the emergent ray is parallel to the incident ray, but no longer moves in the same path. This stick appears “broken”: can you explain why?

Applications of Refraction Inferior Mirages Occur when cooler layers of air lie above warmer air (close to ground!) The index of refraction of air decreases from sky to ground As light travels downwards, it refracts away from the normal Light is eventually totally internally reflected upward from one of the layers We perceive a reflective (shiny) surface, which is actually an image of the sky reflected upward “from the road”. Why does the road look wet in this photograph?

Applications of Refraction Superior Mirages Occur when denser layers of air lie below less dense air The index of refraction of air increases from sky to ground As light travels downwards, it refracts towards the normal We perceive a virtual image A photo of a superior mirage taken in Finland.

Applications of Refraction Sunsets The index of refraction of the atmosphere increases moving from the outermost layer to the ground Light travelling at a large angle to the layers of the atmosphere causes greater refraction than when the sun is directly overhead Refraction causes the sun to appear flattened when it is close to the horizon Refraction also prolongs sunsets

Applications of Refraction More on sunsets… Images of the rising sun near Colorado Springs, Colorado. Earth Science Picture of the Day, http://epod.usra.edu/

Snell’s Law SNELL’s LAW n1 sin θ1 = n2 sin θ2

Snell’s Law n1 sin θ1 = n2 sin θ2 Predicts the amount that light bends due to a change in speed θ1 n1 sin θ1 = n2 sin θ2 n1 n2 θ2 Example: Light travels from air into an unknown solid at an angle of incidence of 45˚. The angle of refraction is 27˚. Determine the index of refraction of the solid. Identify the unknown solid.