Refraction Refraction Refraction the change in direction of

Refraction

Refraction �Refraction – the change in direction of a wave as it crosses the boundary between two media in which the wave travels at different speeds �Wave Fronts – lines that represent the position of different crests �At each point along a wave front, the wave is moving perpendicular to the wave front �The direction of motion is best represented by a ray

Refraction

Refraction of Sound �Sound waves are refracted when parts of a wave front travel at different speeds �This happens in uneven winds or temperatures �Sound waves tend to bend away from warm ground, since it travels faster in warmer air �On a cold night, the speed of sound is slower near the ground than above, so we can hear over larger distances

Refraction of Sound

Refraction of Light �A pond or swimming pool may appear shallower than it actually is.

Refraction of Light �A pencil in a glass of water will appear bent

Refraction of Light �An object can appear in a completely different place when perceived through two different media.

Refraction of Light �The air above a hot road seems to be moving due to the difference in densities of hotter and cooler air.

Refraction of Light �All of these effects are caused by changes in the speed of light as it passes from one medium to another, or through varying temperatures and densities of the same medium – which changes the directions of light rays

Refraction of Light

Atmospheric Refraction �On hot days there may be a layer of very hot air in contact with the ground, the light will travel faster through this air and will bend, creating a mirage �When you watch the sun set, you can still see the sun for several minutes after it has sunk below the horizon, because light is refracted by Earth’s atmosphere

Dispersion in a Prism �Light of frequencies closer to the natural frequency of the electron oscillators in a medium travels more slowly in the medium �Since different frequencies of light travel at different speeds in transparent materials, they will refract differently and bend at different angles �When light is bent twice at nonparallel boundaries, as in a prism, the seperation of the different colors is apparent �Dispersion – the separation of light into colors arranged according to their frequency

Dispersion in a Prism

Dispersion in a Prism

The Rainbow �The rainbow takes the concept of dispersion and multiples it through the atmosphere �The sun shines on water droplets in a cloud or when it is raining �The light is dispersed by the raindrop into its spectral colors

The Rainbow

Total Internal Reflection �Critical Angle – the minimum angle of incidence for which a light ray is totally reflected within a medium �Total Internal Reflection – the 100% reflection of light that strikes the boundary between two media at an angle greater than the critical angle �Optical fibers utilize the concept of total internal reflection to feed light from one location to another, these cables are very useful for communications

Total Internal Reflection

Index of Refraction (n)

Snell’s Law: n 1 sin θ 1 = n 2 sin θ 2 (where n 1 and n 2 are the indices of refraction of the media on either side of the boundary, and θ 1 and θ 2 are the respective angles of incidence and refraction)

Practice Problem 1 �Light travels from air (n=1. 00) into water (n=1. 33). If the angle of refraction is 30. 0 o, what is the angle of incidence?

Practice Problem 2 �A ray of light in air (n=1. 00) strikes a block of quartz at an angle of incidence of 30 o. The angle of refraction is 20 o. What is the index of refraction of the quartz?