Refraction. Why do we sometimes see a 'mirage'? See picture below~!

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1 Refraction Level : Physics Teacher : Kim Why do we sometimes see a 'mirage'? See picture below~! Warm-up) The picture below is a top view of a beach. If you were a life-guard and you see the person in trouble on the water, which path would you choose? Path A,B or C? (hint: choose the shortest path to reach their destination) life-guard!! sand!! A B C water!! drowning person!! 1

2 Refraction of Light *~Light wave always choose the shortest path possible! - When light wave or light ray passes from one medium to another, light changes speed. For example, light travels the fastest in a vacuum, then slows down when entering the atmosphere, then slows down more if it enters water, etc... The speed of light slows down to 60% in lead! - When light wave enter a medium in which their speed decreases, as when passing from air into water, the rays bend toward the normal line - When light wave enter a medium in which their speed increases, as when passing from water into air, the rays bend away from the normal line => Refraction is the bending of a wave going into a second medium! Law of Refraction When light wave enters a medium that is denser, then light bends closer to the normal line. ex) air to water When light wave enters a medium that is less dense, then light bends away from the normal line. ex) water to air Demo1) Toy Car Observe what happens to the direction of a toy car when it enters a different surface Q1) Draw a diagram of light ray and describe how wave refracts when entering a different medium Light ray Normal Normal AIR AIR WATER WATER Light ray 2

3 Q2) If you were spearing a fish with a spear, would you aim above(a), directly at the observed fish(b) or below the fish(c) to make a direct hit? What about when you use a laser beam to spear the fish? i) When you use a spear: C B A actual position of the fish ii) When you use a laser beam: hint: laser behaves the same way as light rays Demo2) How well can you see through water? Place a block or any object at one end of the tank and have the observer look through the tank at the other end. How well can the observer recognize the object? water tank! view Critical Angle(θc) Let's say there is lamp underwater. The lamp will emit light rays in all directions. However, some rays will refract, but some rays will reflect back into the water. At a certain incident angle, the light ray no longer emerges out to air but reflects back. We call that angle critical angle(θc). Any incident angle greater than the critical angle will always reflect back. (imaginary) normal line! view Air! Water! θc Light source! 3

4 Observe video Total Internal Reflection Inside a Stream of Water Light through a Prism The light emitted by the Sun is called 'white' light. However, white is actually combination of various colors. The colors are violet, blue, green, yellow, orange and red light. The different colors all have different frequency. When light rays enters a denser medium, the angle of refraction will be determined by the frequency of the wave. That is, the higher the frequency, the greater the angle of refraction. For example, since blue light has a higher frequency(=greater wavelength) than red light, the blue light will refract at a greater angle than red light. See below! If white light is sent into a triangular glass, then all the different colors of light will refract at different angles! 4

5 Atmospheric Refraction What is a mirage?! - Although the speed of light in air is only 0.03% less than its speed in a vacuum, in some situations atmospheric refraction is quite noticeable. - We experience this when see observe a mirage on a highway pavement on hot summer day Q3) Why do we sometimes see water above the road on a hot sunny day at a distance? SUN Layers of hot air road!! 5