reflection, refraction and diffraction · reflection •law of reflection: 𝜃𝑖=𝜃 •all...
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Reflection
• Law of reflection: 𝜃𝑖 = 𝜃𝑟
• All waves, including light, sound and water waves, obey this law
©cgrahamphysics.com 2016
Reflection of a single pulse Fixed end
• Inverted
• Smaller amplitude of reflected pulse as some energy was absorbed at support
• 1800 phase change
Free end
• No phase change
©cgrahamphysics.com 2016
Reflection and transmission A wave encountering a denser medium part of energy reflected larger part transmitted
• A wave coming from a denser medium to a less dense medium part reflected, part transmitted reflected not inverted energy is conserved
• Since speed is less in dense medium, λ must be shorter ©cgrahamphysics.com 2016
Refraction
• A wave incident on a boundary between two media will result in a small part reflected and a larger part transmitted
• Speed of wave only depends on medium through which wave travels
• The change in direction of a wave results from the change in speed of the wave
©cgrahamphysics.com 2016
Ripple tank
• Since frequency does not change, speed and λ changes 𝑣𝑑
𝑣𝑠=
λ 𝑑λ 𝑠
• 𝑣𝑑= speed in deep water • 𝑣𝑠= speed in shallow water • λ 𝑑= λ in deep water • λ 𝑠= λ in shallow water
λ 𝑑 > λ 𝑠
Deep water shallow Deep water
©cgrahamphysics.com 2016
Wave front incident on boundary
• If the wave front is incident at an angle on a boundary, the direction of the wave changes as well
• λ is smaller
• By the time A reaches barrier original wave at B has refracted and just reached C, since the wave is traveling slower
barrier
Deep water
©cgrahamphysics.com 2016
Refraction of light waves • The reflected and refracted rays are in the same plane as the normal
• The ratio of the sine angle of incidence to the sine angle of refraction is constant and called the refractive index n
• Light bends towards the normal when traveling from less dense to more dense material
•sin 𝜃1
sin 𝜃2= 1𝑛2 for light going from medium 1 to medium 2
Medium 1 Refractive index 𝑛1
Medium 2 Refractive index 𝑛2
𝑛1
𝑛2
©cgrahamphysics.com 2016
Diffraction
• A wave passes through a gap
• Frequency, λ and v remain the same
• Direction of propagation and wave pattern changes
• Effect most obvious when aperture ~ λ of waves
• Amplitude of diffracted waves less than incident waves because energy distributed over larger area
©cgrahamphysics.com 2016
The amount of diffraction depends on the ratio of λ to a, the size of the diffracting object
•λ𝑎
~1 (or more) very significant diffraction and almost
circular diffracted waves
•λ𝑎
< 1 less diffraction, centre part is almost
unaffected
•λ𝑎
<< 1 no significant diffraction *superposition will occur
©cgrahamphysics.com 2016
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