Reflection, Refraction and Diffraction · Reflection •Law of reflection: 𝜃𝑖=𝜃 •All waves, including light, sound and water waves, obey this law ©cgrahamphysics.com 2016

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Reflection, Refraction and Diffraction

Book page 146, 151 ©cgrahamphysics.com 2016

Explanation

©cgrahamphysics.com 2016

Reflection

• Law of reflection: 𝜃𝑖 = 𝜃𝑟

• All waves, including light, sound and water waves, obey this law


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


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


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


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


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


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


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


Example • A wave with a speed of 60𝑐𝑚 𝑠−1 and λ=10cm passes

into a second medium in which its speed becomes 40𝑐𝑚 𝑠−1. What will be the wavelength in the second medium.

Solution

•𝑣2

𝑣1=

λ 2

λ 1

• λ2 =λ1𝑣2

𝑣1=

40×10

60= 6.67𝑐𝑚


Documents

Reflection, Refraction and Diffraction · Reflection •Law of reflection: 𝜃𝑖=𝜃 •All waves, including light, sound and water waves, obey this law ©cgrahamphysics.com 2016