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Physics 9: 1. Waves

Unit

Waves

 

Theory

Reflection

reflection

Angle of incidence = Angle of reflection

Refraction

Refraction is the change in direction of a wave when it transmits from one medium to another.

8097906_orig

The angle of incidence and the angle of refraction can be determined by Snell’s law given by the following formula

snell2

where n1 and n2 are the refractive indexes for their respective mediums

1-2-reflectionrefraction-014

1-2-reflectionrefraction-016

Fast-to-slow: towards normal; slow-to-fast: away from normal

In addition, the refractive index n1 and n2 are related by the following equation

snells_law

where v1 and v2 are the speed of the waves in their respective mediums and λ1 and λ2 are the wavelength of the waves of their respective mediums

changingmedia-new

Total Internal Reflection

internal

The refractive index and the critical angle are related by the following equationcrit-angle-5Total internal reflection only occurs when the light ray propagates from a optically denser medium to an optically less dense medium.

Diffraction

When waves pass through a gap in a barrier, they spread out into space beyond the barrier. This spreading of the waves is called diffraction.

The waves are diffracted the most when the width of the gap is similar to the wavelength of the waves.

(The effect is strongest in c and weakest in a)  

  • Amplitude, wavelength, frequency, period and wave speed

  Amplitude Period (T) Frequency (f)  
  Maximum displacement of the oscillating object Time taken for one complete oscillation (in seconds) Number of times the object oscillates per unit time (usually one second)

f=1/T

 

simple_harmonic_motion1

waveparameters

Wave speed can be calculated by the following equation

velocityformula

  • Transverse and longitudinal waves

Transverse wave Longitudinal wave
The direction of oscillation is perpendicular to the direction of the energy transfer2-13-waves-transverse

 

 

The direction of oscillation is parallel to the direction of the energy transfer2-13-waves-longitudinal
Example:

 

Ÿ   Water waves

Ÿ   Wave in a string flicked up and down

Ÿ   Light (electromagnetic waves)

Example:

 

Ÿ   Wave produced in a spring

Ÿ   Sound waves

Ÿ   Earthquake P-waves

FYI

Transverse wave Longitudinal wave
A point with maximum positive displacement is called a crest.

 

A point with minimum displacement is called a trough.

A region where particles are closed to each other is called a compression.

 

A region where particles are furthest apart from each other is called a rarefaction.

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  • The nature of electromagnetic waves

All EM waves travel in vacuum at the same speed of 3*10^8m/s.

EM waves are transverse waves.

em_spectrum

  • The nature of sound waves

The speed of sound in 20 degrees Celsius dry air is approximately 343.2m/s.

Sound waves are longitudinal waves.

lwav2

Visuals

 

Gap is large compared to wavelength                                                       Gap is small compared to wavelength

                        

                         

Explanation                                                                         In glass fiber

    

 

In water

Dispersion of light through a prism

 

Angle of incidence is not 90°                                                     Angle of incidence is 90°

Longitudinal Waves

 

Transverse waves

Water waves