3. Waves

• Waves transfer energy between points without transferring matter.
• Amplitude (A) is the maximum displacement from the original position.
• The SI unit for amplitude is in meters.
• Wavelength (λ) is the horizontal distance between two points that are in phase.
• The SI unit for wavelength is in meters.
• The period (T) is the time taken for the wave to complete a cycle or return to its original displacement.
• The SI unit for periods is seconds.
• Frequency (f) is the number of complete cycles in a second.
• The SI unit for frequency is hertz (Hz).
• The relationship between frequency (f) and period (T) is:

  f=1/T
• The speed of a wave can be calculated using the following equation:

  Speed (m/s) = Frequency (Hz) x Wavelength (m)

  v = f x λ
• There are two types of waves, longitudinal waves and transverse waves.
• In a transverse wave, particles vibrate perpendicular the lines of motion and consists of a series of “peaks” and “valleys”.
• In a longitudinal wave, particles vibrate along the lines of motion and consists of a series of compression and rarefaction.
• Examples of transverse waves include; electromagnetism, water waves and S-seismic waves.
• Examples of longitudinal waves include: sound waves and P-seismic waves.
• A wave must be able to demonstrate these three phenomena in order to be considered as a wave: reflection, refraction and diffraction.
• Reflection is the change of direction when a wave collides with a reflective barrier.
• Refraction is the change of direction when the wave goes through a change of medium.
• Refraction occurs when the direction of motion is not perpendicular to the border between the deep and shallow regions.
• The speed of the water changes when there is a change in the depth of the water. From deep to shallow waters, the wave’s speed decreases as the wavelength becomes shorter. From shallow to deep waters the wave’s speed increases as the wavelength becomes longer (Hint: recall v = fλ).
• Diffraction is shown when a wave spreads when the wave passes through an opening or an edge.
• Diffraction increases when the size of the gap decreases or the wavelength of the waves increases.
• Light is a wave because it undergoes reflection, refraction and diffraction.
• Label the following:

  A) Incident ray

  B) Normal

  C) Reflected ray

  
• Types of mirrors:

  A) Plane

  B) Convex

  C) Concave

  
• Refraction is the bending of light ray at the boundary of two medium as the light ray propagates from a medium to another with different density.
• Choose the correct labels:

  A) Incident ray

  B) Refracted ray

  C) Angle of refraction

  
• When light passes through a medium which is denser, i > r
• When light passes through a medium which is less dense, i < r
• Snell’s law states that the value of (sin i)/(sin r) is constant for light passing from one given medium into another:

  n = (sin i)/(sin r)

  Here n is the refractive index. Remember that n>1.
• Another equation for refractive index is:

  Refractive index = speed of light in vacuum/speed of light in medium

  or n=c/v
• The greater the refractive index, the denser is the medium. Hence, the speed of light in the medium will be slower.
• Total internal reflection and the critical angle:

  • Total internal reflection (TIR) occurs when the angle of incidence is greater than the critical angle and the incident material is denser than the second material.

  
• • Sometimes, when light is moving from a denser medium towards a less dense one, instead of being refracted, all of the light is reflected. This phenomenon is called total internal reflection.
• Formula for critical angle:

  sin(C)=1/n

  where C is the critical angle and n is the refractive index.
• For a converging lens (convex lens), when parallel rays of light pass through a lens, they are brought to focus at a point known as the principal focus (f).
• The distance of the principal focus from the lens is called the focal length which depend on the curvature of the lens.
• The characteristics of the image form using a convex lens is always either virtual or real; upright or inverted; magnified or diminished.
• When light is refracted by a prism, the incidence ray is not parallel to the emergent ray, since the prism’s sides are not parallel.