Waves

Activity In your notebook, make a list of some things you know about waves. Include examples of waves, and any terms that you know that apply to waves.

Waves Waves are repeated disturbances that move along in a certain direction.

Wave Motion Below is a picture of water waves. What is the direction of motion for water waves?

Wave Motion Water waves travel in a horizontal direction. The parts of the waves that travel in a horizontal direction are called the wavefronts. Wavefronts

Wave Motion If you are floating in water, what is your direction of motion? Objects floating in water move in a vertical direction as the wavefronts pass by.

Measuring Waves The highest point of a wave is called the crest or the peak. The lowest point is called the trough. crest or peak trough

Measuring Waves Speed – the distance traveled by a crest or a trough every second. Symbol – v Unit – meters/second or m/s v

Measuring Waves Speed – the distance traveled by a crest or a trough every second. The speed of sound in air is 340m/s. The speed of light in a vacuum is 300,000,000m/s. v

Measuring Waves Wavelength – the distance from one wave crest to the next. Symbol – λ(Greek letter pronounced “lambda”) Unit – meters or m (or in cm, mm, or nm) wavelength

Measuring Waves Amplitude – the height of the wave crest from the middle. Symbol – none Unit – meters or m (or in cm, mm, or nm) amplitude

Measuring Waves Frequency – the amount of complete waves passing a point in one second. Symbol – f Unit – hertz (Hz)

The Wave Equation v = fλ speed = frequency x wavelength v – the speed of the wave in m/s f – the frequency of the wave in Hz λ – the wavelength of the wave in m

Summary Tasks Copy Figure 7.1.4 from your textbook on page 105. Use the glossary of your textbook and write the definitions for the following key words: wavefront, frequency, wavelength, and amplitude. Answer questions 1 and 2 on page 105.

Transverse and Longitudinal Waves

Transverse Waves Recall that the direction a wave is moving is the direction the wavefront is traveling. In transverse waves the vibrations are perpendicular to the direction in which the waves are traveling.

Polarised Waves Transverse waves are polarised if the vibrations are always along the same line. The following diagram shows two different polarised waves.

Polarised Waves If we use an object that only allows one direction of wave to pass through, we can turn unpolarised waves into polarised waves.

Polarised Waves Light waves from a lamp are unpolarised transverse waves. A Polaroid material only allows light waves that vibrate in the same direction.

Polarised Waves If a second Polaroid filter is place at a right angle to the first one, the polarised waves will not be able to pass the second one.

Longitudinal Waves The vibrations of a longitudinal wave are parallel to the direction in which the waves are traveling. Sound waves are created when an object vibrates. The vibration repeatedly pushes and pulls the air. As each layer of air

Comparing Transverse and Longitudinal Waves On transverse waves the high point is the crest and the low point is the trough. On longitudinal waves, the high density point is called a compression and the low density point is a rarefaction.

Summary Transverse waves vibrate at right angles to the direction of travel of the waves. Longitudinal waves vibrate parallel to the direction of travel of the waves. Light waves and waves on a rope are transverse waves. Sound waves are longitudinal. Transverse waves are polarised if the vibrations are always along the same line. Pg. 107 #1,2

Reflection and Refraction

Reflection Waves travelling towards a barrier will be reflected. The waves moving towards the barrier are called incident waves and the waves travelling away are called reflected waves. The reflected waves are at the same angle from the barrier as the incident waves.

Reflection

Refraction Refraction occurs when Waves change from one medium to another, which changes the speed The waves approach the barrier at a non- zero angle

Refraction

Diffraction

Diffraction Diffraction is the spreading of waves when they pass through a gap or move past an obstacle.

Gap Size A narrow gap makes the wave spread out more. A wider gap makes the waves spread out less.

Note: For diffraction to be noticeable the gap must be similar in width to the wavelength of the waves. The wavelength does not change when diffraction occurs.

Summary Diffraction is the spreading of waves when they pass through a gap or round an obstacle. For noticeable diffraction the gap must be similar in size to the wavelength. The wavelength does not change on diffraction. The narrower the gap, the more the waves spread out.