Ch 4: What is a Wave? A WAVE is a disturbance that travels through a medium. A wave transports energy; it does not transport matter. Ex. Ripples on water, a guitar string producing vibrations which travel to our eardrum, an earthquake creating vibrations through the Earth’s crust

Wave Characteristics Waves come in a variety of shapes and sizes. We can distinguish them by their Amplitude (A), Wavelength (λ) and Frequency (f) Waves are distinguished by the two ways they propagate (move) through a medium Transverse wave Longitudinal wave

Amplitude (A) The amplitude (A) of a wave is the distance from a crest or trough to where the wave is at equilibrium. The amplitude is used to measure the energy transferred by the wave. The bigger the distance, the greater the energy transferred for transverse waves. For longitudinal waves, the denser (shorter) the region of compression the more energy transferred. *For a diagram of Amplitude in a Longitudinal wave, see image 4.9 on p 94 of textbook Transverse wave

Wavelength (λ) A WAVELENGTH is the length of a wave’s complete cycle. Wavelength is symbolized by the Greek letter λ “lambda”. High energy waves have a short wavelength. wavelength

Frequency (f) FREQUENCY (f) corresponds to the number of cycles of a wave per unit of time The frequency (f) of a wave is measure in hertz (Hz) which corresponds to the number of cycles per second (the pitch of the sound) The speed (distance over time) of a wave can be calculated with: Speed = f x λ

Moving from one medium to another Once a wave (incident wave) has reached the end of a medium, part of the energy is transferred to the medium that is immediately next to it (transmitted wave) and part is reflected backward (reflected wave).

Two Types of Waves: 1-Mechanical waves 1- MECHANICAL and 2- ELECTROMAGNETIC 1-Mechanical waves are caused by a localized disturbance changing the physical state of the medium (ex. seismic, sound, water) Need a medium or an environment to move through (liquid, solid, gas). These waves can be: A TRANSVERSE wave: moves perpendicular to the motion of its medium (ex. water) A LONGITUDINAL wave: moves parallel to the motion of its medium (ex. sound)

2-ELECTROMAGNETIC WAVES are transverse waves that can travel in both a medium and a vacuum A vacuum is a space that is essentially empty of matter They carry radiant energy and have many categories classified by frequency and wavelength The greater the frequency of the wave, the more energy it transports The ELECTROMAGNETIC SPECTRUM organizes all electromagnetic waves according to their wavelength and frequency

Electromagnetic spectrum **pg. 98-99 in textbook**

Sound waves SOUND is a longitudinal mechanical wave produced by the vibration of an object and transmitted to the object’s environment. A vibrating source pushes molecules in air back and forth, creating areas of compression and rarefaction (due to air pressure changes). When air molecules move, they collide with the surrounding ones and make them move as well. Only the sound wave travels away from the source through these collisions. The air particles only move back and forth slightly. Anything that creates sound produces a longitudinal mechanical wave. (ex. drums, speakers, our voice) Sound cannot travel through a vacuum (no sound in space)

Sound waves (cont’d) Rarefaction The speed at which sound waves travel changes greatly depending on the medium. Sound waves propagate through liquids and solids much faster than in gases *Youtube video: Going past the speed of sound

Sound intensity and the decibel scale As the amplitude (A) of a sound wave increases, so does its energy and intensity (volume) Intensity of sound is measured in decibels (dB). The DECIBEL SCALE is a relative scale that represents the perception of the intensity of sound by the human ear. The Threshold of Hearing (TOH – the faintest sound a human can hear) is measured as 0 dB. A sound like rustling leaves that is 10 times more intense is 10 dB. Something that is 10x10 (or 100 times) more intense, like a whisper, is 20 db. Prolonged exposure above 100 dB can cause long-term damage. Can be immediate above 120 dB.

Question: A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?

Frequency and Sound Sound can also have different tones (higher or lower pitches) A low-frequency wave produces a low pitched sound and a high frequency wave produces a high pitched sound Humans hear from 20 to 20,000 Hz. Different in other animals. Sounds under 20 Hz are called infrasounds. Above 20,000 Hz are ultrasounds. Since sound waves will bounce back when they hit an object, they can be used for imaging and echolocation (ex. ultrasounds, sonar in submarines)

Light LIGHT is an electromagnetic wave that is visible to the human eye Light waves travel in straight lines called light rays. Light waves striking an object are either reflected, refracted or absorbed. Light is composed of elementary particles called photons.

Reflection of light REFLECTION is the rebounding of light that occurs when a light ray hits a different medium and ‘bounces back’ to the medium from which they came. All objects reflect light to some degree based on their colour and light source. We can see objects because reflection brings light to our eyes. Incident ray: the ray that contacts the surface of object Reflected ray: the ray that rebounds Normal: an imaginary line perpendicular to the surface. Angle of incidence: angle formed by the incident ray and the normal. Angle of reflection: is the angle formed by the reflected ray and the normal.

Reflection of Light (cont’d) The angle of incidence is always equal to the angle of reflection. The incident ray and the reflection ray are always on the same plane. Diffuse reflection: when parallel light rays strike an uneven surface, they are reflected back in all directions thus not obeying the laws of reflection. Ex: paper, rock Specular reflection: when parallel light rays contact a smooth surface, their reflections are parallel, producing a true mirror image. Ex: mirror, lake surface.

Refraction of Light REFRACTION is the deviation of a light ray as it passes from one transparent medium to another. This change is due to the change in the speed of light as it passes through each medium. Ex: looking at an object ½ submerged in water.

2 Types of Lenses Converging Lenses Diverging Lenses Transparent objects with at least one curved surface that can refract light. 2 types: Converging lens: bring together light rays that pass through them. Diverging lens: disperse light rays passing through them. Converging Lenses Diverging Lenses Uses for Lenses Lenses can be used to focus light. A single convex lens mounted in a frame with a handle or stand is a magnifying glass Lenses are also used in the correction of visual impairments

Focal point of a lens Optical centre (OC): centre of the lens which the principle axis passes through. FOCAL POINT OF A CONVERGING LENS: the real point where the refracted rays actually meet when the incident rays run parallel Converge/focus on a specific spot on the axis, behind the lens. FOCAL POINT OF A DIVERGING LENS: the virtual point from which the refracted light rays appear to emanate when the incident rays run Parallel. As if the rays originated in the same spot

Images Produced by a Converging Lens Rays must be drawn from a specific point on an object to determine the location of the image formed by a lens. (*2 rays needed) 1) A ray travelling parallel to principle axis is refracted through principal focus point. 2) A ray travelling through the OC is not refracted. 3) A ray travelling through the secondary focal point is refracted parallel to the principal axis. 1 Depending on the location of the object and its relation to the lens, the image has different characteristics - *pg. 114 in textbook 2 3

Images Produced by a Diverging Lens 1) A ray running parallel to the principal axis is refracted, appearing to originate from the focal point. 2) A ray passing through the OC does not deviate. 3) A ray travelling toward the secondary focal point is refracted parallel to the principal axis. The image from a diverging lens is always virtual, not inverted and smaller than the object. Image is always located between the principal focal point and the lens. 1 3 2

Using lenses to correct vision disorders Normal: inverted image of an object forms on the retina. Myopia: image of far away object forms in front of retina (too soon). Hyperopia: image of a near object forms behind retina (too far). Normal vision Myopia (nearsighted) Hyperopia (farsighted) Far sighted eye Light focuses past the retina Corrected far sighted eye Convex lens in glasses adjusts the focus of light rays to fall on the retina