Chapter 4 The Perception of Light & Sound Text Book pp. 91 to 116 Related concepts: Waves, Vision, Hearing, Telescopes Radio IR UV X Rays Gamma

Waves A Wave is a disturbance that travels through a medium. A wave carries energy A wave does not transport matter.

Transverse Waves The movement or vibration is “up and down” A transverse wave’s motion is perpendicular to its travel Travel Vibration

Longitudinal or Compression Wave The vibration is “forward and backward” The direction of vibration is the same as the direction of travel Vibration Direction of Travel

Properties of a Wave s c τ ν λ A Property Name Symbol Unit Frequency Hertz (Hz) Wavelength λ Metres (m) Amplitude A Decibels (dB)* Speed (mechanical waves) s Metres/sec (m/s) Speed (of light) (ElectroMagnetic Radiation) c 300 000 km/s Period τ Seconds (s)

Mechanical Waves Mechanical waves need a substance or “medium” to travel through. Examples: Sound Waves Seismic Waves Ocean Waves

Electromagnetic Waves Electromagnetic waves do not need a substance or medium in order to travel. They may travel through empty space. Examples: Radio waves Infrared waves Visible light Ultraviolet light X rays Gamma rays

EMR Spectrum How EMR is organized WAVELENGTH Longest to shortest across the spectrum X-ray, Yellow visible light, purple vis. Light, Radio waves, UV, Gamma Ray, Microwaves, Infrared

Sound Sound is carried by longitudinal, mechanical waves Sound needs air to travel through In the vacuum of space, sound will not travel. The speed of sound through air is about 340 m/s or over 1000 km/h That seems fast, but it is way slower than light, which travels 299000 km/s

The Decibel Scale The decibel scale measures the intensity, or loudness of sound as perceived by the human ear. Sound Decibels Breathing, 3 m away 10 Murmur or whisper, 2 m away 20 Calm classroom 40 Intense road traffic, 3 m away 70 Motorcycle, without muffler, 2m away 100 Rock Concert / Jet engine (14m away) 120 Space Shuttle launch (50 m away) 200

Frequency & Wavelength Sound has the related properties of frequency (how fast the vibration that created the wave was) and wavelength (how long the waves are) Frequency is measured in Hertz (Hz), also called cycles per second. Its how many vibrations there were per second. Wavelength is measured in metres or centimetres The higher the frequency of a sound, the shorter its wavelength is.

Frequency and Pitch of Sound High frequency sound wave (>1000Hz) have a very high pitch sound (high treble). Low frequency sound waves (<100 Hz) have a low pitch (bass) Mid-range sounds (100Hz to 1000Hz) are the most comfortable to the ear. The mid-point of most musical compositions is C4 or “middle-C” (262 Hz). The octave that contains middle C runs from 220 Hz (A4) to 440Hz (A5) The range of human hearing is said to be: 20Hz to 20000Hz (on the average)

Infrasound and Ultrasound Frequencies below 20Hz are called infrasound. Human’s can’t hear them, but some animals, like elephants, can. Frequencies above 20000 are called ultrasound. Human’s can’t hear them, but some animals, like dogs and bats, can Bats use ultrasound to locate objects in the dark (echolocation) Hospitals use ultrasound to “see” the development of a fetus.

Different light frequencies are interpreted as colours Visible Light Light is a form of electromagnetic radiation (EMR) that humans can see with their eyes. The different frequencies of light are interpreted by our brains as different colours Different light frequencies are interpreted as colours

400 THz 700 THz

Reflection Normal Incident Ray Reflected Ray Mirror

Refraction Refraction is the deviation (bending) of light as it passes from one transparent medium to another. For example, light refracts when it passes from air into glass or water. The spoon looks bent Due to refraction Air Water

Lenses Lenses use refraction to collect, focus or project light. Lenses are made of transparent material (like glass) and have at least one curved surface. The two main types of lens are converging and diverging. All of these devices use lenses: Contact lens, eye eyeglasses telescope projector camera photographic lens

Converging, or convex, lenses focus light at a single focal point. Optical Centre Converging, or convex, lenses focus light at a single focal point. Converging lenses can be used to create “real” images… Images that can be projected on a screen. Converging lenses are often used to enlarge or magnify images, Focal point Diverging, or concave, lenses do not focus light at a single point. The so-called focal point of a diverging lens is a virtual point where the image would appear to be when viewed through the lens. Diverging lenses cannot project images onto a screen. Diverging lenses usually make things look small Focal point Optical Centre

How to Find the Image Size (Projected Image, Large) 2. Tip of object, parallel to base line Lens Plane Base line 3. Tip of object through F’. F’ 2a. From lens plane, through Focal point (F) Object 1. Tip of object through OC Image Converging (convex) Lens In this case, when the object is close to the lens, the image is larger than the object, and upside down, and real

How to Find the Image Size (Projected Image, small) 2. Tip of object, parallel to base line 1.Tip of object through OC Lens Plane 2a. through Focal point (F) 3. Tip of object through F’ Base line F’ Upside down Smaller Real Image Object 3a. Parallel to base line Converging (convex) Lens

How to Find the Image Size (Virtual Image, large) 2b. From focal point through lens 2. From lens, parallel to first line Lens Plane Base line . F’ Image Right side up Larger Virtual Object 1. Tip of object through O.C. Converging (convex) Lens

Object Image Small, Virtual Right side up Diverging Lens (concave) seen through a diverging lens, the image is smaller than the object, but right-side up.

Glasses Myopia: Near-sighted people need diverging lenses in their glasses. Near sighted people can see well close up, but not far away Hyperopia: far-sighted need converging lenses in their glasses Far-sighted people see things far away well, but cannot see close things well.

Exercises Workbook, pages 59, 60, 61