Properties of Waves, Including Light and Sound

General Wave Properties

  • Waves transfer energy without transferring matter.

  • Examples of wave motion include:

    • Water Waves

    • Ropes

    • Springs

  • Frequency: the number of waves passing any point per second measured in hertz (Hz)

  • Period: time taken for one oscillation in seconds

  • Wavefront: the peak of a transverse wave or the compression of a longitudinal wave

  • Speed: how fast the wave travels measured in m/s

  • Wavelength: distance between a point on one wave to the corresponding point on the next wave in length

  • Amplitude: maximum displacement of a wave from its undisturbed point.

  • Transverse Waves

  • Travelling waves in which oscillation is perpendicular to direction of travel

  • Has crests and troughs

  • For example, light, water waves and vibrating string

  • Longitudinal Waves

    • Travelling waves in which oscillation is parallel to direction of travel.

    • Has compressions and rarefactions

    • For example, sound waves

  • Refraction:

    • Speed and wave length is reduced but frequency stays the same and the wave changes direction

    • Mechanical waves slow down when they pass from a denser to a rarer material and vice versa

    • Note: Electromagnetic waves like light increase in speed from an optically denser to a rarer medium.

    • When wave is slowed down, it is refracted towards normal

    • When wave is sped up, it is refracted away from normal

    • Deep water is denser than shallow water

    • Deep water to shallow water: speed decreases, wavelength decreases, and frequency remains constant

  • Shallow water to deep water: speed increases wavelength increases, and frequency remains constant

  • Reflection:

    • Waves bounce away from surface at same angle they strike it

    • Angle of incidence = angle of reflection

    • The incident ray, normal and reflected ray all lie on the same plane.

    • Speed, wavelength and frequency are unchanged by reflection

  • Diffraction:

  • Waves bend round the sides of an obstacle or spread out as they pass through a gap.

  • Wider gaps produce less diffraction.

  • When the gap size is equal to the wavelength, maximum diffraction occurs

Reflection of Light

  • Plane (flat) mirrors produce a reflection.

  • Rays from an object reflect off the mirror into our eyes, but we see them behind the mirror.

  • The image has these properties:

    • Image is the same size as the object

    • Image is the same distance from the mirror as object

    • A line joining corresponding points of the image and object meet the mirror at a right angle

    • Image is virtual: no rays actually pass through the image and the image cannot be formed on a screen

  • Laws of reflection:

    • Angle of incidence = angle of reflection

    • The incident ray reflected ray and normal are always on the same plane (side of mirror)

  • Critical angle: angle at which refracted ray is parallel to the surface of material.

  • If the angle of incidence is greater than the critical angle there is no refracted ray, there is total internal reflection.

  • If the angle of incidence is less than the critical angle the incidence ray will split into a refracted ray and a weaker reflected ray.

Refraction of Light

  • Refraction is the bending when light travels from one medium to another due to the change in speed of the ray of light.

  • Note:

    • The emergent ray is parallel to the incident ray only if the sides of the glass are parallel.

    • i = angle of incidence, r = angle of refraction

  • Light put in at one end is totally internally reflected until it comes out the other end.

  • Application: Optical Fibres

    • Used in communications: signals are coded and sent along the fiber as pulses of laser light

    • Used in medicine: an endoscope, an instrument used by surgeons to look inside the body; contains a long bundle of optic fibers.

Thin Converging Lens

  • Principal focus: the point where rays parallel to the principal axis converge with a converging lens.

  • Focal length: distance from principle focus and the optical center.

  • Principal axis: line that goes through optical center, and the 2 foci.

  • Optical center: the center of the lens

  • Real: image can be caught on a screen

  • Virtual: image cannot be caught on a screen

Real Image

  • When object is further away from the optical centre than F' is

A) A ray through centre of the lens passes straight through the lens.

B) A ray parallel to the principal axis passes through the focus on the other side of the lens

C) A ray through F' will leave the lens parallel to the principal axis

Virtual Image

  • When the object is closer to the optical centre than F' is

  • Magnifying glass: when a convex lens is used like this - an object is closer to a convex (converging) lens than the principal focus (like the diagram above), the rays never converge. Instead, they appear to come from a position behind the lens. The image is upright and magnified, it is a virtual image.

  • Images can be:

    • Enlarged: The image is larger than the object.

    • Same size: The image is the same size as the object.

    • Diminished: The image is smaller than the object.

    • Upright: The image is in the same vertical orientation as the object.

Dispersion of Light

Refraction by a prism:

  • 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.

  • If a beam of white light is passed through a prism it is dispersed into a spectrum.

  • White light is a mixture of colors, and the prism refracts each color by a different amount -- red is deviated least & violet most

  • Monochromatic light is that of a single frequency and colour.

  • The visible spectrum of light is acronymed as ROYGBIV

Electromagnetic Spectrum

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  • All electromagnetic waves:

    • Travel at the speed of light: approximately 3 × 108m/s.

    • They travel at around the same speed in air too.

    • Don't need a medium to travel through (travel through a vacuum)

    • Can transfer energy

    • Are produced by particles oscillating or losing energy in some way

    • Are transverse waves

  • Applications:

    • Radio waves: radio and television communications

    • Microwaves: satellite television and telephones

    • Safety issue: cause internal heating of body tissues

    • Infrared: electrical appliances (radiant heaters and grills), remote controllers for televisions and intruder alarms

    • X-rays: medicine (x-ray photography and killing cancer cells) and security

    • Safety issue: is a mutagen, it cause cancer (mutations)

    • Monochromatic: light of a single wavelength and color (used in lasers)

Sound

  • Sound is a mechanical wave.

  • Sound waves come from a vibrating source e.g. loudspeaker

  • As the loudspeaker cone vibrates, it moves forwards and backwards, which squashes & stretches the air in front.

  • As a result, a series of compressions (squashes) and rarefactions (stretches) travel out through the air, these are sound waves

  • Humans can hear frequencies between 20 and 20 000Hz.

  • Properties:

    • Sound waves are longitudinal: they have compressions and rarefactions and oscillate backwards and forwards.

    • Sound waves need a medium to travel through as it moves due to oscillating particles.

  • Ultrasound Waves: high frequency sound waves, medically used to look at structures and organs inside the human body, i.e. to form an image of a fetus in a pregnancy

  • Compression: High pressure section of a longitudinal wave

  • Rarefaction: Low pressure section of a longitudinal wave

  • The higher the frequency, the higher the pitch.

  • The higher the amplitude, the louder the sound

  • If a sound is repeated 0.1 seconds or more after it is first heard, the brain senses it again.

  • Therefore, given the adequate distance, if sound reflects off a surface, and comes back, an echo is produced.

Speed of sound in varrious media
Medium
Concrete
Pure Water
Air
V in Gas V in Liquid V in solid

Finding the speed of sound

  • When sound reflects off of a wall, it will come back to you; echo

  • If you know the distance between you and the wall, and measure how long it takes for the echo to sound, you can figure out the speed of sound in air.

  • Remember to take into account that sound has gone there & back