WAVES Types of waves There are three types

WAVES

Types of waves • There are three types of waves: • Mechanical waves require a material medium to travel (air, water, ropes). These waves are divided into three different types. – Transverse waves cause the medium to move perpendicular to the direction of the wave. – Longitudinal waves cause the medium to move parallel to the direction of the wave. – Surface waves are both transverse waves and longitudinal waves mixed in one medium.

Types of waves • Electromagnetic waves do not require a medium to travel (light, radio). • Matter waves are produced by electrons and particles

Measuring waves • Any point on a transverse wave moves up and down in a repeating pattern. • The shortest time that a point takes to return to the initial position (one vibration) is called period, T. • The number of vibrations per second is called frequency and is measured in hertz (Hz). Here's the equation for frequency: f=1/T

Amplitude • The amplitude of a wave is the distance from a crest 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

Example • A radio wave has a frequency of 93. 9 MHz (93. 9 * 106 Hz). What is its period? f = 93. 9 * 106 Hz f=1/T T=1/f T = 1 / 93. 9 x 106 Hz T = 1. 06 x 10 -8 s

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).

Sound waves • Sound waves are longitudinal waves produced by variations in air pressure. A vibrating source pushes molecules in air back and forth, creating areas of compression and rarefaction. When a molecule moves, it collides with the next one and makes it move too. • The energy of a sound wave travels away from the source trough a series of molecule collisions parallel to the direction of the wave. Sound cannot travel through a vacuum(no sound in space)

Threshold of hearing (TOH) • This faintest sound which a human ear can detect is known as the threshold of hearing. The most intense sound which the ear can safely detect without suffering any physical damage is more than one billion times more intense than the threshold of hearing.

Intensity and decibel scale • Since the range of intensities which the human ear can detect is so large, the scale which is frequently used by physicists to measure intensity is a scale based on multiples of 10. • The scale for measuring intensity is the decibel scale. The threshold of hearing is assigned a sound level of 0 decibels (0 d. B); this sound corresponds to an intensity of 1*10 -12 W/m 2. A sound which is 10 times more intense ( 1*10 -11 W/m 2) is assigned a sound level of 10 d. B

• A sound which is 10*10 or 100 times more intense ( 1*10 -10 W/m 2) is assigned a sound level of 20 db. • A sound which is 10*10*10 or 1000 times more intense ( 1*10 -9 W/m 2) is assigned a sound level of 30 db • And so on. . .

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

Electromagnetic spectrum • The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that object. • The electromagnetic spectrum extends from below the frequencies used for modern radio (at the longwavelength end) through gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to a fraction the size of an atom.

Electromagnetic spectrum

Types of radiation • Electromagnetic radiation can be described in terms of a stream of photons, each traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy. • The only difference between radio waves, visible light, and gamma-rays is the energy of the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and gamma-rays.

Light • Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400– 700 nm). • Light is composed of elementary particles called photons.

Colours • Red, green and blue are known as primary colors, because when they are added together white light is formed • By mixing primary colors in pairs we obtain secondary colors. Red and green produce yellow. Blue and red produce magenta, and blue and green produce cyan

Reflection of light • Incident ray is the ray moving toward the surface • Reflected ray is the ray moving from the surface • Normal is an imaginary line perpendicular to the surface • When a ray of light hits the surface of an object, part of the light is reflected

Refraction of light • When a ray of light passes from one medium to another, it bends. Depending of the new medium the light will travel faster or slower. • If the light travels faster in the second medium, then this medium is called the rarer medium. • On the other hand, the medium in which the light travels slower, in this case the first one, is called the denser medium.

Index of refraction • There is an index of refraction (n) between the two mediums. To get a value, we have to divide the sine of the angle in vacuum or air by the sine of the angle in the denser medium. In the example above, the index of refraction would be • n = sin a / sin b

Types of lenses

Focal point of a lens • A beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens

Uses of lens • Lenses can be used to focus light. • A single convex lens mounted in a frame with a handle or stand is a magnifying glass • Lens are also used in the correction of visual impairments

ICW • Finish p. 118 (# 15 to 18) • P. 117 # 1 to 4 • P. 118 # 8, 9, 13