WAVES, LIGHT AND SOUND

WAVES• A wave is a disturbance that transfers (carries)

energy through matter (a medium) or space (no medium).

• Most waves require a medium to travel through but light waves (electromagnetic waves) do not need any matter to travel through (why we can get light from the sun).

• Two main shapes of waves Transverse (light) and Longitudinal (sound)

http://www.physicsclassroom.com/Class/waves/u10l1c.cfm

PARTS OF A WAVE

Crests

Trough

Amplitude: the height of a wave above or below the equilibrium (relates to the amount of energy)

Crest: the top of the wave

Equilibrium: the original position of the medium before the waves move through it

Trough: the lowest point of the wave

Wavelength: the length of a wave (cycle) crest to crest or trough to trough

Frequency: the number of waves (cycles) that pass a point in one second

WAVELENGTH, FREQUENCY, AND SPEED

• A long wave has a low frequency and low energy. A short wave has a high frequency and energy.

• As - inverse relationship• The speed of a wave depends on the wavelength

and frequency and medium. • Velocity = wavelength x frequency

• v = • Example: What is the speed of a wave that has a

wavelength of 1m and a frequency of 2 Hz?• v = (1m) x (2 Hz) = 2 m/s

SOUND WAVES• Sound waves are longitudinal waves.• Sounds are caused by vibrations.• The speed of sound depends on the medium. Sound

travels faster in denser materials.• vsolids > vliquids > vgases • The speed of sound also depends on temperature of

the medium. As the temperature increases in a gas the wave speed increases.

• To increase loudness of a sound you must increase the amplitude (amount of energy).

• Frequency is related to pitch (high frequency = high pitch and a low frequency = low pitch)

DOPPLER EFFECT• The change in a waves frequency whenever

there is motion between the source and its receiver.

• Ex: Sound waves – a car honking it’s horn as it moves down the street. As the car approaches, the waves become pushed together creating shorter wavelength and higher frequencies so the pitch goes up. As the car moves away, the wavelengths become longer, the frequencies are lower, and pitch goes down.

ELECTROMAGNETIC WAVES

• Light waves are Electromagnetic waves.

• They do not need a medium to travel through (though they can)

• They are transverse waves.

• The speed of light waves is 300 million m/s

• Only a small portion of the Electromagnetic Spectrum is visible to us.

The Electromagnetic SpectrumLeast Energy

Most Energy

RADIO WAVES: lowest frequency and longest wavelength, used for communication (radio and TV)

MICROWAVES: used in cooking and for RADAR

INFRARED WAVES: cannot be seen, felt as heat, “below” red, used for cooking, medicine, and night vision

VISIBLE LIGHT: the only portion your eyes can detect; consists of 7 main frequencies R(ed)O(range)Y(ellow)G(reen)B(lue)I(ndigo)V(iolet); red violet

ULTRAVIOLET: present in sunlight, “beyond” violet, energy is enough to kill living cells, used for sterilization

X-RAYS: energy is enough for photons to pass through the skin, for medicine

GAMMA RAYS: tremendous ability to penetrate matter, used in the treatment of cancer but can be very dangerous as well

WAVE INTERACTIONS• When a wave strikes a barrier it can either be

absorbed or reflected. • There are 4 types of interactions:

1) Reflection: the bouncing back after a wave strikes an object that does NOT absorb the wave’s energy

2) Refraction: the bending of waves due to a change in speed.

3) Diffraction: The bending of waves around a barrier.

4) Interference: The phenomenon which occurs when two waves meet while traveling along the same medium.

REFLECTION• The Law of Reflection states that the angle of the

incidence is equal to the angle of reflection.

Examples:

Sound waves: Echoes

Light waves: Mirrors

MIRRORS• There are 3 types of mirrors:

Plane: flat surface, reflected image is the same size

Concave (converging mirror): curves inward, reflected image is either enlarged (object is close to mirror) or image is smaller and up-side down (object is far from mirror)

Convex (diverging mirror): curves outward, reflected image is always smaller and right side up; provides a wide field of vision (car side mirrors, surveillance mirrors)

PLANE MIRROR

CONVEX MIRROR

A

B

A and B: CONCAVE MIRRORS

http://sol.sci.uop.edu/~jfalward/reflection/reflection55.html

REFRACTION• Waves move at different speeds in different types of

matter (medium). This causes the waves to bend. Temperature can also affect the speed of a wave.

• Examples: Sound in air (below); light in air and water (below); light and lenses

www.hk-phy.org/iq/sound_night/sound_night_e.html

http://www.splung.com/content/sid/4/page/snellslaw

LENSES• Lens: a transparent object that refracts light

rays such that they converge or diverge to create an image

• Concave (Diverging Lenses): produce virtual, reduced and upright images (near-sighted corrective lenses)

• Convex (Converging Lenses): produce real or virtual images (in microscopes; magnifying glasses, far-sighted corrective lenses)

c. Diffraction - the bending of a wave around the edge of an object.

1) Water waves bending around islands

2) Water waves passing through a slit and spreading out

http://www.lexington1.net/technology/instruct/ppts/Science/Physical/Waves%20and%20Sound.ppt#264,9,Slide 9

d. Interference -two or more waves overlapping to form a new wave.

http://www.lexington1.net/technology/instruct/ppts/Science/Physical/Waves%20and%20Sound.ppt#267,12,Slide 12

There are two main types of wave interference:

1)Constructive

2)Destructive

1) Constructive (in phase)

Sound waves that constructively interfere are louder

2) Destructive (out of phase)

Sound waves that destructively interfere are not as loud

http://www.lexington1.net/technology/instruct/ppts/Science/Physical/Waves%20and%20Sound.ppt#268,13,Slide 13

GHSGT PRACTICE QUESTIONS• 1) A violin is generating sounds that have the same

frequency. Which property of the sound waves needs to be increased to produce louder sounds?a) wavelength b) period c) amplitude d) speed

• 2) What property does a wave exhibit when it bends as it crosses a barrier?a) diffraction b) reflection c) absorption d) refraction

• 3) The distance between two consecutive troughs is the a) frequency b) amplitude c) medium d) wavelength

ANSWERS• 1) C: AMPLITUDE (greater amplitude =

more energy = louder sound)

• 2) D: REFRACTION

• 3) D: WAVELENGTH (length of one wave or cycle; crest to crest (peak) or trough to trough (lowest point on a wave))

GHSGT PRACTICE PROBLEMS• 4) Which type of wave has the shortest

wavelength?

a) radio b) gamma c) ultraviolet d) microwave

• 5) Through which of the following does sound travel at the greatest speed?

a) air b) steel c) water d) a vacuum

• 6) Which of these light waves has the highest energy waves?

a) infrared b) microwaves c) ultraviolet d) visible light

ANSWERS

• 4) B: GAMMA (highest frequency and energy)

• 5) B: STEEL (sounds travel faster through denser materials – solids)

• 6) C: ULTRAVIOLET

GHSGT PRACTICE PROBLEMS• 7) The bouncing back of a wave when it hits a

barrier is called

a) diffraction b) interference c) reflection d) refraction

• 8) When light waves travel through a vacuum, they always have the same

a) amplitude b) frequency c) speed d) wavelength

• 9) A sound wave travels at a speed of 343 m/s and has a wavelength of 1.4 m. The frequency is

a) 245 Hz b) 480 Hz c) 0.004 Hz d) 344.4 Hz

ANSWERS• 7) C: REFLECTION

• 8) C: SPEED (speed changes with the medium)

• 9) A: 245 Hz (v = = v/ = (343 m/s)/ 1.4 m) = 245 Hz

GHSGT PRACTICE QUESTIONS

• 10) Which items would best be used to demonstrate refraction of light?

a) a mirror and a black cloth

b) clear glass and clear plastic

c) a flashlight and a white wall

d) shiny foil and a shiny floor

ANSWER

• 10) B: clear glass and clear plastic

• Refraction is the bending of light as it passes from one transparent (see through) material to another