Light and sound

Light waves

• light waves are transverse waves

• they can be reflected and refracted

Luminous object

object that gives out its own light

Non-luminous object

object that does not produce its own light, but we can see it because it reflects light from other sources.

Law of reflection

angle of incidence = angle of reflection

Refractive index

The amount that a light wave bends of refracts depends on the refractive index.

equation for refractive index

n= sin i/sin r

when will a wave change direction?

• a wave will change direction if it meets a surface of a different density at an angle.

• one part of the wavefront will slow down before the other.

wavefront

• wavefronts are at right angles to the ray/direction of travel.

Optical fibre

• a thin strand of glass of plastic.

• the light ray travels along the fibre, reflecting off the inside surface by total internal reflection.

Total internal reflection

Complete reflection of a light ray reaching a boundary greater than the critical angle.

Conditions required for total internal reflection

• the light must be travelling from a more dense medium into a less dense medium (ie glass to air)

• the angle of incidence must be greater than the critical angle.

Critical angle equation

sin c = 1/ n (refractive index)

Sound waves are…

longitudal

describe an experiment to investigate the refraction of light:

1. rectangular block (glass)

2. semicircular block (glass)

3. triangular prism (glass)

1. adjust slit on ray box until it produces a narrow ray going through the object.

2. mark the path of the rays: AB and CD

3. trace around the glass block.

4. remove the glass block from paper.

5. use ruler to draw the rays AB, BC, CD. shows that AB and CD are parallel.

6. use protractor to mark the 2 normals.

7. measure angle of incidence and refraction.

conclusions from light ray experiments for semicircular blocks and triangular prisms

• light ray is bent towards the normal when it goes into glass.

• angle of incidence > angle of refraction

• light ray is bent away from normal when it leaves glass block.

• if the block has parallel sides, angle of incidence = angle of refraction

What is sound?

• sound is caused by vibrating particles.

• the ‘medium’ is the air, water or gas in which the sound travels through.

amplitude

the height of the peak

amplitude and volume

a loud sound has a higher amplitude (taller waves)

frequency and pitch

low frequency wave has a lower pitch (the waves are longer and aren’t close together.)

amplitude and energy

the greater the amplitude of the wave, the more energy it carries.

How can an oscilloscope be used to display a sound

• An oscilloscope is a device that displays the microphone signal as a trace on a screen.

• The microphone picks up sound waves travelling through the air and converts the sounds to electrical signals.

frequency range for human hearing

20Hz - 20 000 Hz

experiment: describe how to obtain a value for the speed of sound waves in air, using a 100m outdoor running track

1. 2 students stand at the furthest end of the 100m track with wooden clappers. One operates the clappers, and the other lifts their hand in the air.

2. 10 students stand at the other end of the track, each with stop watches.

3. the ‘arm’ student lowers their arm as the ‘clapper’ student bangs the wooden clappers together.

4. the ‘stopwatch’ students start the stop watches as soon as they see the arm being lowered.

5. they stop the stopwatches when they hear the band from the clapper.

6. repeat the experiment 3 times.

7. calculate the average speed of all of the stopwatches over the 3 experiments.

8. speed = distance/time

9. issues include differences in reaction time

10. still air required.

• a sound wave and a radio have the same wavelength

• state why they have different frequencies

the 2 waves travel the same distance (1 wavelength) in different times.

state 3 similarities between transverse and longitudal waves

1. particles in both waves vibrate.

2. both waves transfer energy.

3. both carry out reflection and refraction