1. 2 REFLECTION OF WAVES

NURUL FADHILAH BINTI ALIAS

MRSM KUALA KLAWANG, NEGERI SEMBILAN

IntroductionWhat will happen to the waves after they hit the retaining (tembok) wall?

Water waves hitting a wall

When an object, like a ball, is thrown against a rigid wall it will…?

ball

wall

This "reflection" of the object can be analyzed in terms of momentum and energy conservation

If the collision between ball and wall is perfectly ELASTIC, then all the incident energy and momentum is reflected, and the ball bounces back with the same speedIf the collision is INELASTIC, then the wall (or ball) absorbs some of the incident energy and momentum and the ball does not bounce back with the same speed.

Waves also carry energy and momentum

whenever a wave encounters an obstacle, they are reflected by the obstacle

reflection of waves is responsible for echoes, radar detectors, and for allowing standing waves which are so important to sound production in musical instruments.

Reflection

When a wave reaches a boundary it is:•Partially reflected (bounces off surface)

•Partially transmitted through surface.

Law of ReflectionAngle of incidence = Angle of reflection

WATER WAVE (REFLECTED WAVES PATTERN)

Wave propagation

Reflected wavesFlat wave reflection from a convex barrier/obstacle/ reflector

Circular wave reflection from flat barrier/obstacle/ reflector

Let’s look at the detail…

Circular wave reflection from a concave

barrier/obstacle/ reflector

Source far from barrier

Circular wave reflection from a concave

barrier/obstacle/ reflector

Source near to the obstacle

Circular wave reflection from a convex barrier/obstacle/ reflector

Source further from obstacle

SOURCE IS FLAT (BAR VIBRATOR)

SOURCE IS CIRCULAR

(CIRCLE VIBRATOR)

LIGHT WAVES (REFLECTED WAVES)

• When rays of light strike any surface the rays are reflected , unless the surface is black, when they are absorbed.

• The reflection depend on how smooth the surfaces are. Good mirrors reflect well over 90% of the light that reaches them, with only a small amount being absorbed.

• A plane mirror is a flat smooth surface which reflects regularly most of the light falling on it.

• The phenomenon of reflection of light obeys the law of reflection.

(a) The angle of incidence is equal to the angle of reflection.

(b) The incident wave, the reflected wave and the normal lie in the same plane.

• It also can be seen that

(a) The size of the image = the size of the object

(b) The distance of the image = the distance of the object

Specular Reflection

• A reflection produced by a smooth surface.

Diffuse Reflection

• Reflection from a rough surface.

http://micro.magnet.fsu.edu/primer/java/reflection/specular/specularjavafigure1.jpg

Reflection from a Plane Mirror

di=do

hi=hoVirtual image(dotted lines show virtual)

Step by step to Draw image…Please refer to the text book…

Learn and practice step by step how to draw the images…

Type of Image formed by a plane mirror

• Laterally inverted• Same size as the object• Virtual and• As far behind the mirror as

the object is in front of it

Refer pg. 133 in your txt bk.

(English Ver.)

What is the minimum mirror height required for a person to see their full self in the mirror?

Hint: Compare AB to CD

A

D

B

C

CURVED MIRRORS (^^,)

Curved mirrors can make things look smaller or BIGGER.

C = center of curvature (center of sphere of the mirror)

R = The distance between C and the mirror is called the radius of curvature

f = FP (FP = ½ CP)

F = Principal focus

Principal Axis = connecting line from the centre of curvature to point P

Application Concave Mirrorstorches,

search-lights and vehicles headlights to get powerful parallel beams of light.

They are often used as shaving mirrors to see a larger image of the face.

The dentists use concave mirrors to see large images of the teeth of patients.

Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Image formed by a convex mirror

Application Convex Mirrors

commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving.

Convex mirrors are preferred because they always give an erect, though diminished, image.

Also, they have a wider field of view as they are curved outwards.

Thus, convex mirrors enable the driver to view much large area than would be possible with a plane mirror

Assignment (A):

If you were a sound engineer, what is the modification that you have to do to improve a recording studio. In your explanations , include steps to overcome sound reflection problems.  Assignment (B):  You are engaged by an interior design company to help to design a café in the town. You are required to make the café look more spacious. Explain how to make the hall look bigger by fixing mirrors on the wall.

REFLECTION OF SOUND WAVE

Assignment (C):

Make a mind map of reflection of wave. Make sure all important points are included. Use a manila card or a mahjong paper.

Assignment (D):

Make a table of comparison of reflection waves of water waves, light waves and also sound waves.

Experiments of Reflection of Waves

Aim : To observe and describe the reflection of water waves in terms of the angle of incidence, angle of reflection and the wavelength.

Apparatus : ripple tank, straight dipper, straight reflector, stroboscope, white mahjong paper, metre rule, power supply, rheostat.

Procedure :1. Set up the apparatus as shown in the figure

above.2. Pour water into the ripple tank to a depth of

about 1 cm and adjust the stands to ensure the depth of the water in the tank is uniform.

3. Lower the straight dipper until it just touches the surface of the water.

4. Place a straight reflector parallel to the dipper, 30 cm away.

5. Switch on the lamp and the motor. Adjust the frequency of the motor by sliding the rheostat so that the image of the wave is clear.

6. Use the stroboscope to freeze the image of the wave form.

7. Mark the image on the white paper.8. Measure the angle of incidence, angle of

reflection, wavelength of the incident waves and reflected waves.

9. Replace the paper, repeat steps 4 – 8 with the angle between the reflector and the dipper at 15º, 30º ,45º and 60º.

10. Tabulate your data.11. Deduce the relationship between

i. the angle of incidence and angle of reflection;ii. The wavelength of the incident waves and reflected

waves.

Appendix 1

Discussion•From the observation, the incident angle i, is equal to the reflection angle.•The length of incident wave is the same as the length of reflection wave after it is reflected

ConclusionThe angle of incidence, i is equal to the angle of reflection , r

A simple experiment of reflection of sound wavesEquipment and materials:Stopwatch, 2 cardboard tube with length 50 cm, smooth wall, smooth/soft wood

Figure 1Procedure1. Place a stop watch at the end of a hollow tube.2. The tube is placed at an angle to a reflecting surface as shown in the above figure.3. Keep a smooth wood vertical on the table top.4. Place another hollow tube on the other side of the smooth wood and hold it close to ear.5. Adjust the angle of the second tube until a loud ticking is heard.Make a conclusion of this activity.

Appendix 2 (Sound Wave)  ResultsIt is observed that the loudest ticking sound is heard when the angle of incident is the same as the angle of reflection. Discussion:1.Soft board/wood is used to avoid the ticking sound of the stopwatch from being heard directly by the listener’s ears.2.Smooth wall reflects sound waves more efficiently. ConclusionThe angle of incidence , i is equal to the angle of reflection , r. 

WORKSHEET 1 Draw the pattern of reflected waves and describe its characteristic. Make a comparison between incident waves and reflected waves in terms of :

i. angle of incidence , iii. angle of reflection , riii. wavelength, frequency and speediv. direction of propagation of the wave