**Example Problems of Sound and Light Waves [+Pembahasan] – Light Wave** is electromagnetic wave radiation that can be detected by the human eye and felt its effect.

**Characteristics of Light Waves: **

- Dispersion – The event of the decomposition of white light (polychromatic) into its components due to refraction.
- Light Interference – Interference is the blending of two or more waves into one new wave.
- Light Diffraction – the event of scattering or bending of waves by a narrow slit as an obstacle.
- Polarization of Light – the process of limiting vector waves that form a transverse wave so that it becomes one direction.

**sound wave** Longitudinal waves are waves that consist of particles that oscillate in the direction of the wave forming high and low pressure areas.

**There are 3 conditions for sound to occur, namely: **

- sound source
- medium and
- listener

## 1 – 10 Examples of Sound and Light Wave Problems

1. Determine the speed of sound in air at 17^{o}! Match it if it is considered that sound propagates in air at a temperature of 15^{0}C is 340 m/s! What’s your conclusion? (𝛾 = 1.4; R = 8.31 /𝑚𝑜𝑙 ; = 29 × 10^{3} /𝑚𝑜𝑙)

**Answer: **

**Discussion: **

Is known:

𝑇_{1} = 17^{𝑂}= 290

𝑇_{2} = 15^{𝑂}= 288

= 1.4

R = 8.31 /𝑚𝑜𝑙

= 29 × 10^{3} /𝑚𝑜𝑙

Wanted: = 17^{𝑂}= ?

**Answer: **

So, the speed of sound in air at 17^{0}C is 341.1 m/s, while at a temperature of 15^{0}C of 339.9 m/s

2. Train A with a speed of 20 m/s and train B with a speed of 60 m/s are moving toward each other. The speed of sound in air is 340 m/s. If the train driver A whistles with a frequency of 1500 Hz, what is the frequency of the whistle heard by the train driver B?

**Answer: **

**Discussion: **

Is known:

𝑣_{𝑠} = 20 /𝑠

𝑣_{𝑝} = 60 /𝑠

= 340 /𝑠

𝑓_{𝑠} = 1500

Asked:_{𝑝} = ?

**Answer: **

So, the frequency of the whistle heard by the train driver B is 1875 Hz.

3. A string 3 m long is tied at both ends. The resonant frequency of the first overtone of the string is 80 vibrations/second. If the mass per unit length of the string is 0.01 g/cm, what is the tension in the wire?

**Answer: **

**Discussion: **

Is known:

= 3

𝑓_{1} = 80

= 0.01 /𝑐𝑚 = 10^{3}/𝑚

Asked: = ?

**Answer: **

So, the magnitude of the tension in the wire is 57.6 N

4. The length of a closed organ pipe is 4 m. If the speed of sound in air at 15 suhu^{0}C is 340 m/s, what is the frequency of the first overtone?

**Answer: **

**Discussion: **

Is known:

L = 4 m

= 340 /𝑠

Asked:_{1} = ?

**Answer: **

So, the frequency of the first overtone is 63.75 Hz.

5. A sound source transmits sound with an output power of 0.16𝜋 watts. Assuming the sound wavefront is spherical, determine: (𝐼_{0} = 10^{12} /𝑚^{2} )

- (a) Sound intensity at a distance of 2 m from the source
- (b) The level of sound intensity at a distance of 2 m from the source

**Answer: **

**Discussion: **

Is known:

= 0.16𝜋

𝐼_{0} = 10^{12} /𝑚^{2}

Asked:

- a) at 2𝑚 =…?
- b) at 2𝑚 =…?

**Answer: **

So, at a distance of 2 m from the source the sound intensity is 0.01 /𝑚^{2} and the sound intensity level is 100 dB.

#### See also: Examples of Mechanical Wave Problems

6. Monochromatic light falling on two slits 0.06 mm apart produces a fourth bright band at an angle of 8^{o} . What wavelength of light is used?

**Answer: **

**Discussion: **

Is known:

= 0.06 = 0.06 × 10^{3}𝑚

= 4

= 8^{0}

Asked: = ?

**Answer: **

So, the wavelength of light used is 2.1 × 10^{6} m

7. Light with a wavelength of 600 nm passes through a slit 2.5 × 10^{3} mm. Determine the third dark band diffraction angle (𝜃_{3})!

**Answer: **

**Discussion: **

Is known:

= 600 = 6 × 10^{4}𝑚𝑚

= 2.5 × 10^{3} 𝑚𝑚

Asked:_{3} = ?

**Answer: **

So, the third dark band diffraction angle is 46^{0}

8. A sound source has an intensity level of 60 dB. When 100 sources of the same sound sound simultaneously, the resulting intensity level is…

**Answer: **

**Discussion: **

**Use the equation:**

TIn = TI1 + 10 log n

TI100 = 60 dB + 10 log 100

TI100 = 60 dB + 10 . 2 dB

TI 100 = 80 dB

9. The intensity level of one bee buzzing is 10 dB. If the buzzing sound of each bee is considered identical and the intensity of the human hearing threshold is 10−12 Wm2, then the intensity of the buzzing sound of 1000 bees is….

**Answer: **

**Discussion: **

Count in advance TI 1000 bees

TI = TI 1 + 10 log n = 10 dB + 10 log 1000 = 40 dB

Counting I 1000 bees

TI = 10 logs (I / I_{o})

40 dB = 10 logs (I / 10^{-12})

log(I / 10^{-12}) = 4

I / 10^{-12} = 10^{4}

I = 10^{4} . 10^{-12} = 10^{-8} Wb

10. The intensity level of a sound source at a distance of 9 m from the observer is 50 dB. If nine identical sound sources are added together, the total intensity level for the observer becomes….

**Answer: **

**Discussion: **

TI = TI 1 + 10 log n = 50 dB + 10 log 10 = 60 dB

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