Comparing Electromagnetic Waves To Water Waves

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Wave model of light The wave model of light describes waves with a

very small wavelength travelling at a high speed in a straight line from its source. It explains why:

• When light passes from one medium to a more optically denser another, some light is reflected and some refracted into the denser medium

• The speed of light can decrease when it moves from one medium to an optically denser one

Remember the electromagnetic spectrum?The electromagnetic waves are grouped into types that have similar wavelengths and so have similar properties.

Electromagnetic waves form a continuous series in order of changing wavelength, frequency and energy. This series is called the electromagnetic spectrum.

gamma raysX-rays ultraviolet

visible lightinfraredmicrowaves

radio waves

0.01 nm100 nm 1 nm0.01 mm1 mm10 cm100 m

smaller wavelength

higher frequency and energy

Water WavesExperiment with the wave tanks1.Record what the waves do when they hit a

convex barrier, a concave barrier, and a straight barrier.

2.Record what the waves do when they hit a gap. Change the gap width and record your observations.

3.Place the glass piece on the bottom to simulate shallow water. Record your observations.

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Curved Reflectors

• Concave reflectors – make the wave converge (meet at a point)

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Curved Reflectors

• Convex Reflectors – make the waves diverge (spread out)

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Diffraction

More diffraction if the size of the gap is similar to the wavelength

More diffraction if wavelength is increased (or frequency decreased)

Diffraction is when waves spread out from the edge of a gap.

What is diffraction?Diffraction is a property of all waves, which can be shown by water waves in a ripple tank.

Here, the waves travel along until they reach a gap. The width of the gap is similar to the wavelength of the waves.

The waves pass through the gap and spread out. This is diffraction.

So, when radio waves travel past an obstacle or through a narrow gap, they bend or spread out due to diffraction.

How does wavelength affect diffraction?The amount of diffraction depends on how the wavelength compares with the size of the gap.

What happens if the wavelength is much smaller than the width of the gap?

In this case, only the edges of the wave front are diffracted.

The amount of diffraction depends on wavelength.

The longer the wavelength, the greater the diffraction.

Why do some radio waves travel far?

Why can I receive radio but not TV?

Diffraction of water waves is observed in a harbor as waves bend around small boats and are found to disturb the water behind them. The same waves however are unable to diffract around larger boats since their wavelength is smaller than the boat. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners, allowing us to hear others who are speaking to us from adjacent rooms. Many forest-dwelling birds take advantage of the diffractive ability of long-wavelength sound waves. Owls for instance are able to communicate across long distances due to the fact that their long-wavelength hoots are able to diffract around forest trees and carry farther than the short-wavelength tweets of song birds.

When have I experienced diffraction?

Sound waves have a wavelength similar in size to the width of the doorway and so are diffracted as they pass through.

How can the teacher hear the band but not see them?

Light waves have a much shorter wavelength than sound waves and so are not diffracted by the doorway.

This means the band can be heard but not seen by the teacher in the corridor.

Summary of Diffraction

• The smaller the wavelength, the less the diffraction (angle) of waves when they pass around a barrier

• Sound waves bend around barriers, unlike light waves. Can you explain why?

Refraction: Waves in Shallow Water

• Waves travel slower in shallow water than in deeper water. This reduces the wavelength. Why?

• Shallow water behaves like an optically denser material, so the velocity of the waves decreases. If the waves are at an angle to the boundary, they will be refracted towards the normal.

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Snell’s Law &Refraction of Water

Waves

• Where:n= refractive indexangle of incidence/refraction

(remember to the normal)v= wave speedwavelengthMedium 1 is the one the wave is leaving.Medium 2 is the one it is entering.

2

1

2

1

1

2

sin

sin

v

v

n

n

r

i

DeepShallow

Deep to shallow waves bend towards the

normal

Still Applies:Remember the soldiers!!

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Snell’s Law – depth of water

• Where:– n= refractive index– = angle of incidence/refraction– v= wave speed– = wavelength– Medium 1 is the one the light is leaving.– Medium 2 is the one it is entering.

2

1

2

1

1

2

sin

sin

v

v

n

n

r

i

Answer this problem.Waves in the deep region of a ripple tank

have a velocity of 20mms-1 and a wavelengthof 10mm. They pass into the shallow regionwhere their speed reduces to 15mms-1

Find:

i) The frequency of the waves in the deep and shallow regions.

ii) The wavelength of the wave in the shallow

iii) If the angle of incidence from the deep is 350 what is the angle of refraction?

2Hz

7.5mm

250

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Wave diagrams1) Reflection

4) Diffraction3) Refraction

2) Refraction