# Comparing Electromagnetic Waves To Water Waves

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Comparing Electromagnetic Waves To Water Waves

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?
Boardworks GCSE Science: Physics Electromagnetic Waves Remember the electromagnetic spectrum? The electromagnetic waves are grouped into types that have similar wavelengths and so have similar properties. radio waves visible light gamma rays microwaves infrared X-rays ultraviolet 0.01 nm 100 nm 1 nm 0.01 mm 1 mm 10 cm 100 m smaller wavelength higher frequency and energy Electromagnetic waves form a continuous series in order of changing wavelength, frequency and energy. This series is called the electromagnetic spectrum.

Water Waves Experiment with the wave tanks
Record what the waves do when they hit a convex barrier, a concave barrier, and a straight barrier. Record what the waves do when they hit a gap. Change the gap width and record your observations. Place the glass piece on the bottom to simulate shallow water. Record your observations.

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

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

Diffraction Diffraction is when waves spread out from the edge of a gap. More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased)

Boardworks GCSE Science: Physics Radio Waves and Microwaves
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?
Boardworks GCSE Science: Physics Radio Waves and Microwaves 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?
Boardworks GCSE Science: Physics Radio Waves and Microwaves Why do some radio waves travel far? Teacher notes This true-or-false activity could be used as a plenary or revision exercise on radio waves, or at the start of the lesson to gauge students’ understanding of the subject matter. Coloured traffic light cards (red = false, yellow = don’t know, green = true) could be used to make this a whole class exercise.

Boardworks GCSE Science: Physics Radio Waves and Microwaves Why can I receive radio but not TV? Teacher notes This two-stage animation illustrates how wavelength effects diffraction. While viewing the animation, it should be stressed the longer wavelengths of radio waves allows diffraction to occur.

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?
Boardworks GCSE Science: Physics Radio Waves and Microwaves When have I experienced diffraction? How can the teacher hear the band but not see them? Sound waves have a wavelength similar in size to the width of the doorway and so are diffracted as they pass through. Teacher notes This illustration shows a school band (called “INTER-FEAR-ANCE”) carrying out an after-school practice in the main hall. To illustrate that sound waves can travel around corners, the teacher in the corridor (on the far right of the scene) has his hands over his ears as he can hear the band before even reaching the doorway, but he cannot 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.

Snell’s Law & Refraction of Water Waves
Still Applies: Remember the soldiers!! Shallow Deep Where: n= refractive index q= angle of incidence/refraction (remember to the normal) v= wave speed l= wavelength Medium 1 is the one the wave is leaving. Medium 2 is the one it is entering. Deep to shallow waves bend towards the normal

Snell’s Law – depth of water
Where: n= refractive index q= angle of incidence/refraction v= wave speed l= wavelength Medium 1 is the one the light is leaving. Medium 2 is the one it is entering.

Answer this problem. 2Hz 7.5mm 250
Waves in the deep region of a ripple tank have a velocity of 20mms-1 and a wavelength of 10mm. They pass into the shallow region where their speed reduces to 15mms-1 Find: 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

Wave diagrams 1) Reflection 2) Refraction 3) Refraction 4) Diffraction