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Waves 6. Comparing reflection, refraction, diffraction, transmission, absorption

Introduction to waves: 6. More on the general properties of any waves - their reflection, refraction, diffraction

Doc Brown's Physics exam study revision notes

The need for a scientific model of waves and their behaviour.

6. More on the general properties of any waves - reflection, refraction, diffraction

(all dealt with in more details for specific examples like light or sound)

All waves undergo reflection, refraction and diffraction and a general description of them is explained below.

Appreciate and understand that all three wave effects can be successfully modelled in the way described in the manner described below and predictions can be made on the basis of these models.

What can happens when waves meet a boundary between two media?

When waves meet an interface between two materials there are three possible outcomes.

The outcome depends on the properties of the wave and the nature of the two materials involved.

(i) The wave, its energy, is absorbed by the second material, so the energy store of the 2nd material is increased - this usually equates to increasing the thermal energy store of the 2nd material.

Soft materials easily absorb sound wave energy.

Rough matt black surfaces absorb most of visible light that's why they look black!

Infrared radiation from the Sun heats up surfaces.

(ii) The wave is reflected back off the second material without losing any significant energy. In this case there is little wave energy absorbed or transmitted.

If the surface is particularly smooth-shiny and flat, very little wave energy is absorbed and you can observe a high quality reflected image e.g. looking at yourself in a silver surfaced mirror!

In the case of sound you get echoes.

If the surface is rough, much of the wave energy is absorbed or scattered in all directions.

(iii) The wave is transmitted through the second material without being absorbed and the waves may change direction (refraction).

This can happen if the material is transparent and the waves can continue passing through the 2nd material

You see this by the 'bent' image at the 'wrong' angle when observed putting a stick in water.

Refraction effects are used in the lenses of optical equipment like reading glasses and cameras.

(iv) There are situations where waves partially meet a barrier and bend round corners or pass through a gap in a barrier and then spread out - radiating from the gap.

These effects are called diffraction and does not involve passing from one medium to another.

You can hear sound round a corner - that's due to a combination of reflection and diffraction effect.

(v) You should be aware that in some situations (some already mentioned) you may get a combination of effects.

e.g. when a light beam in air then passes through a glass block, some rays refract at the interface and others are reflected.

What actually happens at an interface depends on the wavelength/frequency of the wave and the properties of the materials the waves strike.

The five points (i) to (v) are discussed in detail using the scientific models of waves in further sections.

Keywords, phrases and learning objectives for waves

Know that when waves meet a surface or boundary they may be reflected, refracted, diffracted, absorbed or transmitted, and sometimes several things go on at the same tome!

Appreciate the need for a scientific model to explain the behaviour of waves.

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