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Visible light 4. Comparing reflecting, transparent, translucent, opaque surfaces

Reflected light: 4. Other points about reflection including scattering surfaces and comparing transparent, translucent and opaque materials

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INDEX of physics notes: Light reflection and the uses of plane and curved mirrors

4. Other points about reflection including scattering surfaces

 and a short note on comparing transparent, translucent and opaque materials

Apart from luminous objects (which give out their own light), we see objects by reflected light.

Light is reflected by different boundaries in different ways.

If the light is reflected from a very smooth 'shiny' surface we see a clear and coherent mirror image e.g. a silvered glass mirror, aluminium foil, even a shop window (though a much small % is reflected, most is transmitted through the glass) etc.

The top left of diagram above shows what happens when parallel incident waves hit a smooth surface to give a clear reflection of parallel reflected rays.

This is called regular reflection or specular reflection - to give a perfect 'mirror' image.

All the 'normals' are parallel at 90o to the mirror surface.

Note that whatever the wavelength e.g. the colours of visible white light, all the rays of colours bounce off the mirror with the same angle of reflection.

You do NOT get any splitting of the light into colours which happens when light enters or leaves a prism.

If the surface is uneven (e.g. rough or matt, top right of diagram), the light is scattered in all directions eg you don't see a clear mirror image looking tissue paper or a frosted glass surface.

You cannot get a clear reflection from a rough surface.

The right diagram above shows what happens when parallel light waves are reflected by an uneven surface.

This is called diffuse reflection or scattered reflection.

You get this with all non-smooth surfaces e.g. carpet, soil, paper etc.

At any point on the surface the 'normal' may be at any 'random' angle from 0o to 90o with respect to the line of the surface of the material.

So, although the incident rays come in parallel, they are reflected at lots of different angles of reflection, but for every ray the angle of incidence = the angle of reflection.

Note on some technical terms

Opaque materials do not allow the light through - not transparent at all, but light may reflect off them.

Transparent materials allow light through giving a clear image e.g. looking through a glass window pane, though the material may absorb certain colours from white light.

Translucent materials allows some light through but the rays broken up and are scattered on exiting e.g. holding up a sheet of paper to a light source. You cannot see a clear image of what is on the other side of the paper.

INDEX notes: Light reflection and uses of plane & curved mirrors

Keywords, phrases and learning objectives for the reflection of visible light rays

Be able to describe and explain aspects of the reflection, reduced reflection, absorption or scattering of light rays depending on the nature of the material surface,

Be able to compare the effect of light rays on reflecting, transparent, translucent or opaque surfaces using diagrams.


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INDEX of physics notes on light reflection and mirrors

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INDEX notes: Light reflection and uses of plane & curved mirrors