VISIBLE LIGHT - REFLECTION and MIRRORS

See also REFRACTION experiments

 This page will help answer many questions e.g. 

How do you draw ray diagrams for reflection?  How do explain why waves reflect?   What do we use mirrors for?

Sub-index for this page

(a) Illuminated, self-luminous objects and transmission of light

(b) Investigating REFLECTION

(c) The scientific wave model of reflection of light rays

(d) Other points about reflection including scattering surfaces

(e) The Uses of curved and plane mirrors 

(f) The characteristic properties of the image in a plane mirror

(g) Uses of total internal reflection

(h) How a reflecting telescope works

(a) Illuminated, self-luminous objects and transmission of light

Self-luminous objects are those that create and emit their own light e.g. torch bulb, the Sun, candle flame etc.

However, we see most objects by the illumination of them from a source of light and the subsequent reflection of that light from the surfaces of the objects we are viewing.

The reflection of visible light from objects allows us see them from evolution's development of that wonderful light sensitive organ we call the eye.

Very few surfaces do not reflect light. Even the blackest of surfaces do reflect a tiny amount of light.

White light is a mixture of all possible colours (see prisms - refraction - visible spectrum page).

Visible light is part of the electromagnetic spectrum and therefore usually travels in straight lines at 3 x 10⁸ m/s.

It will take a slightly curved path if passing through a medium of varying density - that's how you get mirages in the desert.

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.

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(b) Investigating REFLECTION

When light waves meet the boundary between two materials, some of the wave energy might be reflected, absorbed or transmitted (and then maybe refracted).

Light waves are readily reflected off smooth flat surfaces e.g. light reflected off a mirror.

Some of the light might also be absorbed by the surface.

Set out a white sheet of paper with a line marked on it, as shown in the above diagram.

Draw a 'normal' at 90^o (perpendicular) to this line.

The normal is an imaginary line where the light ray hits the 'surface' and helps construct ray diagrams

Place the mirror adjacent to this line at 90^o to the normal.

You need a light box with a slit to give a narrow beam of light.

Place the light box on the sheet of white paper so the beam of light shines onto a mirror at the point on the mirror where the previously marked normal line is.

Mark out a series dots on the white paper coincident with the thin rays of light for the incident ray and reflected ray.

You can then join the dots up and measure the angle of the incident ray and reflected ray with respect to the normal with a protractor (NOT with respect to the plane mirror surface).

The experiment is best done in a darkened room and make sure the light beam skims over the surface of the paper.

You repeat the experiment and changing the angle of incidence (i on the diagrams), so you then also change the angle of reflection (r on the diagrams).

For a fair test use the same mirror and ray box beam to keep any variables constant.

Hopefully you see the reflected ray as thin and bright as the incidence ray - a quality plane mirror should give a clear reflection with little if any of the light absorbed.

Typical results are described, analysed and explained below.

Reflection ray diagram

Reminder - the vertical dotted line is called the 'normal', it isn't a ray, but helps in the construction and interpretation of ray diagrams.

All angles are measured with respect to the 'normal' which is at 90^o to the reflective surface.

A plane mirror means one with a perfectly flat surface.

Angle 2 = angle i is defined as the angle of incidence of incident ray.

Angle 3 = angle r is defined as the angle of reflection of the reflected ray.

You will always find hat Angle 2 = Angle 3, angle of incidence equals the angle of reflection for a plane mirror.

This is the law of reflection.

The law applies to all mirrors whatever the shape of the reflecting surface

The convex mirror disperses the rays and the concave mirror concentrates the rays at specific focus point (see reflecting telescope).

So, appreciate that the reflection rule (angle i = angle r) applies whatever the shape of the mirror!

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(c) The scientific wave model of reflection of light rays

Visible light is a transverse wave (diagram shown above).

The fine black lines (below) represent the wavefronts of light, so think of the wavefronts as the points of maximum amplitude of the light waves (the crests).

When the waves meet the flat smooth surface they are 'bounced' off symmetrically at the same angle with respect to the normal - see the first reflection diagram.

You can readily see this with ripple tank experiments - just put a barrier in the way of the waves at 45^o to their direction and the way direction is changed by 90^o. In this case the crests of the waves correspond to the wavefronts.

General introduction to the types and properties of waves including ripple tank experiments

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(d) 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 90^o 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 0^o to 90^o 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

A transparent material lets light through so you see a clear image e.g. regular glass window.

A translucent material lets light through but you can't see a clear image e.g. white tissue paper or frosted glass because of the randomness of the surface causing scattering effects.

An opaque material does not let light through - the light may be absorbed or reflected.

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(e) The uses of curved and plane mirrors and other reflecting surfaces

(remember angle of reflection i always equals the angle of reflection r with respect to the 90^o normal)

Reminder: The reflection rule (angle i = angle r) applies whatever the shape of the mirror.

However, the shape of the mirror surface is important for what you want to do with the mirror!

Mirrors can be all shapes and sizes depending on their uses, including distorting your shape at a fun-fair!

 

The most familiar use is a plane mirror in the home - you see a 'perfect' image of yourself, but it is laterally inverted - your left become right and right becomes left! Its called lateral inversion. However, 'top' and 'bottom' are still the same!

Note on the use of mirrors and safety equipment

Good reflectors used on bicycles and safety clothing because they are designed so that the light is directed back in one direction from where it came from e.g. the headlamps of a car.

The reflectors cover a wide range of angles so you cannot see a clear reflected image.

They are often made of red plastic and act as a colour filter, so you see the red warning colour.

A periscope is a simple method of observing something from a different height than that of your eye.

It is used to observe things when there is a barrier or other obstacle in the way.

You can buy a simple one using plane mirrors (left diagram above) for watching golf with spectators in front of you!

The periscopes of submarines require something a little more sophisticated.

The right-hand diagram shows how you use 45^o triangular prisms instead of mirrors.

Prisms have a higher optical quality and note that the inside surfaces of solid '3D' prisms can act as a mirror.

This phenomena is called 'total internal reflection'. (For more see total internal reflection)

You can use these 45^o prisms to reverse the direction of a light beam - can you figure out how and sketch the ray diagram?

Comparing concave and convex mirrors

A concave mirror can focus light rays to a common point F in front of the mirror (F is called the principal focus). The distance from F to the centre of the mirror is called the focal length.

You come across the same terms when you study lenses.

This type of lens is used in reflecting telescopes (example further down). 

A concave mirror is described as a converging mirror, for example it can converge the Sun's rays to a focus point to provide a workable solar heating system.

A shaving mirror is a concave mirror because it can produce an upright magnified image.

A convex mirror disperses the rays and the focal point F is behind the mirror.

Convex mirrors give you a wide field of view and collect light over a wide angle.

Convex mirrors are used by the driver on a bus, shop security and side-mirrors on cars to give a wide view of the road behind and to the side of a vehicle.

Use of a concave parabolic mirror in head lamps of floodlights

A parabolic concave mirror is used to produce a powerful beam of light.

In car headlamps the light from the bulb (filament or LED) is collected by the mirror and reflected to produce an approximately parallel beam of rays to illuminate a narrow field of view ahead of the vehicle - the mirror acts with a small angle of divergence.

So, in reality the diagram isn't quite correct because you want the rays to diverge a little to produce a wider beam to illuminate more of the road ahead.

You can make small changes to the parabolic shape to change the dispersion of the beam.

A parabolic array of mirrors can be used to make a solar furnace, reaching temperatures of over 2000^oC.

French scientists have been experimenting with solar furnaces since 1949.

Hot countries like Spain are doing increasing research, no doubted prompted lately by climate change, since solar energy is free and doesn't produce carbon dioxide!

See https://en.wikipedia.org/wiki/Solar_furnace

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(f) The characteristic properties of the image in a plane mirror

The image produced in a plane mirror is virtual, upright and laterally inverted (ray diagram below).

The ray diagram for the formation of a mirror image

The construction of the ray diagram to show the formation of a virtual image by a plane mirror and from which you can deduce the characteristics of the virtual image.

You are expected to be able to construct the diagram of the virtual image formed in a plane mirror.

The features of the virtual image formed by a plane mirror are ...

The image is the same size as the object.

The image is as far behind the mirror as the object is in front of the mirror.

The image is upright - the same way up as the object (if not it would be inverted, and you would look upside down!).

The image is virtual because the image appears to be from behind the mirror.

The image is laterally inverted, the 'left' of the object now appears on the 'right' side and the 'right' of the object appears to be the 'left' side of the image - this is called lateral inversion of the image

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(g) Uses of total internal reflection

The internal surface of optical fibres acts as a mirror - another case of total internal reflection, this time in fine glass strands which allow the transmission of visible light rays and information signals over long distances with minimum loss of intensity (amplitude).

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(h) How a reflecting telescope works

A reflecting telescope uses a concave mirror

A relatively large concave mirror collects as much light as possible from distant astronomical object e.g. a star.

The collected light is reflected by a small plane mirror at ~45^o into an eyepiece or camera.

By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star or any other distant object.

Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

experiments - investigation of reflection using a ray box

reflecting light off a plane mirror at different angles

IGCSE revision notes reflection ray diagrams uses of mirrors KS4 physics Science notes on reflection ray diagrams uses of mirrors GCSE physics guide notes on reflection ray diagrams uses of mirrors for schools colleges academies science course tutors images pictures diagrams for reflection ray diagrams uses of mirrors science revision notes on reflection ray diagrams uses of mirrors for revising physics modules physics topics notes to help on understanding of reflection ray diagrams uses of mirrors university courses in physics careers in science physics jobs in the engineering industry technical laboratory assistant apprenticeships engineer internships in physics USA US grade 8 grade 9 grade10 AQA GCSE 9-1 physics science revision notes on reflection ray diagrams uses of mirrors GCSE notes on reflection ray diagrams uses of mirrors Edexcel GCSE 9-1 physics science revision notes on reflection ray diagrams uses of mirrors for OCR GCSE 9-1 21st century physics science notes on reflection ray diagrams uses of mirrors OCR GCSE 9-1 Gateway  physics science revision notes on reflection ray diagrams uses of mirrors WJEC gcse science CCEA/CEA gcse science 

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