VISIBLE LIGHT - REFLECTION and MIRRORS
Illuminated and self-luminous objects, reflection of light, experiments investigating mirror, how to do ray diagrams for mirrors, uses of mirrors including plane (flat) and curved mirrors
See also REFRACTION experiments
Doc Brown's Physics Revision Notes
Suitable for GCSE/IGCSE Physics/Science courses or their equivalent
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?
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 the 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, it 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.
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.
Light waves are readily reflected off smooth flat surfaces e.g. light reflected off a smooth surface like a mirror.
Set out a white sheet of paper with a line marked on it, as shown in the above diagram. Draw a 'normal' at 90o to this line. Place the mirror adjacent to this line at 90o 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 mirror surface).
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.
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.
You should also appreciate that the reflection rule (angle i = angle r) applies whatever the shape of the mirror!
The scientific wave model of reflection of light rays
Visible light is a transverse wave (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.
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 45o to their direction and the way direction is changed by 90o. In this case the crests of the waves correspond to the wavefronts.
Other points about reflection.
Apart from luminous objects (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 etc.
If the surface is uneven (e.g. rough or matt), the light is scattered in all directions eg you don't see a mirror image looking at tissue paper or a sliced section of apple. You cannot get a clear reflection from a rough surface.
The Uses of Mirrors (remember angle of reflection i always equals the angle of reflection r)
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!
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 45o 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'.
You can use these 45o 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 from a wide angle. Convex mirrors are used by the driver on a bus, shop security, side-mirrors on cars
Use of a concave parabolic mirror
A parabolic concave mirror doesn't collect light, but quite the opposite! 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. 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 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 collect light is reflected by a small plane mirror at ~45o into an eyepiece.
By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star.
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.
The characteristic properties of the image in a plane mirror (not needed for GCSE 9-1 physics?)
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 ...
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'.
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