2. 2Everything Science
www.everythingscience.co.zaIntroductionGeometrical optics is the
study of how light interacts with materials and in particular with
theshapes of the materials and the angles at which light rays hit
it.Light rays are lines which are perpendicular to the lights
wavefronts. In geometrical opticswe represent light rays with
arrows with straight lines.Light rays reflect off surfaces.
Theincident ray shines in on the surface andthe reflected ray is
the one that bouncesoff the surface. The normal is the
lineperpendicular to the surface where thelight strikes the
surface.The angle of incidence is the anglebetween the incident ray
and the surface,and the incident ray, reflected ray, andthe normal,
all lie in the same plane. 3. 3Everything Science
www.everythingscience.co.zaLaw of reflectionThe Law of reflection
states the angle of incidence ( ) is equal to the angle
ofreflection ( ) and that the reflected ray lies in the plane of
incidence.ir 4. 4Everything Science
www.everythingscience.co.zaProperties of lightLight can be
reflected, refracted, absorbed or transmitted. Reflection and
refraction arecovered in more detail elsewhere in this
chapter.Absorption of light is closely linked to reflection. Some
wavelengths of light are reflected offobjects while others are
absorbed by the object. This is what gives objects their
colour.Light can be transmitted through objects. Objects that allow
the transmission of light arecalled transparent and objects that do
not are called opaque.The last property of light that we will look
at is the speed of light. Light also has a maximumspeed at which it
can propagate, and nothing can move faster than the speed of
light.The speed of light, c, is constant in a given medium and has
a maximum speed in vacuumis3108ms1 5. 5Everything Science
www.everythingscience.co.zaRefractionRefraction occurs at the
boundary of twomedia when light travels from one mediuminto the
other and its speed changes but itsfrequency remains the same. If
the light rayhits the boundary at an angle which is
notperpendicular to or parallel to the surface,then it will change
direction and appear tobend. 6. 6Everything Science
www.everythingscience.co.zaRefractive index and optical densityThe
refractive index (symbol n) of a material is the ratio of the speed
of light in a vacuum toits speed in the material and gives an
indication of how difficult it is for light to get throughthe
material.If the refractive index, n, increases, the speed of light
in the material, v, must decrease.Therefore light travels slower
through materials of high refractive index, n.Optical density is a
measure of the refracting power of a medium.n=cv 7. 7Everything
Science www.everythingscience.co.zaTerminology The normal to a
surface is the line which is perpendicular to the plane of the
surface. The angle of incidence is the angle defined between the
normal to a surface and theincoming (incident) light ray. The angle
of refraction is the angle defined between the normal to a surface
and therefracted light ray. 8. 8Everything Science
www.everythingscience.co.zaSnells lawSnells Law gives the
relationship between the refractive indices, angles of incidence
andreflection of two media. Light travelling from one medium to
another of lighter optical density will be refracted towardsthe
normal. Light travelling from one medium to another of lower
optical density will be refracted awayfrom the normal.n1 sin 1=n2
sin2 9. 9Everything Science www.everythingscience.co.zaCritical
angleThe critical angle is the angle of incidence where the angle
of refraction is 90. The light musttravel from an optically more
dense medium to an optically less dense medium.c=sin1(n1n2) 10.
10Everything Science www.everythingscience.co.zaTotal internal
reflectionTotal internal reflection takes place when light travels
from one medium to another of loweroptical density. If the angle of
incidence is greater than the critical angle for the medium,the
light will be reflected back into the medium. No refraction takes
place.Total internal reflection is used in optical fibres in
telecommunication and in medicine inendoscopes. Optical fibres
transmit information much more quickly and accurately
thantraditional methods. 11. 11Everything Science
www.everythingscience.co.zaFor more practice
see:www.everythingscience.co.zaShortcode: ESBNC
