Lenses are one of the most important
devices in optics. Lenses are used is many applications requiring
the manipulation of light waves.
Fig 2.2. shows the six primary
types of lens used in optics. the lenses are further sorted
into NEGATIVE lenses and POSITIVE lenses.
Positive lenses are lenses which
are thicker at the centre than at the edge. The combined effect
of refraction at the front and back surfaces of the lens
will bend light to a point or Focus - The light from this type
of lens is focused to a point IN FRONT of the lens.
Positive lenses produce a
REAL IMAGE, - the image produced by this type of lens
CAN be projected onto a screen.
The three basic positive lenses
|Double Convex lens
|Plano - Convex lens
|Positive Meniscus lens
Fig 2.3 shows some important
aspect of a positive lens, and its behaviour on light.
Negative lenses are lenses
which are thinner at the centre than the edge. The same optic
laws apply to negative lenses as they do with positive lenses,
but a negative lens works in a somewhat different way than a
The focal lengths of this type of lens are negative, this can
be worked out mathematically by assigning a negative radii of
curvature to a concave lens surface. If you pass parallel
light rays through a negative lens, they seem to spread out
from a point behind the lens, the distance from this point to
the centre line of the lens is the focal length; this is given
a negative value, as it is on the opposite side of the lens
from the focal point of a positive lens.
Because negative lenses do not
bring parallel light rays together, they DO NOT produce a real
image, but they do produce a VIRTUAL IMAGE.
The three basic negative lenses
|Double Concave lens
|Plano - Concave lens
|Negative Meniscus lens
Fig 2.4 shows some of the important
aspects of a negative lens, as you can see, a negative lens
produces a virtual image, and the focal point is behind the
Other Lenses And Aberration
There are many variations on
lenses and design. An important factor with lenses that can
cause problems is Lens aberration. When dealing with the conceptual
scientific principles, discussions are based around 'Ideal'
'perfect' model scenarios. In the real world, the 'ideal' models
are non-existent due to physical properties and variables which
play a part.
When dealing with lenses, the
basic materials from which they are made have variations which
have an effect on the way light behaves, these mainly related
to refractive index and dispersion of the glass. Dispersion
is the change of index of refraction with wavelength.
As we studied previously, white
light is made up of many colours or wavelengths. Red light (long
wavelength) has less energy than Blue light (short wavelength),
and therefore when passing through a medium, the refractive
index will cause the light waves to slow down, Red is bent or
refracted the least, and blue is refracted the most.
When light passes through a lens,
refraction of light will occur. When dealing with the Ideal
single convex lens model, we know that light rays are focused
to a finite point. In reality due to refraction and dispersion,
the lens will not be able to focus the different wavelengths
at the exact same point as the focal point is dependant on refraction
and the index of refraction of the lens.
This type of effect is known
as 'Chromatic Lens Aberration' The result of chromatic aberration
is the image produced by the lens will have a coloured boundary,
and will appear blurry
Another type of lens aberration
is known as spherical aberration. As lenses are made with spherical
surfaces light rays that are parallel but at different distances
from the optical axis, will fail to converge at the same point.
Fig 2.5 shows the details of
chromatic and spherical aberration. There are other forms of
aberration such as astigmatism and distortion. All forms of
aberrations will cause a decrease in quality of the resolved
The effects of aberration can
be corrected by utilising multiple lens systems known as compound
lenses. Compound lens systems are widely used to yield long
and short focal lengths and or high magnification.
Another type of lens is the Graded
Index (GRIN) lens and the spherical lens. These lenses are made
using materials with varied refractive indexes. Such lenses
are used to couple light sources into very small apertures such
a optical fibres.