Light
Lens: A lens is a transparent medium bounded by two spherical surfaces.
Centre of curvature:
The centre of the curved surface of a lens is
called centre of curvature. A lens has two
centers of curvatures, which are denoted by C₁
and C₂ as shown in figure (i).
Principles axis:
The line passing through the two centers of
curvature of a lens is known as Principle axis as
shown in figure (i).
Optical centre:
The geometrical centre of a lens is called its
optical centre. A ray of light passing through it
does not bend. It is denoted by ‘O’.
Principle Focus:
When narrow parallel rays of light converge to a
point or appear to come from a point after
refraction. The point is known as Principle focus
or focus. It is denoted by F.
Focal length:
The distance between principle focus (F) and
optical centre (O) is known as focal length. It is
denoted by f . It is taken as +ve for convex lens
and -ve for concave lens.
In concave lens the image formed is
virtual, erect ad diminished. When the object is
at infinity the image will be formed at focus.
When the object is in between optical centre and
infinity, the image will be formed in between
focus and optical centre on the same side of the
lens as shown in the figure.
Magnification:
The process of variation of the size of image
produced by a lens, of position of object is called
magnification. Mathematically, it is equal to the
ratio of height of the image to the height of the
object. It is denoted by ‘m’
The power of lens:
The power of lens is the ability to converge or to
diverge a beam of light. Mathematically, it is
equal to the reciprocal of focal length (in meter)
of the lens. It is denoted by P. It is unit is Dioptre.
Optical Instrument:
Optical instrument is that instrument in which
mirrors, prisms, lens etc. are used. It is of two
types.
i. Real image forming: Camera,
film projector, slide projector, eye, etc.
ii. Virtual image forming: Binoculars,
microscope, telescope, etc.
Camera:
It is an optical instrument used for taking
photographs.
The inner surface of the camera is made black in
order to absorb maximum light which passes
through the lens.
Eye:
The main parts of the human eyes are
(i)cornea
(ii) iris
(iii) lens
(iv) ciliary muscle
(v)optic nerve
(vi) retina, etc.
Light coming from an object enters in the lens
through the cornea. After refraction through the
lens, it is focused on the retina, where inverted
image of the object is formed. The optic nerves
transmit the image impulse (making it erect) to
the brain.
The nearest point
The nearest point, up to which an object can be
seen clearly, by an eye, is known as near point.
(25cm) of that eye.
The farthest point
The farthest point, up to which objects can be
seen clearly, by an eye, is known as far point.
(Infinity) of that eye.
Accommodation:
The ability of the eye lens to focus objects at
various positions on the retina is known as
accommodation of the eye. Ciliary muscles
conduct it by its contraction and relaxation.
The size of the pupil becomes small in bright
light.
The size of the pupil becomes big in dim light.
The ordinary sun glass should not be used for a long time even in sunny days. The sunglass makes shade to the eyes due to which the size of the pupil increases even in bright light. The ultra- violet rays passing through the glass falls on the retina. It may damage the retina.
Defect of vision:
It is a defect due to which the image of distance
or nearer object is not formed at retina. Hence
the object appears unclear.
Short sightedness (myopia):
The defect of vision, in which a person cannot
see the distant objects, clearly, is known as short
sightedness. The image of distant objects is
formed in front of retina as shown in figure(i).
It is caused by elongation of the eye ball or
shortening of focal length of eye lens. The removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper at proper distance from his/her eyes, while reading without using lens.
Long Sightedness (hypermetropia):
The defect of vision which a person cannot see the near object clearly, is known as long sightedness. The image of near objects is formed behind the retina as shown in figure (i). It is caused by shortening of the eye ball or increasing focal length of the eye lens. The
removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper a little bit away from his/ her eyes, while reading without using lens.
Differences between a compound microscope and a
telescope:
Compound Microscope
It is used for viewing microscopic objects
clearly.
It is used by pathologists or scientists.
The focal length of objective lens is
shorter than that of eye lens.
The image formed by the objective lens
lies within the focus of the eyepiece lens.
The final image is formed beyond the
objective lens.
Telescope
It is used for viewing heavenly bodies
clearly.
It is used by astronomers.
The focal length of objective lens is longer
than that of eye lens.
The image formed by the objective lens lies
exactly at focus of the eyepiece lens.
The final image is formed at infinity.