BS Physics (Foundation) Series

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Module # 40
Camera, Eye & Spectroscopy
Camera
It is an optical instrument by which the permanent image of an
object or scenery is obtained on a sensitive screen. This image is
called photograph or picture.
Construction
A camera is basically a light proof container which consists of a
closed box blackened from inside to absorb stray light. On one
side of this box, there is a convex lens or (in costly cameras) a
system of lenses which acts as a single convex lens. On the other
side of this box, there is a sensitive film. A clear and well focussed
image of an object is formed on the film. This image is real,
inverted and diminished. There is a shutter between the film and
the lens. For sharp image of the object to be photographed, the
camera is focussed by moving its lens in and out with the help of
mechanical mount carrying the lens.
Working
A shutter of variable speed and a diaphragm with a variable
aperture controls the duration of time and quantity of light entering
the lens. Normally, the shutter is closed. It opens only for a

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fraction of a second when the button is pressed to take the
photograph. In this way, light falls on the film for a very short
interval of time after passing through the lens. The amount of light
passing through the lens should be so adjusted that it may have
proper impact on the film during which the shutter remains open.
Fig: Internal structure of Camera
Factors affecting the Image
(1) Diameter d of the aperture of the lens. It adjusts the suitable
amount of light entering the camera.
(2) Focal length f of the lens of the camera. It adjusts the area
and size of the image formed on the film.
(3) The time for the opening of shutter. It is so adjusted that the
film is exposed to the light properly.

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PIN Hole Camera
A pin-hole camera is one of the applications of rectilinear
propagation of light. It was invented to observe solar eclipses
without damaging the eye sight. It consists of a rectangular box
having a very small hole on one side and a frosted glass plate,
tracing paper or a photographic plate on the opposite side.
When a narrow pencil of rays of light coming from an object
passes through the pinhole, an inverted and real image of the
object is formed on the back of the camera. For observing a clear
image, external light is blocked by covering the box with a dark
cloth.
Human Eye
Fig: Internal Structure of Eye

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Optically speaking, the human eye operates in the same manner
as camera. The eye ball is nearly spherical in structure with a
diameter of about 2.5 cm. The eye ball consists of three layers
namely, (1) Sclera (2) Choroid (3) Retina, enclosed in a cavity
filled with fluid called the vitreous humor.
The whole structure is pulled into various positions by muscles
attached around the eyeball. The outer layer of the eyeball, the
sclera, is thick and impermeable to light rays as in the case of a
camera. But, the front part of it called the cornea is transparent.
The cornea allows the light to enter and the aperture of this
camera is determined by the pigmented middle layer of the eye
called the choroid which forms a colored curtain called the iris
which is just in front of the lens. This curtain is muscular and
when it contracts, the pupil which is an opening in the centre of
the iris enlarges and more light can enter. It contracts in bright
light and dilates in dim light. Its diameter varies from about 2 mm
in daylight to about 6 mm in darkness. Its diameter is thus
automatically adjusted to control the amount of light entering the
lens of the eye. A transparent and flexible convex lens is held just
behind the iris and its focal length can be altered by the pressure
of the ring shaped ciliary muscles surrounding it. The process of
changing the shape of the lens to see nearby or far away objects
clearly is called accommodation.

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The rays of light from an object are focussed and converged by
the lens to the sensitive internal layer of the eyeball called the
retina which is situated at the back of the eye chamber. The retina
is a nerve tissue sensitive to light. It contains over a million nerve
fibers. These fibers transmit electric signals along the optic nerves
to the brain. Although the image formed on the retina is inverted
and reversed, the brain interprets this and the images are seen
the right way up.
The shape of the eye is maintained by the pressure of colorless
transparent fluid in the eye. The fluid between the cornea and the
lens is a water-like liquid called the aqueous humor. The rest of
the eye is filled with a clear jelly-like substance known as vitreous
humor.
The farthest distance at which an eye can see objects clearly is
called the far point. The point closest to the eye at which objects
can be seen clearly is called the near point. For a normal eye, the
near point is at 25 cm. This distance is called least distance of
distinct vision.
Defects of Vision
The following are the defects of eye:
(1) Short Sightedness or Near Sightedness (Myopia).

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(2) Long Sightedness or Far Sightedness (Hypermetropia)
(3) Astigmatism
(4) Lack of Accommodation (Presbyopia)
Least Distance of Distinct Vision
The minimum distance at which an eye can see an object clearly
and distinctly is known as least distance of distinct vision. For
normal and healthy eye, this distance is about 25 cm. For children
this distance is small and it increases to 25 cm with the growing
age.
Persistence of Vision
The lingering effect of the image of an object, after it is removed,
is called persistence of vision. If an object is removed from the
seen of eye, the impression of the image persists for about I/10th
of a second in our eye. The cinema movies are made on this
principle.
Long Sightedness or Far Sightedness or Hypermetropia
If an eye is unable to see near objects but is able to see distant
objects, it is suffering from a disease known as long sightedness
or far sightedness or Hypermetropia.

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A long sighted person can see distant objects clearly but cannot
see near objects distinctly. The defect is either due to the focal
length of the eye lens being too long or the eye ball being too
short. The eye lens of such an eye is less converging. The light
rays from near objects are, therefore, focussed behind the retina
and the object is not seen clearly.
Removal of Defect
This defect of an eye can be corrected by using a convex lens of
suitable focal length in front of the eye.
In other words, the remedy or treatment for this defect is to wear
spectacles or contact lenses with convex lenses of suitable focal
lengths. These converge the light rays so that the eye lens can
focus the image clearly on the retina.
Astigmatism
If the cornea or the surface of the eye is not perfectly spherical,
then, the eye has different focal points in different planes and the
image of an object is not focussed clearly on the retina. To correct
this defect, asymmetrical lenses which have different radii of
curvature in different planes are used.

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Comparison of a Camera and an Eye
(1) The lens of a camera forms an image of the object on a
sensitive plate whereas the eye lens forms an image on the retina
of the eye.
(2) The iris and pupil of eye act similar to the shutter of a
camera and control the amount of light entering the eye.
Short Sightedness or Near Sightedness or Myopia
If an eye is unable to see distant objects but is able to see near
objects, it is suffering from a disease known as short sightedness
or near sightedness or Myopia. A short sighted person can see
near objects clearly but distant objects cannot be seen by him
clearly. This defect is either due to the focal length of the eye lens
being too short or the eye ball being too elongated. The eye lens
of such an eye is too converging. The light rays from a distant
object are, therefore, focused in front of the retina.
Removal of Defect
This defect of the eye can be corrected by using a concave lens
of suitable focal length in front of the eye.
In other words, the remedy or treatment for this defect is to wear
spectacles or contact lenses with concave lenses of suitable focal
lengths. These diverge the light rays so that the eye lens can

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focus the image clearly on the retina.
Lack of Accommodation (or Presbyopia)
At old age, the eye lens losses its elasticity and ability to change
its shape and the ciliary muscles become weak. These factors
result in a lack of accommodation. The defect called presbyopia
can be corrected by using convex leases. But, for looking at
distant objects, one will have to use concave lenses. This is the
reason why old people use spectacles having bifocal lenses, i.e.,
convex part in the lower side to see near objects and concave
part in the upper side to see the distant objects.
Fig: Bifocal Spectacles
Spectrometer
It is an optical instrument which is used to study light from
different sources or for the minute study of spectra.
Construction
It essentially consists of three parts:
(1) a collimator
(2) a telescope, and

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(3) a turn-table, as shown in the figure below:
Turntable
Levelling Screws
Fig: The Spectrometer.
The collimator C consists of a slit, the width of which can be
adjusted. It is placed at the focal plane (focus) of a convex lens L1
(as shown in figure above), so that light from the slit may become
parallel after passing through the lens. The lens and the slit are
placed at the ends of a metallic tube and the distance between
them can be altered (changed or adjusted).
The telescope is an ordinary refracting telescope fitted with cross-
wires at the focal plane of the objective lens L2. The telescope
can be rotated about the central axis of the spectrometer. The
rotation of the telescope can be read on a circular scale
graduated in degrees with the help of a vernier arrangement
Telescope
Collimator
Slit

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The turn table supporting the prism can be raised or lowered. It
can also be rotated about a vertical axis through any desired
angle. The turn table is provided with three leveling screws S1, S2
and S3.
Working
(1) The telescope is pointed towards a white wall and the eye-
piece is slowly drawn out until the cross-wires are distinctly
visible. That is, the eye-piece is focussed on the cross-wires so
that they are clearly visible.
(2) The telescope is adjusted for parallel rays so that a parallel
beam falling on the objective of the telescope produces an image
at the cross-wires.
To achieve this, parallax is removed between the image of a far
off object and the cross-wires.
(3) The telescope is turned and brought in front of the collimator.
The collimator is also adjusted to give a parallel beam of light.
This is done by illuminating the slit with mono-chromatic light
(sodium light) and viewing image of the slit through the telescope.
The distance of the slit from the collimator is adjusted till the slit is
in focus of the telescope.
The parallax is removed between the image of the slit and the

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cross-wires. When this is so, the collimator gives a parallel beam
of light.
Uses
The spectrometer is used for the determination of the refractive
index of transparent materials and also for the determination of
wavelengths of different light from different sources.
Spectroscopy
It is that branch of physics which is concerned with the
production, measurement and interpolation of electromagnetic
spectra produced by either emission or absorption of radiant
energy by various substances.
In atomic spectroscopy, we study (i) photon emission and photo
absorption spectra of atoms, and (ii) ejected-electron spectra of
atoms following electron, ion and photon impact excitation /
ionization.
Spectrum
We know that dispersion of sunlight or white light is produced with
the help of a prism and a band of red, orange, yellow, green, blue,
indigo and violet colors is observed. A band of colors formed in
this way is called a spectrum. Each color in this spectrum
represents a certain range of frequency and wave length.