RAY OPTICS CRASH COURSE
OPTICAL INSTRUMENTS
Simple microscope : It is a convergent lens.
When the object is placed between the focus and the optical centre a virtual, magnified and erect image is
formed.
Compound Microscope
Compound microscope is used to get more magnified image as compared to a simple microscope. Object is
placed in front of the objective lens and the image is seen through the eye piece. The aperture of objective lens
is less as compared to eye piece because object is very near so collection of more light is not required. Generally
object is placed between F – 2F due to this a real, inverted and magnified image is formed between 2F – ¥. It
is known as intermediate image A'B'. The intermediate image acts as an object for the eye piece. Now the
distance between both the lens are adjusted in such a way that intermediate image falls between the optical
centre of eye piece and its focus. In this condition, the final image is virtual, inverted and magnified
Astronomical Telescope
A telescope is used to see distant objects. The objective forms the image A'B' at its focus. This image A'B' acts as
an object for eyepiece and it forms the final image A"B".
Cassegrain's telescope
This telescope consists of a paraboloidal mirror M as the
objective, and a convex elliptical mirror m called the
secondary mirror. F1 and F2 are the two conjugate foci of
the mirror m.
It is easy to see that the angular magnification of the
telescope
Galilean telescope
If in an astronomical telescope, the convergent eye–piece is L
replaced by a divergent lens which is placed in such a way
that rays from objective are directed towards its focus
(Figure), final image will be erect, enlarged and virtual. This
telescope is also used to see distant terrestrial objects and is
called Galilean telescope and for it
DEFECTS OF VISION
(i) Distant objects are not clearly visible, but nearby objects are clearly visible because image is formed before the retina.
(ii) To rectify the defect concave lens is used
* The maximum distance which a person can
see without the help of spectacles is known as
far point of distinct vision.
* If the reference of object is not given then it is taken as infinity.
* In this case image of the object is formed at the far point of the person
LONG–SIGHTEDNESS OR HYPERMETROPIA
(i) Nearby objects are not clearly visible.
(ii) The image of nearby objects is formed behind the retina.
(iii) To remove this defect convex lens is used.
SOME NATURAL PHENOMENON DUE TO SUNLIGHT
* Rainbow
After a light drizzle, an observer with the sun facing his back, sees a number of concentric coloured arcs looming
in the sky, with the common centre of these arcs lying on the line joining the sun and the observer. These
arcs constitute the primary rainbow.
The inner edge of the primary rainbow is violet and the outer edge is red. Besides the primary rainbow, a
bigger but a fainter rainbow is also seen. This is called the secondary rainbow. The sequence of colours in
the secondary rainbow is the reverse of that in the primary rainbow, i.e., the inner edge is red and the outer
edge is violet.
Both these rainbows are formed by :
(i) Dispersion and
(ii) Internal reflection of the Sun's rays in the rain drops suspended in the atmosphere
Scattering of light
The deflection of light energy by fine particles of solid, liquid or gaseous matter from the main direction of
the beam is called the scattering of light.
The basic process involved in scattering is the absorption of light by the molecules followed by its re-radiation
in different directions. The intensity of the scattered light depends on :
(i) the wavelength (l) of light (ii) the size of the particles causing scattering.
Depending upon the size of the scatterers, the following two situations arise :
(a) If the scattering particles (air molecules) are of size smaller than the wavelength of light, the intensity
of the scattered light (I) varies inversely as the fourth power of the wavelength of light,
This statement, which holds for elastic scattering, is known as the Rayleigh's law of scattering. Since the wavelength
of blue light is less than that of red light, blue light is scattered most while the red light least. Blue colour
of sky, reddish appearance of Sun during sunrise and sunset are due to this phenomenon as discussed below.
Further, it is due to this reason that red signals are used to indicate danger. Such signals go to large distances
without an appreciable loss due to scattering.
(b) If the scattering particles are of sizes greater than the wavelength of light (e.g., dust particles, water
droplets), Rayleigh's law of scattering is not applicable and all colours are scattered equally. It is due
to this reason that clouds generally appear white
Blue colour of the sky
If an observer (O) looks at the sky when the Sun is overhead at noon as shown by position S in figure it is
the scattered light that is received by the observer. Since blue light is scattered more (almost six times) than
red, the sky appears blue to the observer
During sunrise (S1) and sunset (S2), the light coming from the sun has to travel a larger distance through the
atmosphere (than it does at noon) before entering the observers eye. As a result of this, most of the blue
light is scattered on its way to the observer. The transmitted light (sunlight minus the scattered light), which
reaches the observer, is rich in red and orange colour and makes the Sun appear reddish orange
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