General Physics Interview Preparation Guide

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Some substance become luminous when light of a certain wave
length falls upon them. The wave length of the light emitted
by them is always different from that by which they are
illuminated. This phenomenon is called fluorescence e.g.
floorsper, chlorophylle.

Dispension:-Light consists of seven columns. They are
violet, indigo, blue, green, yellow, orange and red. This
coloured bend i.e. called spectrum and the phenomenon of
white light splitting up into seven constituent columns is
called dispersion

Spectrum:-To get a pure spectrum light from a slit is
allowed to pass through a convex lens placed in such a
position that a clear image of the slit is formed on the
screen. A spectrum will appear on the screen on inserting a
prism between the screen and the lens. On rotating the
prism, the spectrum will move towards the original image and
after sometime will become stationary

The essentials for pure spectrum are:-

1.The slit must be very narrow.

2.The prism should be paced in the position of minimum deviation

3.The rays of light must be paralled to each other as they
fall on the prism

4.A convex lens must be used to focus rays of different
column on the screen

Refraction:-When a pencil of light passes obliquely from one
transparent medium into another, it undergoes some
deflection from its straight path. This change in direction
is called refraction

Law of Refraction:-

1. The incident ray the refracted ray and the normal to the
surface separation at the point of incidence, all lie in the
same plane

2. Irrespective of the obliquity of the incident ray, the
ration of the sine of the angle of incidence to the sine of
the angle of refraction is a constant quality of any two
given media. This is called Snell?s law of refraction

Total internal reflection:-Whenever a ray of light traveling
from a denser into a rare medium is incident at an angle
greater than the critical angle for the two media, the ray
is totally reflected back into the same medium

Mirage:-In this optical illusion inverted images of distant
objects are seen, as if reflects from a pool or as if
suspended from atmosphere. In sandy districts objects are
seen reflected from a pond while in colder regions they are
seen suspended from atmosphere. In sandy deserts the payers
of air near surface of earth get heated so soon that density
adjustments do not take place. Hence densities and
refractive indices of the layers just above sand are lower
than the layers higher up. The rays of light form a distant
object after passing through layers which are gradually less
refracting, bend more and more still they fall on layer at
an angle greater than the critical angle for it and get
totally reflected. These rays then travel and undergo a
series of refractions but in the opposite direction for now
they pass through layers which are gradually more and more
refraction, till they reach the eye of the observer, who
sees an image of the object as though reflected from surface
of the water.

Twinking of Stars:-A ray coming from a star at night has to
pass through layers of air having different refractive
indices. It is therefore bent different till the observer
sees it in a particular position. The densities and hence
the refractive indices of the payers go on changing due ti
wind. Due t this the star appears to be at different place
at the very next moment. The difference of place being very
small, therefore the star appears to be moving in a small
area and hence it twinkles

Planets do not appear to twinkle as due to their being
nearer the amount of light received from them id greater and
so the variation in their brightness is not appreciable

Lena:-It is a portion of a transparent refraction medium
bounded by the surfaces one of which is spherical and the
other is either spherical or plant

When a pencil of ray, diverging from a point, after
reflection and refraction, actually passes through a second
point that second point is called the real image of the
first point.

When a pencil of rays after reflection; and refraction only
appear to pass through a point, the image is said to be virtual.

The difference between the two is that if a image can be
received upon a screen while a virtual image cannot be
received on a screen but can only be seen by the eye.

The point where a ray of light strikes the surface of a
mirrors is called the point of incidence.

If a normal is drawn to the surface at this point, the
angle4 which the incident ray makes with it is called the
angle of incidence.

The angle which the reflected ray makes with the normal is
called the angle of reflection.

The plane containing normal and incident ray is called the
plane of incidence and the plane containing normal and
reflected ray is called the pane of reflection.

Law of Reflection :- They are

1. The incident ray, the reflected ray and the normal to be
surface at the point of incidence. All lie in the same plane.

2. the angle of incidence is equal to the angle of reflection.

It deals with measurement of illuminating power and involves
measurement of the quantity of light.

Illuminating power of two sources of light is directly
proportional to the squares of the distances at which they
can produce equal intensities of illumination.

The instruments used for comparing the illuminating powers
of two sources of light are called photometers. They are.

1.Runford?s Shadow Photometer

2. Bunsen?s Grease-Spot Photometer

3. Flicker Photometer.

When light is incident on an opaque object the space behind
it is dark as light cannot penetrate thought opaque body.
This dark space is called the shadow of the object.

Light travels in a straight line through an homogenous
medium. It can be proved by an experiment given below.

Arrange two pieces of cardboard, having pin holes at the
centre of each, vertically, one behind the other. They
should be arranged in such a way that the two holes are
exactly in the same level with each other. Now place a
candle behind and in level with the hole of the second
cardboard. It will be visible when viewed through the hole
of the first cardboard. Now displace one of the cardboards
slightly to its life or right candle would no longer be
visible. This shows that light travels in a straight line.

The straight line along which light travels is called a ray
of light.

A collection of rays of light is called a beam of light.

A narrow beam is called a pencil of light.

If the rays converge to a point, the beam is to be convergent.

Rays coming from a distant object like a star are called
paroled.

It is substance which does not allow any light to pass
thought it e.g. metal or stone.

Terms transparent and opaque are only relative terms as
there is no substance which is perfectly transparent or opaque.