LIGHT
Light - a type of energy - transmitted in the form of an electromagnetic wave - wavelength between
3900Å to 7800Å.
Light - an electromagnetic wave which moves in a straight lines - a transverse wave and does not
require a medium to move
The ability of a medium to reflect light is also expressed by its Light density / Optical density
When light enters the denser medium through the rare medium, its speed decreases
Ole Roemer measured the speed of light for the first time in history in 1676
Speed of light varies in different mediums
Speed of light in:
Vacuum 3 ×108 m/s GAS > LIQUID > SOLID
Water 2.25 × 108 m/s
Glass 2× 108 m/s
Max Planck gave quantum theory of light
Energy of a quanta,
E = hν = hc/λ h is the plank constant and ν is the frequency of incident light.
Object which do not emit light on their own are called non-luminous objects.
Objects that give out or emit light or give/produce their own light are called as luminous objects.
Moon is a non-luminous object because it does not emits its own light and glows by reflecting the light
of the sun.
Sun, bulb and candle give out or emit their own light and hence, are luminous objects
Light transmits through:
TRANSLUCENT TRANPARENT OPAQUE
Materials through which Materials through which Materials through which
objects can be seen, objects can be seen objects cannot be seen
but not clearly
GLASS | WATER | AIR WOOD | METAL
BUTTER PAPER
When an opaque object is placed between an extended source of light and a screen, we obtain, on the
screen, a shadow consisting of two parts:
UMBRA – an inner part PENUMBRA – an outer part
The region from which the occluding body appears entirely within the disc of the light source -antumbra
(from the Latin ante "before" and umbra "shadow")
REFLECTION OF LIGHT
When a ray of light collides with some medium and returns to the same medium again, this phenomenon
is called reflection of light.
The colour of the grass appears green to us, because it reflects green light back to our eyes.
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The laws of refection include all mirrors whether it is a plane mirror, concave mirror, or convex mirror
and also for all other reflecting surfaces which are not mirrors but act like one for example water, steel
etc.
Laws of Reflection
Ist Law of Reflection IInd Law of Reflection
The angle of incidence The incident ray, reflected ray and
is equal to the angle of normal lie in the same plane, and
reflection the incident ray and the reflected
ray are one opposite ray.
REFRACTION OF LIGHT
When rays of light penetrate from one transparent medium to another
transparent medium, they distract out of their original direction -
'refraction of light’
The light ray will bend towards the normal as it passes from a rarer
medium to a denser medium.
Refraction has two associated laws
When light travels from rarer to When light travels from denser to
denser medium, it bends towards rare medium it bends away from
normal normal
When a ray of light enters from a denser medium (glass) to a rarer medium (air) it bends away from the
normal, because angle of refraction will be greater than the angle of incidence
Each beam of light with its own particular wavelength (or colour) is delayed differently by glass
When light travels from one medium to another medium then incident ray, normal at the point of
incidence and refracted ray all are in the same plane
A ray of light travelling in air enters a glass slab.
(i) Angle of incidence is greater than the angle of refraction.
(ii) Angle of incidence is equal to angle of emergence
(iii) The emergent ray is parallel to the incident ray
Snell’s Law
𝑛 𝑆𝑖𝑛 𝜃 = 𝑛 𝑆𝑖𝑛 𝜃
The ratio of the sine of the angle of incidence to the sine of
the angle of refraction is a constant, for the light of a given
colour and for the given pair of media
A ray of light bends to wads the normal while travelling from medium A to medium B, then speed of light
is more in medium A than medium B.
Refractive index of medium B is more than refractive index of medium A
Refractive index of a material can be linked with relative speed of propagation of light in different media
- the measure of bending of a light ray when passing from one medium to another.
Refractive Index (µ) of a medium is defined as the ratio of the velocity of light in air (c) and the velocity
of light in that medium (v).
If the refraction of light occurs in a medium from vacuum, then the ratio of the sine of the incidence
angle and the sine of the angle of refraction is called the absolute refractive index of that medium.
Absolute refractive index =
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Absolute refractive index of air 1.0003
Absolute refractive index of water 1.333
Absolute refractive index of diamond 2.417
The value of absolute refractive index of a medium is always more than 1.
/
Refractive index of matter =
The refractive index of a medium is different for different colours of light.i.e.dispersion
The value of refractive index decreases as the wavelength increases – property of dispersion
The refractive index of a medium relative to air is approximately equal to the absolute refractive index
of that medium
Ascending order of the refractive index ice (1.31) < kerosene (1.44) < benzene (1.5) < rock salt (1.54)
Refractive is not related to the flow of heat, when the rays of light enter from one transparent medium
to another transparent medium, the light ray deviate from their original path. It is not related to heat flow
this phenomenon is refraction.
There is no SI Unit for refractive index. The refractive index is defined as the ratio between the speed
of light in a vacuum and its speed in a particular medium.
The light which has more wavelength will deviate less
Refractive index of the medium is inversely proportional to the speed of light in it. As the refractive index
of medium increases the speed of light passing through it decreases.
When light travels from one medium to another, it changes the speed at which the light travels
A beam of white light undergoes dispersion though a triangular glass prism forming a band of seven
colours. The red coloured compound has minimum refractive index
As violet light has a shorter wavelength it is delayed more than longer wavelengths of red light.
consequently, violet light is bent most while red light in bent the least.
When a light is incident along to the normal and from air to water, then the direction of light will not be
change because when a light ray incident perpendicular on the plane which separates two medium then
there is no refraction of light ray takes place
A ray of light undergoes refraction through a triangular glass prism. The angle between incident ray and
emergent ray is called angle of deviation
The angle of incidence for a ray of light incident on a glass slag along its normal will be zero degree
Applications on Refraction
When a pencil is placed in a glass filled Appearance of a stick bent at the interface
with water, the pencil looks bent when immersed in water
Rays coming from the sun are refracted by Twinkling of stars occurs because change in
the atmosphere - the apparent flattering of the density of different layers of air causes
the sun at sunset and sunrise is due to change in refractive index continuously
refraction
Fluctuations in the light coming from the
The random wavering of objects seen various points of planet due to refraction in
through a turbulent stream of hot air rising atmosphere get averaged out - reason for no
above a fire due to atmospheric refraction twinkling of planets
A rainbow is observed due to refraction of Sun can be seen above the horizon about
sunlight through rain drops - sunlight is two minutes before actual sunrise
polychromatic - refract through rain drops it
disperse into 7 colours When a coin is placed in a tub filled with
water, the coin appears slightly above the
Lemon placed in a glass filled with water base of the tubs floor
appears larger than its actual size when
viewed from the outside The fish lying under the water appears to be
above the actual depth.
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Spectacles use the principle of refraction. Optical instruments such as microscopes, telescopes and
cameras use mirrors and lenses to reflect and refract light and from images
The colour of the sun becomes red while setting is not an example of refraction of light but of scattering
of light - at sunrise and sunset, the Sun ray has to travel a relatively greater distance through the
atmosphere
In this way most of shorter wavelength light is separated by scattering, so only the red colour (the least
scattering part of the light) reaches our eyes.
Change in refractive index in atmospheric gases bents the light rays, coming from the star, many times
- reason behind the apparent position change of a star
The ability of a medium to refract light is also expressed in terms of its "optical density".
Optical density is not as same as mass density. Optical Density is also referred as "absorbance".
Diamond's low refractive index does not contribute to the sparking of diamonds.
Snell's law is used to calculate the value of Refractive index
TOTAL INTERNAL REFLECTIONS
Travelers in deserts often tend to have an optical illusion of a sheet of water where none actually exists.
It is a called mirage.
Mirage happens when the ground is very hot and the air is cool. The hot ground warms the layer of air
just above the ground. When the light travels through the cold air and enter into the layer of hot air it is
refracted. A layer of very warm air, near the ground refracts the light and thus mirage occurs.
When a light ray travelling from a denser medium towards a rarer medium is incident at the interface at
an angle of incidence greater than critical angle, then light rays are reflected back into the denser
medium (i.e. same medium). This phenomenon is called total internal reflection.
Optical fibre works on the principal of total internal reflection
Daimond sparkles due to Total internal reflection
Periscope, endoscope works on this principle
SCATTERING OF LIGHT
Scattering of light - the phenomenon in which light rays get deviated from their straight path on striking
an obstacle like dust or gases molecule water droplets etc.
Applications of Scattering of Light
Blue colour of the sky and sea
Blue is scattered more than other colours because it travels as shorter and smaller waves
When a car is moving through a dusty road during night, a path of beam from the head
light is clearly visible due total scattering effect of light
White colour of clouds – water droplets scatter all colours of light almost equally - makes
the sunlight appear white
Reddish appearance of sun during sunrise and sunset
The atmosphere is the main cause of scattering of sunlight
In the absence of atmosphere, there will be no scattering of sunlight at all - no scattered light will enter
our eyes - the sky will look black or dark - Thus, the sky appears dark instead of blue to an astronaut.
When a photograph of earth is taken from space its background looks dark because of no scattering of
light
Tyndall effect is related to scattering of light.
The phenomenon of scattering of light by particles present in a colloidal solution is called Tyndall effect
- also be seen by a solution containing small suspended particles - this effect was named after John
Tyndall.
Red light is more visible than Blue light, because Red light has a higher wave Length
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Clouds can be seen as white because the small water droplets of the cloud scatter very high amount of
red and blue wave length of visible light.
As the red colour has longest wavelength among all the visible parts of light that's the reason red colour
is scattered least by fog on smoke.
Red, Green and Blue - the primary or main colours - Secondary colours are obtained by mixing primary
colours
Red colour deviation angle is the lowest - maximum wavelength in all colours that are present in rainbow
Polarization occurs only in transverse waves, which are related to the direction of their oscillations. The
moving sound waves in a gas or liquid do not exhibit the properties of polarization
Sunlight is white light which is the mix of seven colours namely violet, indigo, blue, green, yellow,
orange, and red. We usually called it as VIBGYOR. The sunlight enters into the atmosphere and
scattered. Blue light is scattered more than the other colours because it travels as shorter, smaller
waves. This is why we see a blue sky most of the time. If the atmosphere is not present then the sky is
seen black in colour
Sir CV Raman - Indian physicist - awarded the prestigious Nobel Prize in Physics in 1930 - for his
outstanding work on the scattering of light.
Sir CV Raman – 1954 Bharat Ratna - 1957 Lenin Peace Prize
During the formation of a rainbow, the phenomena of dispersion, refraction and internal reflection are
involved.
DISPERSION OF LIGHT / RAINBOW
The splitting of light of sum into various colours of rainbow is called dispersion of light.
Among the 7 constituent colours of light, violet light has the least wavelength and hence it deviates the
most and red light having the largest wavelength deviates the least.
The order of obtained spectrum of light is VIBGYOR
The colours of VIBGYOR arranged in the increasing order of their refractive indices will be
Red < Orange < Yellow < Green < Blue < Indigo < Violet
Red colour light has the lowest frequency and longest wavelength and vice-versa for violet
A rainbow is formed by water droplets suspended in the atmosphere after the rain shower
When blue and orange lights, parallel to each other, are passed through prism then blue will bend more
because the wavelength of blue light is less than orange light.
A spectrum is formed when each colour of the white light is refracted in the prism at a different angle
OPTICAL INSTRUMENTS
Optical devices can be used to produce a parallel beam of rays from a point source of light - concave
mirror or a convex lens
LEDs and CFLs are replacing ordinary bulbs for illumination due to - energy efficiency
The microscope uses only convex lenses
Bioscope is used for the study of biology, rock science, metrology, crystal science and metals and
plastics and the car's headlight and flashlight use concave mirrors.
When watching 3D movies in the theater, we have to wear special glasses, because 3D movies use
special colors, which cannot be felt by human eyes
Plane mirrors are used in a reflecting periscope.
Two plane mirror are fixed at the bends of the tube at an angle of 45˚ with the side of the tube.
A stethoscope does not have an optical lens.
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