Refraction of Light
Refraction of Light : Happens in Transparent medium when a light
travels from one medium to another, refraction takes place.
A ray of light bends as it moves from one medium to another Refraction is due to change in the speed of light as it enters from one transparent medium to another.
A ray of light bends as it moves from one medium to another Refraction is due to change in the speed of light as it enters from one transparent medium to another.
Speed of light decreases as the beam of light travel from rarer
medium to the denser medium.
Some Commonly observed
phenomenon due to Refraction
- Your eyes.
- Rainbows.
- Light bending in a glass of
water.
- Glasses.
- Camera lenses.
- Object dislocation in water.
Refraction through a Rectangular Glass Slab
When a incident ray of
light AO passes from a rarer medium (air) to adenser medium (glass) at point. O
on interface KL, it will bends towards the normal. At ptO1, on interface NM the
light ray entered from denser medium(glass) to rarer medium (air) here the
light ray will bend away from normal OO1 is a refracted ray O
B is an emergent ray. If the
incident ray is extended to C, we will observe that emergent ray O1B I parallel
to incident ray. The ray will slightly displaced laterally after refraction.
B is an emergent ray. If the
incident ray is extended to C, we will observe that emergent ray O1B I parallel
to incident ray. The ray will slightly displaced laterally after refraction.
Note : When a ray of light is incident normally to
the interface of two media it will go straight, without any deviation.
Laws of Refraction of Light
The incident ray, the
refracted ray and the normal to the interface of two transparent media at the
point of incidence, all lie in the same plane.
The ratio of sine of angle of
incidence to the sine of angle of refraction is a constant i.e.
For given colour and
pair of media, this law is also known as Snell’s Law
Constant
n is the refractive
index for a given pair of medium. It is the refractive index of the second
medium with repect to first medium.
Refractive Index
The refractive index
of glass with respect to air is given by ratio of speed of light in air to the
speed of light in glass.
c
Speed of light in vacuum
= 
speed of light in air is
marginally less, compared to that in vacuum.
Speed of light in vacuum
= speed of light in air is
marginally less, compared to that in vacuum.
Spherical Lens
A transparent material
bound by two surfaces, of which one or both surfaces are spherical, forms a
lens.
Convex lens / Concave lens
1. Bulging outwards - convex -Converging lens
1. Bulging outwards - convex -Converging lens
2. Bulging inwards - concave - Diverging lens.
Few Basic Terms Related to
Spherical Lens
1. Centre of curvature : A lens, either a convex lens
or a concave lens is combination of two spherical surfaces. Each of these
surfaces forma part of sphere. The centre of these two spheres are called
centre of curvature represented by C1 and C2.
2. Principal axis :Imaginary straight line passing through the two centres of curvature
3. Optical Centre : The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.
4. Aperture : The effective diameter of the circular outline of a spherical lens.
5. Focus of lens : Beam of light parallel to principal axis, after refraction from
2. Principal axis :Imaginary straight line passing through the two centres of curvature
3. Optical Centre : The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.
4. Aperture : The effective diameter of the circular outline of a spherical lens.
5. Focus of lens : Beam of light parallel to principal axis, after refraction from
1. Convex lens, converge to the point on principal axis,
denoted by F, known as Principal focus
2. Concave lens, appear to diverge from a point on the
principal axis known as principal focus.
The distance OF2 and
OF1 is called as focal length
Tips for Drawing Ray
Diagram
- After refraction, a ray
parallel to principal axis will pass through F.
- A ray passes through F, after
refraction will emerge parallel to principal axis
Image formation by a
convex lens for various position of object
Image Formation by Concave Lens
Sign Convention for Refraction
by Spherical Lens
Similar to that of
spherical mirror, only the difference is that all themeasurement are made from
optical centre ‘O’
Lens formula
Magnification
It is defined as the
ratio of the height of image to the height of object.
It is also related to ‘u’
& ‘v’
From equation (1)
& (2)
If magnification
m > 1,then image is magnified
m = 1 ,image is of same size
m < 1, image is diminished
If magnification
m > 1,then image is magnified
m = 1 ,image is of same size
m < 1, image is diminished
Few Tips to Remember Sign
Convention for Spherical Lens
|
f
|
u
|
v
|
|
|
CONCAVE
|
-ve
|
-ve
|
-ve(virtual
image always)
|
|
CONVEX
|
+ve
|
-ve
|
+ve(real)
-ve(virtual) |
h is always +ve
h´ –ve for Real and +ve for Virtual &Errect.
h´ –ve for Real and +ve for Virtual &Errect.
Power of Lens
The degree of
convergence or divergence of light ray achieved by a lensis known as power of a
lens.
It is defined as the reciprocal of its focal length Represented by P.
If F is given in meter, then
It is defined as the reciprocal of its focal length Represented by P.
If F is given in meter, then
If F is given in cm ,
then
SI unit of power of a
lens is “diopter” denoted by ‘D’
I diopter or ID It is the power of lens whose
focal length is I m
I diopter or ID
It is the power of lens whose
focal length is I m
Power of convave lens
or diverging lens is always negative
If any optical
instrument has many lens, then net power will be
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