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Light is a form of energy, which enable us to see the object.
In this chapter we will study the phenomena of reflection and refraction using the property of light i.e. straight line propagation (Light wave travel from one point to another, along a straight line).
Reflection of Light
When the light is allowed to fall on highly polished surface, such as mirror, most of the light gets reflected.
Laws of Reflection
- The angle of incidence is always equal to angle of reflection. —i = —r
- The incident ray, reflected ray and the normal to the reflecting surface at the point of incidence lie in the same plane.
Image formed by Plane Mirror (Plane reflecting surface)
CHAPTER – 10
LIGHT-REFLECTION
& REFRACTION
Points of incidences
Incident ray
Reflected ray
normal
i r
A
B
A^1
— i B 1 — r
Plane Mirror
Object Image
Virtual (imaginary) & Erect (Virtual fiThe image that do not form on screen.)
Laterally inverted (The left side of object appear on right side of image)
The size of image is equal to that of object
- The image formed is as for behind the mirror as the object is in front of it.
Reflection of light by spherical Mirrors
Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical mirror.
For example - Spoon } fiThe curved surface of shinning spoon can be considered
as curved mirror.
If it is curved inward fiAct as concave mirror
If it is curved outward fiAct as a convex mirror.
Reflecting side
Reflecting side
Concave Mirror OR CONVERGING MIRROR
Convex mirror OR DIVERGING MIRROR
Principal Axis
R
Radius of curvature
C F f focal length
P Concave Mirror
R
f F C focal length Convex Mirror
P
Principal Axis
Radius of curvature
Few Basic terms related to Spherical Mirror
Principal Axis
C F^ CONCAVE
MIRROR
Pole (P)
C F
i P r
—i = —r
b) A ray of light which passes through centre of curvature (it is also known as normal at the point of incidence on spherical mirror) will retrace their path after reflection
c) A ray of light falling on pole get reflected at the same angle on the other side of principal axis.
Principal P F C Axis
CONVEX MIRROR
Principal F C Axis
P
—i = —r —i —r (^) F C
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C F
i r P
(passing through c)
normal at pt of incidence
P
C F
C
B^1
A^1
B
A
F
P
object image^ —^ r
— i
A
P
A
B^1 B F
Note : A ray of light passes through centre of cus-valerie reflecting spherical surface is always act as normal at the point of incidence. If we know the normal we can draw angle of incidence and angle of reflection
Note : The image will only form when two or more rays meets at apoint. Image formation by a concave mirror for different position of the object
- Object At infinity
Position of Image At focus
Size of Image Highly diminished (point size)
Nature Real and Inverted
- Object Beyond C
Position of Image Between F&C
Size of Image Small
Nature Real and Inverted
- Object At C
Position of Image At C
Size of Image Same Size of object
Nature Real and Inverted
P F C
— i
— r
100
- Object Anywhere between infinity and pole of the mirror
Position of Image Between P & F
Size of Image Very small
Nature Virtual & erect
Uses of Concave Mirror
- Used in torches, search light and headlight of vehicle.
- Used to see large image of face as shaving mirror
- Used by dentist to see large images of the teeth
- Large concave mirror used to focus sunlight (heat) in solar furnaces.
Uses of Convex Mirror
- Used as rear-view mirror in vehicles because it gives erect image. It also helps the driver to view large area.
Sign Convention for Reflection by Spherical Mirror
- The object is always placed to the left side of mirror.
- All distance should be measured from pole (P); parallel to principal axis.
- Take 'P' as origin. Distances measured
Right of the origin (+ x - Axis) are taken positive Left of the origin (– x-Axis) are taken negative Perpendicular to and above principal axis (+y-Axis) are taken positive Perpendicular to and below principal axis (–y-Axis) are taken negative
o (^) (Cartesian system)
F
B^1
A^1
B
A
P
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MIRROR FORMULA
F
v
= + u
R
where f = 2
f fidistance between F and Pole v fidistance of image from Pole u fidistance of object from Pole R fidistance between centre of curvature and pole.
MAGNIFICATION
It is expressed as the ratio of the height of the image to height of the object
height of image height of object
m = =
h^1 h 1
It is also related to 'u' and 'v'
\from 1 and 2 equation
where h^1 fiimage height from principle axis h^1 fiObject height from principle axis.
h^1 m = (^) h =
It magnitude m > 1 _____ Image is magnified m = 1 _____ Image is of same size m < 1 _____ Image is dimirushed
Few tips to remember sign convention for Spherical mirror
Object height h fialways positive | Image height h^1 Real - negative }Virtual - positive
Object distance from pole u fiis always negative
Image distance from pole v fiReal - Image } (^) Virtual - Image
always negative always positive
Focal length f fiConcave mirror } (^) Convex mirror
always negative always positive
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
When a incident ray of light AO passes from a rarer medium (air) to a denser medium (glass) at point. O on interface AB, it will bends towards the normal. At pt O , on interface DC the light ray entered from denser medium (glass) to rarer^1 medium (air) here the light ray will bend away from normal OO is a refracted ray^1 OB is an emergent ray. If the incident ray is extended to C, we will observe that emergent ray O B is parallel to incident ray. The ray will slightly displaced laterally^1 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 ie. Sin i Sin r =^
constant ( r )
for given colour and pair of media, this law is also known as Snells Law
Constant n is the refractive index for a given pair of medium. It is the refractive index of the second medium with respect to first medium.
Sin i Sin r =
n 2 n 1 = n^21
Refractive Index
The refractive index of glass with respect is air is given by ratio of speed of light in air to the speed of light in glass.
n (^) ga=
ng na^ =^
Speed of light in air Speed of light in glass
c v
C fiSpeed of light in vacuum = 3·10 m/s^8
speed of light in air is marginally less, compared to that in vacuum.
Refractive index of air with respect to glass is given by
a fiair ( (^) g figlass) n (^) ag=
na ng^ =
Speed of light in glass Speed of light in air
v c
Where 2 is for second medium and 1 is for first medium
105
The absolute refractive index of a medium is simply called refractive index
n (^) m=
Speed of light in air Speed of light in the medium
c v
Refractive index of water (n ) = 1.33w Refractive index of glass (n ) = 1.52g
Spherical Lens A transparent material bound by two surface, of which one or both surfaces are spherical, forms a lens.
CONVEX LENS A lens may have two spherical surfaces, bulging outwards, is called double convex lens (or simply convex lens.
It is also known as converging lens because it converges the light.
CONCAVE LENS A lens bounded by two spherical surfaces, curved inwards is known as double concave lens (or simply concave lens)
It is also known as diverging lens because it diverges the light.
Few Basic Terms related to spherical lens.
C 1 O C 2
R f C 1 F 1 O F 2 C 2 Optical centre (O)
or (2F ) 1
Principal Axis or (2F ) 2
Convex Lens
R
f
C 1 F 1 O F 2 C 2
Optical centre (O) Principal Axis
Concave Lens
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b) A ray passes through F, after refraction will emerge parallel to principal axis.
F 1
F 2
O
F (^2) Principal Axis O
Principal Axis
F 1
F (^1) O F 2 F 1 F 2
Principal Axis
c) A ray passes through optical centre 'O', paeses without any deviation.
2F 1 F 1 F 2 2F 2
Image formation by a convex lens for various position of object
- Object At infinity
Position of Image At focus F 2 Size of Image Highly diminished (point size)
Nature Real & inverted
2F 1 F 1 O 2F 2 F 2 A^1
B^1 B
A
2F 1 F 1 O
A
B F 2 2F 2
B^1
A^1
- Object Beyond 2F 1 Position of Image Between F & 2F 2 2
Size of Image Small
Nature Real & inverted
- Object At 2F 1
Position of Image At 2F 2
Size of Image Same size of object
Nature Real & inverted
O
108
2F 1 F 1 O (^) F 2 2F 2
A^1
B B 1
A
2F 1 F 1 O (^) F 2 2F 2
B
A
- Object Between F & 2F 1 1
Position of Image Beyond 2F 2
Size of Image Enlarged
Nature Real & inverted
Object At focus F 1 Position of Image at infinity
Size of Image Highly Enlarged
Nature Real & inverted
2F 1 F 1 B O F 2 2F 2
A
A^1
B^1
- (Special Case) Object Between F and 1 optical centre 'O'
Size of Image Enlarged
Nature Virtual & Erect
Position of Image On the same side of the object
Image formation by concave lens
- Object Alt infinity
Position of Image At F 1
Nature Virtual & Size of Image Erect Highly Diminished
2F 1 F 1 O F 2 2F 2
From equation
h^1 h
m =
v u
If magnitude of m > | fiImage is magnified m = 1 fiImage is of same size m < | fiImage is deminished
Few tips to remember sign convention for spherical lens
Object height (^) h fiis always positive
Image height (^) h^1 Real^ fiis always^ negative Virtual fiis always positive
Object distance from optical centre u fiis always negative
Image distance from optical centre v fi^ Real^ fi positive virtual fi negative
Focal length v fiConvex lens^ fiis always^ positive } Concave lens fiis always negative
}
Power of Lens
The degree of convergence or divergence of light ray achieved by a lens is known as power of a lens.
It is difined as the reciprocal of its focal length Represented by P
f =f
It f is given in meter, then 1 P =f
It f is given in cm, then 100 P = f
SI unit of power of a lens is "dioptre" denoted by 'D'
I dioptre or ID fiIt is the power of lens whose focal length is 1m
1 ID =1m OR ID = 1m–
Power convex lens or converging lens is always positive
Power of concave lens or diverging lens is always negative
O (^) F 2
F 1 O
f is +ve
f is –ve
If any optical instrument have many lens, then net power will be
P = P + P + P .... 1 2 3
EXERCISE
(Question Bank)
Very Short Answers Type Questions (1 Mark)
- If the angle of incidence is O°, what is the angle of reflection?
- What is the nature of image formed by concave mirror if the magnification produced by the mirror is +3?
- Give two uses of concave mirror?
- Find the focal length of a convex mirror, whose radius of curvature is 30 cm?
- What do you understand by magnification of a spherical mirror?
- An object is held at the principal focus of a concave lens of focal length f. Where the image will form?
- Show the angle of incidence and angle of refection.
- Complete the ray diagram.
F
2F 1 F 1 O F 2 2F 2
