Chapter-10
Light-Reflection and Refraction
Light
Light can be simply defined as a form of energy which helps us to see objects in the dark. There are both natural and artificial sources of light. Sunlight is the best example for the natural sources of light. Reflection and refraction of light are the two main properties of light. Some common phenomena associated with lights are image formation by mirrors, the twinkling of stars, the beautiful colours of the rainbow, bending of light by a medium and so on.
Properties Of Light
An electromagnetic wave so does not require any medium to travel
Light tends to travel in a straight line
Light has dual nature i.e., wave as well as particle
Light casts shadow
Speed of light is maximum in vacuum. Its value is 3 x 108 m/s
When light falls on a surface, the following happens
– Reflection
– Refraction
– Absorption
What is Reflection of light?
The natural phenomenon of bouncing back the light in the same medium on striking the surface of an object is termed as the Reflection of light. According to the laws of Reflection:
The angle of reflection is always equal to the angle of incidence.
All the three rays- incident, reflected ray and the normal lie in the same plane.
Virtual and Real Image
Image is a point where at least two light rays actually meet or appear to meet.
Real Image |
Virtual Image |
Formed when light rays actually meet |
Formed when light rays appear to meet |
Can be obtained on the screen |
Cannot be obtained on the screen |
inverted |
erect |
Example – image formed on cinema screen and formed by concave mirror |
Example – image formed by plane mirror or convex mirror |
Image Formed By Plane Mirror
Characteristics of Image formed by Plane Mirror
Virtual and erect
Size of image is equal to the size of object
Image is formed as far behind the mirror as the object is in front of it
Laterally inverted
Lateral Inversion – the right side of the object appears on the left side of the image and vice-versa
Application of Lateral Inversion
The word AMBULANCE is written in reverse direction so that it can be read correctly in the rearview mirror of vehicles going in front of it.
Spherical Mirrors
Mirrors whose reflecting surface is curved. There are two types of spherical mirrors, they are:
Convex mirror
Concave mirror
Properties of Concave Mirror:
Reflecting surface is curved inwards
Converging mirror
Properties of Convex Mirror:
Reflecting surface is curved outwards
Diverging mirror
(a) concave mirror (b) convex mirror
Common Terms for Spherical Mirrors
Principal axis – the line joining pole and the centre of curvature
Pole (P) – the centre of the spherical mirror
Aperture (MN) – it is the effective diameter of the spherical mirror
Centre of curvature (C) – The centre of the hollow glass sphere of which the mirror was a part
Radius of curvature (R) – The distance between the pole and the centre of curvature
Focus (F) – The point of principle axis where all the parallel light rays actually meet or appear to meet after reflection
Focal Length (f) – The distance between the pole and the focus
Relationship between focal length and the radius of curvature. f=R/2
Rules For Making Ray Diagrams By Spherical Mirror
(i) A ray passing to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror
(ii) A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis
(iii) A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path
(iv) A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis
Ray Diagrams For Images Formed by Concave Mirror
(i) When object is at infinity
Image Position – At ‘F’
Nature of image – Real, inverted
Size – Point sized or highly diminished
(ii) When object is beyond ‘C’
Image Position – Between ‘F’ and ‘C’
Nature of image – Real, inverted
Size – Diminished
(iii) When object is at ‘C’
Image Position – At ‘C’
Nature of image – Real, inverted
Size – Same size as that of object
(iv) When object is placed between ‘F’ and ‘C’
Image Position – Beyond ‘C’
Nature of image – Real, inverted
Size – Enlarged
(v) When object is placed at ‘F’
Image Position – At infinity
Nature of image – Real, inverted
Size – Highly enlarged
(vi) When object is between ‘P’ and ‘F’
Image Position – Behind the mirror
Nature of image – Virtual, erect
Size – Enlarged
Uses of Concave Mirror
Used in torches, searchlights and vehicles headlights to get powerful parallel beam of light
Concave mirrors are used by dentists to see large image of teeth of patients. (Teeth have to be placed between pole and focus)
Concave mirror is used as shaving mirror to see a larger image of the face
Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace
Ray Diagrams of Images formed by convex mirror
(i) When object is at infinity
Image Position – At ‘F’
Nature of image – Virtual, erect
Size – Point sized
(ii) When object is placed between pole and infinity
Image Position – Between ‘P’ and ‘F’
Nature of image – Virtual, erect
Size – Diminished
A full length image of a tall building/tree can be seen in a small convex mirror
Uses of Convex Mirror
Convex mirrors are used as rear view mirrors in vehicles because
– they always give an erect though diminished image
– they have a wider field of view as they are curved outwards
Convex mirrors are used at blind turns and on points of merging traffic to facilitate vision of both side traffic
Used in shops as security mirror
Sign Convention for Reflection by Spherical Mirror
The object is placed to the left of the mirror
All distances parallel to the principal axis are measured from the pole of the mirror
All distances measured in the direction of incident ray (along + X-axis) are taken as positive and those measured against the direction of incident ray (along – X-axis) are taken as negative
Distance measured perpendicular to and above the principal axis are taken as positive
Distances measured perpendicular to and below the principal axis are taken as negative
Object distance = ‘u’ is always positive
Focal length of concave mirror = negative
Focal length of convex mirror = positive
Mirror Formula
1/v + 1/u = 1/f
Where v=image distance
u= object distance
f=Focal length
Magnification of Spherical Mirrors
It is the ratio of the height of the image to the height of the object
m=Height of image/Height of the object
=> m = hi/ho
Also, m = -v/u
If ‘m’ is negative, image is real
If ‘m’ is positive, image is virtual
If ‘hi‘ = ho then m = 1, i.e., image is equal to object
If hi > ho then m > 1 i.e., image is enlarged
If hi < ho then m < 1 i.e., image is diminished
Magnification of plane mirror is always + 1
‘+’ sign indicates virtual image
‘1’ indicates that image is equal to object’s size
If ‘m’ is positive and less than 1, it is a convex mirror
If ‘m’ is positive and more than 1, it is a concave mirror
If ‘m’ is negative, it is a concave mirror
Refraction
The natural phenomenon of changing the path of light while moving from one medium to another is termed as the Refraction of light.
As the light travels from its rarer to a denser medium, light rays bend towards normal and while travelling from the denser to rarer medium, light rays bend away from normal. According to the law of Refraction:
The Refraction of light occurs when there is a change in the speed of light and when the light rays enter from one transparent medium to another. Speed of light is maximum in vacuum. It is 3 x 108 m/s. Cause of refraction is a change in the speed of light.
Some examples of refraction
The bottom of swimming pool appears higher
A pencil partially immersed in water appears to be bent at the interface of water and air
Lemons placed in a glass tumbler appear bigger
Letters of a book appear to be raised when seen through a glass slab
Refraction takes place when light strikes on a polished surface like mirror and bounces back
The extent of bending of ray of light at the opposite parallel faces of rectangular glass slab is equal and opposite, so the ray emerges parallel to incident ray
Lateral displacement depends on
– refractive index of glass slab
– thickness of the glass slab
Laws Of Refraction
The incident ray, the reflected ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane
Snell’s law – the ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for a light of given colour and for a given pair of media
sin i / sin r = constant
Refractive index (n) – The ratio of the speed of light in a given pair of media
n = Velocity of light in medium 1/Velocity of light in medium 2
-> n21 means refractive index of the second medium with respect to first medium
n21 = v1/v2
-> n12 means refractive index of first medium with respect to second medium
n12 = v2/v1
Absolute Refractive Index
It is the refractive index of a medium with respect to vacuum or air.
n= (c/v) x c = 3 x 108 m/s
The refractive index of one medium is reciprocal of other’s refractive index in a given pair. n12 = 1/ n21
If the refractive index of medium 1 w.r.t air is given as 1nair and if the refractive index of medium 2 w.r.t. air is given as 2nair . Then the refractive index of medium 1 w.r.t. medium 2 is given by (1nair ) / (1nair )
Refractive index of diamond is highest till date. It is 2.42. It means the speed of light is 1/2.42 times less in diamond than in vacuum.
Optically denser medium – out of two given media, the medium with a higher value of the refractive index
Optically rarer medium – Out of two given media, the medium with lower value to refractive index
When light enters obliquely from a rarer to a denser medium, it bends towards the normal
When light enters obliquely from a denser to a rarer medium, it bends away from the normal
The refractive index of a medium does not depend on physical density.
Spherical Lens – A transparent medium bound by two surfaces, of which one or both surfaces are curved.
Convex lens |
Concave lens |
Thin from corners |
Thick from corners |
Thick at centre |
Thin at centre |
Converging |
Diverging |
Rules for image formation by Convex Lens
(i) A ray of light parallel to the principal axis of a convex lens always pass through the focus on the other side of the lens
(ii) A ray of light passing through the principal focus will emerge parallel to the principal axis after refraction
(iii) A ray of light passing through the optical centre will emerge without any deviation
Ray Diagrams of Image formed by Convex Lens
(i) When object is at infinity
Image position – At ‘F2’
Nature of image – Real, inverted
Size – Point sized or highly diminished
(ii) When object is beyond ‘2F1’
Image position – Between ‘F2’ and ‘2F2’
Nature of image – Real, inverted
Size – Diminished
(iii) When object is at ‘2F1’
Image position – At ‘2F2’
Nature of image – Real, inverted
Size – Same size
(iv) When object is between ‘F1’ and ‘2F1’
Image position – Beyond ‘2F2’
Nature of image – Real, inverted
Size – Enlarged
(v) When object is at ‘F1’
Image position – At infinity
Nature of image – Real, inverted
Size – Highly Enlarged
(vi) When object is between ‘F1’ and optical centre
Image position – On the same side of the lens as object
Nature of image – Virtual and erect
Size – Enlarged
Rules for Image formation by Concave Lens
(i) A ray of light parallel to the principal axis appears to diverge from the principal focus located on the same side of the lens
(ii) A ray of light appearing to meet at the principal focus of a concave lens will emerge parallel to the principal axis
(iii) A ray of light passing through the optical centre of a lens will emerge without any deviation
Ray Diagrams Of Images Formed By A Concave Lens
(i) When object is placed at infinity
Image position – At ‘F1’
Nature of image – Virtual and erect
Size – Point sized or highly diminished
(ii) When object is placed between infinity and optical centre
Image position – between ‘F’ and ‘O’
Nature of image – Virtual and erect
Size – Diminished
Sign Convention For Spherical Lenses
Sign conventions are similar to the one used for spherical mirrors, except the measurements are taken from optical centre of the lens
Focal length of convex lens = Positive
Focal length of concave lens = Negative
Lens Formula
1/v – 1/u = 1/f
Magnification
M = hi/ho = v/u
Power of Lens
It is defined as the reciprocal of focal length in meter
The degree of convergence or divergence of light rays is expressed in terms of power
Power(P) = 1/v – 1/u = 1/f
S.I. unit of Power = Dioptre = D
1 D = 1 m-1
1 dioptre is the power of a lens whose focal length is one metre
Power of convex lens = positive
Power of concave lens = negative
Power = 1/(focal length or thickness)
Power of a lens combination (P) = P1 + P2 + P3 …