Chapter 10 Light- Reflection and Refraction Notes
Light: Light is a form of energy that gives us the sensation of vision.
SOURCE OF LIGHT:
A body that emits light in all directions is said to be the source of light. The sources of light are of two types:
(i) Luminous source: Any object that emits light by itself is called a luminous source. e.g. Sun and Stars, Electric lamps, Candles, and Lanterns.
(ii) Non-luminous source: Those objects that do not emit light but become visible when light from luminous objects falls on them and they reflect it back. They are called non-luminous. e.g. Moon, Planets, Wood, Table.
TYPES OF MEDIUM:
A medium is a substance through which light propagates. Based on these mediums are classified into three categories.
- Transparent: The medium which allows most of the light to pass through it is called a transparent medium. e.g. Air, Water, Glass, etc.
- Translucent: The medium that allows only a part of the light to pass through it is called a translucent medium. e.g. Paper, Ground glass etc.
- Opaque: The medium that does not allow any light to pass through it is called opaque medium. e.g. Wood, Bricks, Metals etc.
RAY:
Ray of light is the line drawn in the direction of propagation of light.
It is represented by an arrowhead on a straight line 
.
The direction of the arrow gives the direction of propagation of light.
BEAM OF LIGHT:
The beam of light is the bundle or group of rays emitted from a source together.
There are three types of beams of light:
1. Convergent beam: A group of rays of light meeting at a common point.
2. Divergent beam: The spreading out of light rays emitted from a source.
3. Parallel beam: A bundle of rays of light parallel to each other.
Terms related to Reflection of Light
- Incident Ray: The light ray striking a reflecting surface is called the incident ray.
- Point of Incidence: The point at which the incidence ray strikes the reflecting surface is called the Point of Incidence.
- Reflected Ray: The light ray obtained after reflection from the surface in the same medium in which the incident ray is traveling is called Reflected ray.
- Normal: The perpendicular drawn to the surface at the point of incidence is called the Normal.
- Angle of Incidence: The angle which the incident ray makes with the normal at the point of incidence is called the angle of incidence.
- Angle of Reflection: The angle which the reflected ray makes with the normal at the point of incidence is called the angle of reflection.
Laws of Reflection
The laws of reflection state that,
i) The angle of incidence is equal to the angle of reflection, and
ii) The incident ray, the reflected ray, and the normal to the mirror at the point of incidence all lie in the same plane.
Types of Reflection:
There are two types of reflection:
Regular Reflection: When the reflecting surface is smooth and well polished, e.g. mirror, the parallel rays falling on it are reflected parallel to one another. Then it is known as a regular reflection.
Irregular Reflection: When the reflecting surface is rough, the parallel rays falling on it are reflected in different directions as shown in the figure. Such a reflection is known as irregular reflection or diffused reflection or scattering of light.
Some Definition :
IMAGES: Image is an optical appearance produced when light rays coming from an object are reflected from a mirror.
REAL IMAGE : The image which can be obtained on a screen is called a real image. In a cinema hall, we see the images of actors and actress on the screen. So, the images formed on a cinema screen is an example of real images.
VIRTUAL IMAGE : The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror. The image of our face in a plane mirror is an example of virtual image.
LATERAL INVERSION : When an object is placed in front of a plane mirror, then the right side of the object appears to become the left side of the image, and the left side of the object appears to become the right side of the image. This changes of the sides of an object and its mirror images called lateral inversion.
E MIRRORS
CHARACTERISTICS OF IMAGES FORMED
BY PLANE MIRRORS
|
E MIRRORS |
The characteristics of images formed by plane mirrors are:
1. The image formed in a plane mirror is always erect.
2. The size of the image in a plane mirror is always the same as the size of the
object.
3. The image formed in a plane mirror is as far behind
the mirror, as the object is in front of the mirror.
4.The image formed in a plane mirror is laterally
inverted i.e. the left side of the objects becomes the right side of the
image and vice-versa.
SPHERICAL
MIRROR
A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass.
The spherical mirrors are of two types: Concave mirror and Convex mirror.
CONCAVE MIRROR: A concave mirror is that spherical mirror in which the reflection of light takes place at the bent-in surface. It converges the light so it is also called converging mirror.
CONVEX MIRROR: A convex mirror is that spherical mirror in which the reflection of light takes place at the convex bulging –out surface. It diverges the light so it is also called a diverging mirror.
TERMS RELATED
TO SPHERICAL MIRRORS
Centre of Curvature(C): The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass of which the spherical mirror is a part. It is represented by letter ‘C’.
Pole(P): The pole of a spherical mirror is the centre of the mirror. It is represented by letter ‘P’.
Radius of Curvature(R): The radius of curvature of a spherical mirror is the radius of the hollow sphere of glass of which the spherical is a part. It is represented by the letter ‘R’.
Principal axis: The principal axis of a spherical mirror is the straight line passing through the centre of curvature C and pole P of the spherical mirror, produced on both sides.
Aperture: The aperture of a spherical mirror is the diameter of the reflecting surface of the mirror.
Focus: The Focus of a Concave Mirror is a point on the Principal Axis at which the light rays incident parallel to the principal axis meet after reflection from the mirror.It is represented as F .
Focal length: The distance of focus from the Pole of the mirror is called the Focal Length of the mirror.It is represented by f.
Relation between Radius of curvature and focal length of a spherical mirror
RULES FOR OBTAINING IMAGES FORMED BY SHPERICAL MIRRORS
The intersection of at least two reflected rays give the position of image of the point object. Any two of the following rays can be considered for locating the image.
1. A ray parallel 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.
3. 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. The light rays come back
along the same path because the incident rays fall on the mirror along the
normal to the reflecting surface.
FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE MIRROR
The type of image formed by a concave mirror depends on the position of object in front of the mirror. There are six positions of the object:
Image
Position − At ‘F’
Nature – Real, inverted
Size – Point sized or highly diminished
Image
Position – Between ‘F’ and ‘C’
Nature – Real, inverted
Size – Diminished
Position – At ‘C’
Nature – Real, inverted
Size – Same size as that of object
Image
Position – Beyond ‘C’
Nature – Real, inverted
Size – Enlarged
Image
Position – At Infinity
Nature – Real, inverted
Size – Highly enlarged
Image
Position – Behind the mirror
Nature – Virtual, erect
Size – Enlarged
USES OF CONCAVE MIRRORS
1. Concave mirrors are commonly used in torches,
search-lights and vehicles headlights to get powerful
parallel beams of light.
2. Concave mirrors
are used as shaving mirrors
to see a larger image of the face.
3. The dentists use concave mirrors to see large images of the teeth of patients.
4. Concave mirrors are used as doctor’s head mirrors to focus light coming from a lamp on to the body parts of a patient to be examined by the doctor.
5. Concave dishes are used in TV dish antennas to receive TV signals from the distant communications satellite.
6.Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONVEX MIRROR
The type of image formed by a convex mirror depends on the position of object in front of the mirror. There are six positions of the object:
(i) When object is placed at infinity :
Image
Position − At ‘F’
Nature – Virtual, erect
Size – Point sized
(ii) When object is placed between pole and infinity
Image
Position – Between ‘P’ and ‘F’
Nature – Virtual, erect
Size – Diminished
Uses of Convex Mirror:
(i) Convex mirrors are used as rear view mirrors in vehicles because
(a) they always give an erect though diminished image.
(b) they have a wider field of view as they are curved outwards.
(ii) Convex mirrors are used at blind turns and on points of merging traffic to facilitate vision of both side traffic.
(iii) Used in shops as security mirror.
Sign Convention for Reflection by Spherical Mirror
1. The object is always placed to the left
side of mirror.
2. All distance should
be measured from
pole (P); parallel
to principal axis.
3. 3. 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
Mirror Formula :
It is a relation between distance of object, distance of image from the pole of the mirror and it’s focal length.
Where, v = Image distance
If 'm' is negative, image is real.
If 'm' is positive, image is virtual.
If h2 = h1 then m = 1, i.e., image is equal to object.
If h2 > h1 then m > 1 i.e., image is enlarged.
If h2 < h1 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 '+ve' and less than 1, it is a convex mirror.
If 'm' is '+ve' and more than 1, it is a concave
mirror.
Laws of Refraction :
(i) The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
(ii) When a ray of light undergoes refraction then the ratio of sine of angle of incidence to the sine of angle of refraction is constant. This law is also known as snell's Law.
Note: The speed of light is different in different
substances. The refraction of light
is due to the change in the speed of light going from one medium to another.
MEDIUM
A transparent substance in which light travels is known as a medium. Medium can be divided into two types:
1. Optically
rarer medium: A medium
in which the speed of light is more is known as optically rarer medium (or less
dense medium)
2. Optically
denser medium: A medium in which
the speed of light is less is
known as optically rarer medium (or more dense medium)
Glass is an optically denser medium than air and water.
RULES OF REFRACTION :
Rule 1: When a light ray travels from a rarer medium to a denser
medium, the light ray bends towards the normal.
Rule 2: When a light ray travels from a denser medium to a rarer
medium, the light ray bends away from the normal.
Refractive Index:
The refractive index of a medium is defined as the ratio of speed of light in vacuum to the speed of light in the medium. It is represented by n.
Refraction by spherical lenses: Lens is a transparent medium that is formed by joining two pieces of spherical glass. There are two types of lenses.
(i) Convex Lens: It is a lens that is thicker at the centre and thinner at the edges. Convex lens converges light rays. Hence it is called converging lens.
(ii) Concave Lens: It is a lens that is thinner at the centre and thicker at the edges. Concave lenses diverge light rays. Hence it is called diverging lenses.
TERMS RELATED TO SPHERICAL LENS
1. Centre of curvature - A lens, either a convex lens or a concave lens has two spherical surfaces. Each of these surfaces form a 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 the principal axis, after refraction from convex lens, converges to the point on the principal axis or appear to diverge from a point on the principal axis in case of concave lens, known as Principal focus.
6. Focal Length: The focal length (f) is the distance between the optical centre and the focal point.
Tips for drawing Ray diagram
iii)A ray passes
through optical centre
'O', passes without
any deviation.
FORMATION OF DIFFERENT TYPES
OF IMAGES BY A CONVEX
LENS
Case–1: Object is at infinity
When the object is placed at the infinity, the image formed is
(i) at the focus F2.
(ii) real and inverted, and
(iii) highly diminished or point sized
Case–2: Object is at beyond
2F1
When the object is placed beyond 2F1 in front of the convex lens, the image formed is
(i) between F2 and 2F2 on the other side of the lens,
(ii) real and inverted, and
(iii) diminished)
Case–3: Object is at 2F1
When the object is placed at a distance 2f in front of convex lens, the image formed is
(i) at 2F2 on the other side of the lens,
(ii) real and inverted, and
(iii)
of the same size as the object.
Case–4: Object
is in between F1 and 2F2
When the object is placed between F1 and 2F1 in front of a convex lens, the image formed is
(i) beyond 2F2,
(ii) real and inverted, and
(iii) larger than the object (or magnified).
Case–5: Object
is at the focus (F1)
(i) at infinity
(ii) real and inverted, and
(iii) highly enlarged
Case–6: Object is in between optical centre(O) and focus (F1)
When the object is placed between optical centre(O) and focus(F1), the image formed is
(i) behind the object (on th left side of lens)
(ii) virtual and erect, and
(iii) enlarged or magnified
FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE LENS
Case–1: Object is at infinity
When the object
is placed at the infinity, the image formed
is
(i) at the focus F1.
(ii) virtual and erect,
and
(iii) highly diminished or point
sized
Case–2: Object is in between
optical centre(O) and infinity
When the object is placed in between optical
centre(O) and infinity, the image formed is
(i) between optical centre(O)
and focus F1.
(ii) virtual and erect,
and
(iii) smaller than the object (or diminished)
When the object is placed at the infinity, the image formed is
(i) at the focus F1.
(ii) virtual and erect, and
(iii) highly diminished or point sized
Case–2: Object is in between
optical centre(O) and infinity
When the object is placed in between optical centre(O) and infinity, the image formed is
(i) between optical centre(O) and focus F1.
(ii) virtual and erect, and
(iii) smaller than the object (or diminished)
SIGN CONVENTION FOR SPHERICAL LENSES
While using the lens formula we must make use of proper sign convention while taking the values of object (u), image distance (v), focal length (f), object height (h) and image height (h’). The sign conventions are as follows:
1. All distances are measured from the optical
centre of the lens.
2.The distances measured
in the same direction as the incident
light are taken positive.
3.The distances measured
in the direction opposite to the direction of incident light are taken negative.
4. Heights measured upwards
and perpendicular to the principal
axis are taken positive.
5. Heights measured downwards and perpendicular to the principal axis are taken negative.
Consequences of new
Cartesian sign convention:
1. The focal length of a convex
lens is positive
and that of a concave
lens is negative.2.Object
distance u is always negative.3.The distance of real image is positive and that of virtual image is negative.4. The object
height h is always positive.
Height h' of virtual erect
image is positive and that of real inverted image
is negative.5.The linear
magnification, m = h'/h is positive for a virtual
image and negative
for a real image.
1. All distances are measured from the optical centre of the lens.
2.The distances measured in the same direction as the incident light are taken positive.
3.The distances measured in the direction opposite to the direction of incident light are taken negative.
4. Heights measured upwards and perpendicular to the principal axis are taken positive.
5. Heights measured downwards and perpendicular to the principal axis are taken negative.
Consequences of new Cartesian sign convention:
1. The focal length of a convex lens is positive and that of a concave lens is negative.LENS FORMULA
Lens formula gives the relationship between object distance (u), image-distance (v) and the focal length (f ). The lens formula is expressed as
where ‘u’ is the distance of the object from the optical centre
(O), ‘v’ is the distance of the image from the optical
centre (O), and ‘f’ is the distance of the principal focus from the optical centre (O).
Points to be remembered
1. If the magnification ‘m’ has a positive value, the image is virtual and erect. And if the magnification ‘m’ has a negative value, the image will real and inverted.
2.A convex lens can form virtual images as well as real images, therefore, the magnification produced by a convex lens can be either positive or negative.
3. A convex
can form images which are smaller than the object, equal to the object or bigger than the object, therefore
magnification ‘m’ produced by a convex lens can be less than 1, equal to 1 or
more than 1.
4.A concave
lens, however, forms only virtual images, so the magnification produced by a
concave lens is always positive.
5.A concave
lens forms image which are always smaller than the object, so the magnification ‘m’ produced by a concave lens is always less than 1.
The degree of convergence or divergence of light ray achieved by a lens is known as power of a lens.
OR
The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
The power of a convex lens is positive and that of a concave lens is negative.
If any optical instrument have many lens, then net power will be P = P1 + P2 + P3 +…
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