FAHAD
AKHTER
PROPAGATION AND REFLECTION OF
LIGHT
CH 13
X-PHYSICS
LIGHT:
“Light is a form of energy which enable us to see various objects and their colors”
RECTILINEAR PROPAGATION OF LIGHT:
“Formation of shadows of an opaque objects place in the path of light proves that light travels in straight
line. This straight line propagation of light is called rectilinear propagation of light.”
REFLECTION:
“Reflection is the turning back of light from the boundary of a medium.”
PIN HOLE CAMERA:
CONSTRUCTION:
It consists of a rectangular box containing a pin hole
on one side and a photographic film on the opposite
side.
WORKING:
When a narrow beam of light rays from an object is
allowed to pass through the pin hole and invert and
real image is formed on the photographic film.
USE:
It was use to observe eclipse of the sun.
1. INCIDENT RAY:
“Incident ray is the ray of light coming from source and striking the surface of second medium”.
2. REFLECTED RAY:
“Reflected ray is the ray which is reflected back from the surface of second medium in the first medium.”
3. ANGLE OF INCIDENCE:
“Angle between incident ray and normal ray is called angle of incidence.”
4. ANGLE OF REFLECTION:
“Angle between reflected ray and normal ray is called angle of reflection.”
FIRST LAW OF REFLECTION:
“The incident ray, reflected ray and the normal to the surface at the point of incidence all lie in the same
plane.”
SECOND LAW OF REFLECTION:
“The angle of incidence is equal to the angle of reflection.”
m< i = m< r
REGULAR REFLECTION:
“When a beam of parallel rays light is incident or a highly
polished regular plane surface such as mirror, the reflected rays
will also be parallel and the reflection is known as regular
reflection.”
X-PHYSICS
FAHAD AKHTER
IRREGULAR REFLECTION:
“When a beam of parallel rays is incident on a rough irregular surface the
reflected rays are scattered in different direction. This type of reflection is
known as irregular reflection.”
ADVANTAGES OF IRREGULAR REFLECTION:
1.
2.
3.
We can see the non-luminous objects due to irregular reflection.
Due to the irregular reflection sun light scattered in all directions
and can reach the place, where the sun light can not reach directly.
Due to the irregular reflection sun light reaches us before sun rise and persists for some time even
after the sun set.
MIRROR:
"Smooth and polished surfaces that reflect light are called mirrors.”
TYPES OF MIRROR:
1.
2.
Plane mirror.
Spherical mirror.
1. PLANE MIRROR:
“A plane flat reflecting polished surface is called plane mirror.”
IMAGE FORMATION (PLANE MIRROR):
Consider a light ray coming from an object ‘O’ reflected by plane mirror
‘M’ and enters in our eye. The line which joins the image ‘I’ and the object
‘O’ makes an angle of 900 with the surface of the mirror. The distance of
object and image from the mirror are equal.
PROPERTIES OF IMAGE FORMED BY PLANE MIRROR:
Image formed by plane mirror are found to be:
 Virtual.
 Inverted.
 Same in size.
 At equal distance.
2. SPHERICAL MIRROR:
“A mirror which is a part of surface of a hollow sphere is called a
spherical mirror.”
TYPES OF SPHERICAL MIRROR:
1.
2.
Concave mirror.
Convex mirror.
1. CONCAVE MIRROR:
“If a spherical mirror is polished such that its reflecting surface is
facing towards the
Center of sphere is called a concave mirror.”
X-PHYSICS
FAHAD AKHTER
2. CONVEX MIRROR:
“If a spherical mirror is polished such that its reflecting surface is facing opposite to the center of sphere
is called a convex mirror.”
SOME USEFUL TERMS
CENTER OF CURVATURE:
“The center of sphere of which the mirror is a
part is called center of curvature.”
It is denoted by ‘C’.
POLE:
“The geometrical center of the curve of
mirror is called pole.”
It is denoted by ‘P’.
RADIUS OF CURVATURE:
“A straight line draw from the center of
curvature to the reflecting curved surface is
called radius of curvature.”
It is denoted by ‘r’.
PRINCIPLE AXIS:
“The straight line passing through the center
of curvature and the pole of the mirror is
called principle axis.”
PRINCIPLE FOCUS:
“When a beam of light parallel to the
principle axis falls on spherical mirror, after
reflection it converges to a single point on
principle axis. This point is called principle
focus.”
It is denoted by ‘F’.
FOCAL ELNGTH:
“The distance between the pole and the principle focus is called focal length.”
It is denoted by ‘F’.
TYPES OF IMAGE FORMED BY MIRRORS:
1.
2.
Real image.
Virtual image.
REAL IMAGE:
“An image is said to be real if it is formed by the actual intersection of reflected rays and can be received
on the screen.”
VIRTUAL IMAGE:
“An image is said to be virtual if it is formed by the apparent intersection of reflected rays and can not be
received on the screen.”
IMAGE FORMED BY CONCAVE MIRROR:
CASE #1:
When an object is placed at infinity.
X-PHYSICS
FAHAD AKHTER
CHARACTERISTIC OF IMAGE:



Real.
Inverted.
Formed at principle focus.
CASE #2:
When an object is placed behind the center of curvature.
CHARACTERISTIC OF IMAGE:



Real.
Inverted.
Formed between ‘C’ and ‘F’.
CASE #3:
When an object is placed at the center of curvature.
CHARACTERISTIC OF IMAGE:




Real.
Inverted.
Equal size.
Formed at center of curvature.
CASE #4:
When an object is placed between the center of curvature ‘C’ and principle focus ‘F’.
CHARACTERISTIC OF IMAGE:




Real.
Inverted.
Magnified.
Formed beyond the center of curvature.
CASE #5:
When an object is placed at principle focus ‘F’.
CHARACTERISTIC OF IMAGE:




Real.
Inverted.
Highly magnified.
Formed at infinity.
CASE #6:
When an object is placed between pole ‘P’ and principle focus ‘F’.
CHARACTERISTIC OF IMAGE:



Virtual.
Erect.
Magnified.
X-PHYSICS

FAHAD AKHTER
Formed behind the mirror.
MIRROR FORMULA
MIRROR FORMULA:
Consider an object AB placed between centre
of curvature ‘C’ and principle focus ‘F’ of the
convex mirror. Ray starting from ‘A’ after
reflection meets at point A′ and forms areal
image A′ B′.
Since ∆APB and ∆FPD are similar. Therefore
𝐀𝐁
𝐏𝐁
=
𝐀′𝐁′
𝐏𝐁′
𝐀𝐁
𝐏
= … … … (𝟏)
𝐀′𝐁′
𝐪
Since ∆ABF and ∆FPD are similar therefore,
𝐀𝐁
𝐁𝐅
=
𝐃𝐏
𝐅𝐏
𝐀𝐁
𝐁𝐅
=
𝐀′𝐁′
𝐅𝐏
𝐀𝐁
𝐏−𝐟
=
… … … (𝟐)
𝐀′𝐁′
𝐟
Combining equation (1) and (2)
𝐏
𝐏−𝐟
=
𝐪
𝐟
X-PHYSICS
FAHAD AKHTER
Dividing both side by ‘P’
𝐏
𝐏−𝐟
=
𝐏𝐪
𝐏𝐟
𝐏
𝐏
𝐟
=
−
𝐪
𝐏𝐟
𝐏𝐟
𝟏
𝟏
𝟏
=
−
𝐪
𝐟
𝐏
𝟏
𝟏
𝟏
=
+
𝐟
𝐏
𝐪
Where ‘P’ and ‘q’ are the distances of object and image from the mirror and ‘f’ is the focal length.
LINEAR MAGNIFICATION:
“The ratio of the size of the image to the size of the object is called linear magnification.”
𝐌𝐚𝐠𝐧𝐢𝐟𝐢𝐜𝐚𝐭𝐢𝐨𝐧 =
𝐇𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐢𝐦𝐚𝐠𝐞
𝐇𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐎𝐛𝐣𝐞𝐜𝐭
𝐌 =
𝐌𝐚𝐠𝐧𝐢𝐟𝐢𝐜𝐚𝐭𝐢𝐨𝐧 =
𝐌 =
𝐡𝐢
𝐡𝐨
𝐈𝐦𝐚𝐠𝐞 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞
𝐎𝐛𝐣𝐞𝐜𝐭 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞
𝐪
𝐩
USES OF SPHERICAL MIRROR:
1. Use in shaving mirror:
A concave mirror produces an erect and magnified image of an object when place with in the focal length
of mirror. For this reason they are use as shaving mirror.
2. Use in Ophthalmic-scope:
Doctors use concave mirror for the examination of ear, nose throat and eyes of the patients.
3. Use in Microscope:
Concave mirror are used to throw light on the slides in a microscope so that the slides can be viewed more
easily.
4. Use in Search lights and head lights:
Concave mirror are used to focus light in case of search lights and head lights of an automobile.
5. Use in Telescope:
The concave mirrors are used as objectives in big telescopes.
6. Use for rear view:
Due to diverging property of convex mirror its field of view increases. As such convex mirror are used in
automobiles rear view.
Great things are done more through courage, than through wisdom.
X-PHYSICS
CH 01
FAHAD AKHTER
MULTIPLE CHOICE QUESTIONS (MCQ’S)
1. If an object is placed between focus and pole of concave mirror the image is formed
i) Beyond center of curvature
ii) At focus
iii) At center of curvature iv) Behind the mirror
2.
When the ray of light travels from the one medium strikes the surface of another medium a part of its of
bounced back along the particular direction in the same medium, this process of bouncing back is
called
of light.
i) Reflection of light
ii) Refraction of Light
3. If q = 8cm and P = 2cm then the magnification will be
i) Two
ii) Zero
iii) Five
iv) Four
4.
If an object is placed between the focus and the center of curvature of the concave mirror the image
is formed
i) Beyond center of curvature
ii) At focus
iii) At center of curvature
5. If q = 4cm and P = 2cm then the magnification will be
i) Two
ii) Zero
iii) Five
iv) Four
6.
If an object is placed at the center of curvature of a concave mirror
&
formed.
i) Erect and enlarge
ii) Invert and small
iii) Inverted and equal
image is
7.
All the rays, parallel to the principle axis falling on the concave mirror passed after the reflection
through it
i) Pole
ii) Principle focus
iii) Center of curvature
8. The ratio of size of the image to the size of the object is called
i) Magnification
ii) Power of lens
CH 01
PAST TEN YEARS QUESTIONS
(2003)
Q: Write down the two of difference between the following: I) Real image and Virtual image.
Q: Show the position, nature and size of the image with the help of ray diagram when an object is placed in
front of a concave mirror. I) At center of curvature. II) Between the center of curvature and the principle
focus.
Q: Write down four characteristics of the image formed by a plane mirror?
(2004)
Q: define regular and irregular reflection of light and write down the two advantages of irregular reflection
of light in practical life.
Q: Write four uses of spherical mirror?
Q: The focal length of the concave mirror is 20 cm where should an object be placed so as to get its real
image magnified four times.
(2005)
Q: Draw the image formed by a convex mirror. Describe its characteristics (the object may be placed at
any point).
Q: Why are concave mirror wad in the head lights of an automobile? [Scientific reason]
Q: Define the following I) Pole II) Magnification
Q: Draw the ray diagram of the image formed by a plane mirror and writes its characteristics.
(BLANK): The image formed by a convex mirror is always virtual, erect and smaller than the object itself.
Q: The focal length of concave mirror is 10 cm where an object should be placed so as to get its real image
magnified twice.
X-PHYSICS
FAHAD AKHTER
(2006)
Q: Define the following: I) Focal length.
Q: An object is placed at a distance of 15 cm from a concave mirror of focal length 10 cm. find the position
and nature of the image formed.
Q: Write two points of difference between the following: I) Real image and Virtual image.
(2008)
Q: A body is kept at a distance of 10 cm from a concave mirror. The radius of curvature of the mirror is 10
cm, find the position and the nature of the image.
Q: Define the following: I) Focal length
Q: Write two points of difference between of the following: I) Real image and Virtual image.
Q: With the help of ray diagram show the nature, size and position of the image formed in a concave mirror
when:
I) The object is place beyond ‘C’.
II) The object is place between C and f.
(2009)
Q: Draw a ray diagram of the image formed by a plane mirror and writes its three characteristics.
Q: Define the following: I) Regular reflection.
(BLANK): Magnification = q/P.
(2010)
Q: Define the following: I) Regular reflection. II) Radius of curvature III) Magnification of concave mirror.
Q: Draw ray diagram only to show the nature position and size of the image formed by a concave mirror
when the object is placed:
I) At the center of curvature of the mirror.
II) Between the center of curvature and the principle focus.
(2011)
Q: Define the following I) Magnification II) Focal length of concave mirror.
Q: Draw ray diagram for image formed by a concave mirror and write its characteristics.
(2012)
Q: Write three uses of concave mirror.
Q: The radius of curvature of concave mirror is 20cm. where should an object be placed so as to get its real
image magnified twice.
Q: Write down three characteristics of the image formed by a plane mirror.