INTRODUCTION — HOW DO WE SEE OBJECTS?
When you enter a completely dark room, you cannot see
anything. But the moment light enters the room, everything becomes visible.
Why does this happen?
Objects become visible because light falls on them and gets
reflected into our eyes. Our eyes receive this reflected light and send signals
to the brain, helping us see objects.
Light usually travels in a straight-line path, called
a ray of light.
Examples around us:
- Formation
of shadows
- Rainbow
colours
- Image
in mirrors
- Twinkling
of stars
- Magnifying
glass
REFLECTION OF LIGHT
The bouncing back of light after striking a surface is
called:
Reflection of Light
Highly polished surfaces like mirrors reflect most of the
light falling on them.
LAWS OF REFLECTION
There are two laws:
1. Angle of incidence = Angle of reflection
i = r
2. Incident ray, reflected ray, and normal lie in the
same plane.
These laws apply to:
- Plane
mirrors
- Concave
mirrors
- Convex
mirrors
IMAGE FORMED BY PLANE MIRROR
Properties:
✔ Virtual and erect
✔ Same size as object
✔ Same distance behind mirror as object is in front
✔ Laterally inverted
ACTIVITY 9.1 — Shining Spoon Experiment
Take a shining spoon and observe your face.
Observation:
Inner curved surface:
- Behaves
like concave mirror
Outer curved surface:
- Behaves
like convex mirror
SPHERICAL MIRRORS
Mirrors whose reflecting surface forms a part of a sphere
are called:
Spherical Mirrors
Two types:
Concave mirror
Reflecting surface curves inward.
Convex mirror
Reflecting surface curves outward.
IMPORTANT TERMS RELATED TO SPHERICAL MIRRORS
Pole (P)
Centre of mirror surface.
Centre of Curvature (C)
Centre of sphere of which mirror is a part.
Radius of Curvature (R)
Distance between pole and centre of curvature.
Principal Axis
Straight line joining pole and centre of curvature.
Principal Focus (F)
Point where parallel rays meet or appear to meet after
reflection.
Focal Length (f)
Distance between pole and focus.
Relationship:
ACTIVITY 9.2 — Finding Focus of Concave Mirror
Steps:
- Hold
concave mirror facing Sun.
- Reflect
sunlight onto paper.
- Move
paper until a bright sharp spot appears.
Observation:
- Paper
may start burning.
Reason:
- Sunlight
converges at one point producing heat.
Conclusion:
That point is the principal focus.
IMAGE FORMATION BY CONCAVE MIRROR
|
Object Position |
Image Position |
Size |
Nature |
|
At infinity |
At F |
Highly diminished |
Real, inverted |
|
Beyond C |
Between F and C |
Diminished |
Real, inverted |
|
At C |
At C |
Same size |
Real, inverted |
|
Between C and F |
Beyond C |
Enlarged |
Real, inverted |
|
At F |
At infinity |
Highly enlarged |
Real |
|
Between P and F |
Behind mirror |
Enlarged |
Virtual, erect |
USES OF CONCAVE MIRRORS
Examples from textbook:
✔ Torches
✔ Searchlights
✔ Vehicle headlights
✔ Shaving mirrors
✔ Dentist mirrors
✔ Solar furnaces
IMAGE FORMATION BY CONVEX MIRROR
|
Object Position |
Image Position |
Size |
Nature |
|
At infinity |
At F behind mirror |
Highly diminished |
Virtual, erect |
|
Anywhere in front |
Between P and F |
Diminished |
Virtual, erect |
USES OF CONVEX MIRRORS
Rear-view mirrors in vehicles
Reason:
✔ Wider field of view
✔ Always forms erect image
✔ Forms diminished image
NEW CARTESIAN SIGN CONVENTION
Rules:
- All
distances measured from Pole (P)
- Distances
towards right → Positive
- Distances
towards left → Negative
- Above
principal axis → Positive
- Below
principal axis → Negative
MIRROR FORMULA
Where:
- u =
object distance
- v =
image distance
- f =
focal length
MAGNIFICATION OF MIRROR
Where:
- h′
= image height
- h
= object height
Positive magnification:
- Virtual
image
Negative magnification:
- Real
image
REFRACTION OF LIGHT
When light travels from one transparent medium to another,
its path changes.
This bending of light is called:
Refraction of Light
Examples from textbook:
✔ Pencil appears bent in water
✔ Coin appears raised
✔ Letters beneath glass appear raised
✔ Lemon appears larger in water
WHY DOES REFRACTION OCCUR?
Refraction occurs because:
Speed of light changes in different media
LAWS OF REFRACTION (SNELL'S LAW)
First Law
Incident ray, refracted ray and normal lie in same plane.
Second Law
This constant is:
Refractive Index
REFRACTIVE INDEX
Absolute refractive index:
Where:
- c
= speed of light in vacuum
- v
= speed in medium
Speed of light in vacuum:
EXAMPLES OF REFRACTIVE INDEX
|
Medium |
Refractive Index |
|
Air |
1.0003 |
|
Water |
1.33 |
|
Kerosene |
1.44 |
|
Glass |
1.52 |
|
Diamond |
2.42 |
Observation:
Diamond has highest refractive index among these examples.
SPHERICAL LENSES
A transparent medium bounded by two surfaces, at least one
spherical, is called:
Lens
Two types:
Convex Lens
- Thicker
at centre
- Converges
light
- Also
called converging lens
Concave Lens
- Thinner
at centre
- Diverges
light
- Also
called diverging lens
ACTIVITY 9.11 — Convex Lens and Sunlight
Observation:
Paper begins to burn after focusing sunlight.
Reason:
Convex lens concentrates sunlight at one point.
IMAGE FORMATION BY CONVEX LENS
|
Object Position |
Image Position |
Nature |
|
At infinity |
At F₂ |
Real, inverted |
|
Beyond 2F₁ |
Between F₂ and 2F₂ |
Real |
|
At 2F₁ |
At 2F₂ |
Same size |
|
Between F₁ and 2F₁ |
Beyond 2F₂ |
Enlarged |
|
Between F₁ and O |
Same side as object |
Virtual, erect |
INTRODUCTION — HOW DO WE SEE OBJECTS?
When you enter a completely dark room, you cannot see
anything. But the moment light enters the room, everything becomes visible.
Why does this happen?
Objects become visible because light falls on them and gets
reflected into our eyes. Our eyes receive this reflected light and send signals
to the brain, helping us see objects.
Light usually travels in a straight-line path, called
a ray of light.
Examples around us:
- Formation
of shadows
- Rainbow
colours
- Image
in mirrors
- Twinkling
of stars
- Magnifying
glass
REFLECTION OF LIGHT
The bouncing back of light after striking a surface is
called:
Reflection of Light
Highly polished surfaces like mirrors reflect most of the
light falling on them.
LAWS OF REFLECTION
There are two laws:
1. Angle of incidence = Angle of reflection
i = r
2. Incident ray, reflected ray, and normal lie in the
same plane.
These laws apply to:
- Plane
mirrors
- Concave
mirrors
- Convex
mirrors
IMAGE FORMED BY PLANE MIRROR
Properties:
ACTIVITY 9.1 — Shining Spoon Experiment
Take a shining spoon and observe your face.
Observation:
Inner curved surface:
- Behaves
like concave mirror
Outer curved surface:
- Behaves
like convex mirror
SPHERICAL MIRRORS
Mirrors whose reflecting surface forms a part of a sphere
are called:
Spherical Mirrors
Two types:
Concave mirror
Reflecting surface curves inward.
Convex mirror
Reflecting surface curves outward.
IMPORTANT TERMS RELATED TO SPHERICAL MIRRORS
Pole (P)
Centre of mirror surface.
Centre of Curvature (C)
Centre of sphere of which mirror is a part.
Radius of Curvature (R)
Distance between pole and centre of curvature.
Principal Axis
Straight line joining pole and centre of curvature.
Principal Focus (F)
Point where parallel rays meet or appear to meet after
reflection.
Focal Length (f)
Distance between pole and focus.
Relationship:
ACTIVITY 9.2 — Finding Focus of Concave Mirror
Steps:
- Hold
concave mirror facing Sun.
- Reflect
sunlight onto paper.
- Move
paper until a bright sharp spot appears.
Observation:
- Paper
may start burning.
Reason:
- Sunlight
converges at one point producing heat.
Conclusion:
That point is the principal focus.
IMAGE FORMATION BY CONCAVE MIRROR
|
Object Position |
Image Position |
Size |
Nature |
|
At infinity |
At F |
Highly diminished |
Real, inverted |
|
Beyond C |
Between F and C |
Diminished |
Real, inverted |
|
At C |
At C |
Same size |
Real, inverted |
|
Between C and F |
Beyond C |
Enlarged |
Real, inverted |
|
At F |
At infinity |
Highly enlarged |
Real |
|
Between P and F |
Behind mirror |
Enlarged |
Virtual, erect |
USES OF CONCAVE MIRRORS
Examples from textbook:
IMAGE FORMATION BY CONVEX MIRROR
|
Object Position |
Image Position |
Size |
Nature |
|
At infinity |
At F behind mirror |
Highly diminished |
Virtual, erect |
|
Anywhere in front |
Between P and F |
Diminished |
Virtual, erect |
USES OF CONVEX MIRRORS
Rear-view mirrors in vehicles
Reason:
NEW CARTESIAN SIGN CONVENTION
Rules:
- All
distances measured from Pole (P)
- Distances
towards right → Positive
- Distances
towards left → Negative
- Above
principal axis → Positive
- Below
principal axis → Negative
MIRROR FORMULA
Where:
- u =
object distance
- v =
image distance
- f =
focal length
MAGNIFICATION OF MIRROR
Where:
- h′
= image height
- h
= object height
Positive magnification:
- Virtual
image
Negative magnification:
- Real
image
REFRACTION OF LIGHT
When light travels from one transparent medium to another,
its path changes.
This bending of light is called:
Refraction of Light
Examples from textbook:
WHY DOES REFRACTION OCCUR?
Refraction occurs because:
Speed of light changes in different media
LAWS OF REFRACTION (SNELL'S LAW)
First Law
Incident ray, refracted ray and normal lie in same plane.
Second Law
This constant is:
Refractive Index
REFRACTIVE INDEX
Absolute refractive index:
Where:
- c
= speed of light in vacuum
- v
= speed in medium
Speed of light in vacuum:
EXAMPLES OF REFRACTIVE INDEX
|
Medium |
Refractive Index |
|
Air |
1.0003 |
|
Water |
1.33 |
|
Kerosene |
1.44 |
|
Glass |
1.52 |
|
Diamond |
2.42 |
Observation:
Diamond has highest refractive index among these examples.
SPHERICAL LENSES
A transparent medium bounded by two surfaces, at least one
spherical, is called:
Lens
Two types:
Convex Lens
- Thicker
at centre
- Converges
light
- Also
called converging lens
Concave Lens
- Thinner
at centre
- Diverges
light
- Also
called diverging lens
ACTIVITY 9.11 — Convex Lens and Sunlight
Observation:
Paper begins to burn after focusing sunlight.
Reason:
Convex lens concentrates sunlight at one point.
IMAGE FORMATION BY CONVEX LENS
|
Object Position |
Image Position |
Nature |
|
At infinity |
At F₂ |
Real, inverted |
|
Beyond 2F₁ |
Between F₂ and 2F₂ |
Real |
|
At 2F₁ |
At 2F₂ |
Same size |
|
Between F₁ and 2F₁ |
Beyond 2F₂ |
Enlarged |
|
Between F₁ and O |
Same side as object |
Virtual, erect |
IMAGE FORMATION BY CONCAVE LENS
A concave lens always forms:
irrespective of object position.
IMPORTANT RAYS FOR LENSES
- Parallel
ray → passes through focus
- Ray
through focus → emerges parallel
- Ray
through optical centre → no deviation
IMAGE FORMATION BY CONCAVE LENS
A concave lens always forms:
✔ Virtual image
✔ Erect image
✔ Diminished image
irrespective of object position.
IMPORTANT RAYS FOR LENSES
- Parallel
ray → passes through focus
- Ray
through focus → emerges parallel
- Ray
through optical centre → no deviation
