LIGHT REFLECTION AND REFRACTION
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Reflection of Light :-
The Bouncing back of light when it hits a polished surface like mirror.
Law of Reflection :-
- ∠i = ∠r Angle of incidence = Angle of reflection
- The incident ray, the reflected ray & the normal, all lie in the same plane.
Spherical Mirrors
Concave Mirror
Convex Mirror
Key Terms
- Pole (P)
- Centre of Curvature (C)
- Principal Axis (PA)
- Radius of Curvature (R)
Principal Focus (F) and Focal Length (f)
For spherical mirror: R = 2f
Concave (Converging)
Convex (Diverging)
Image Formation and Characteristics
At least two rays of light meet ⟹ Image
If rays of light actually meet ⟹ Real → ulti inverted ↓
If rays of light appear to meet ⟹ Virtual → Erect, Seedhi ↑↑
Image Formation : Concave Mirror – Rules :-
Parallel → through F
Through F → parallel
Through C → retraces
At Pole → equal angles
Image Formation for Concave Mirror :-
Object at ∞
- Image at Focus
- Real, Inverted, Highly Diminished
- Point-sized
Object Beyond C
- Image between C & F
- Real, Inverted, Diminished
Object at C (special)
- Image at C
- Real, Inverted, Same Size
Object Between C and F
- Image beyond C
- Real, Inverted, Magnified (Enlarged)
Object at F
- Image at Infinity
- Real, Inverted, Highly Magnified
Object Between F and P (v. special)
- Image behind mirror
- Virtual, Erect, Magnified
As object moves from ∞ → F (towards mirror), image moves from F → ∞ (away from mirror). Most magnified image forms when object is just beyond F.
Summary for Concave Mirror
| Position of Object | Position of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At infinity | At the focus F | Highly diminished, point-sized | Real and inverted |
| Beyond C | Between F and C | Diminished | Real and inverted |
| At C | At C | Same size | Real and inverted |
| Between C and F | Beyond C | Enlarged | Real and inverted |
| At F | At infinity | Highly enlarged | Real and inverted |
| Between P and F | Behind the mirror | Enlarged | Virtual and erect |
Use of Concave Mirror :-
- Car Headlight
- Torch
- Dentist mirror
- Shaving Mirror
- Solar Furnace → To concentrate sunlight to produce heat → Object at ∞
Solar Furnace
Image Formation : Convex Mirror – Rules
Parallel → appears from F
Aimed at F → parallel
At Pole → equal angles
Aimed at C → parallel
Image Formation of Convex Mirror → Seedhi choti image
Object at finite distance (anywhere except ∞)
- Image between F and P
- Virtual, Erect (Upright), Diminished
Object at infinity ∞
- Image at F
- Virtual, Erect, Highly Diminished (Point-sized)
Summary – Convex Mirror
| Position of Object | Position of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At infinity | At the focus F | Highly diminished, point-sized | Virtual and erect |
| Between infinity and pole P | Between F and P | Diminished | Virtual and erect |
Use of Convex Mirror :-
Rear View Mirrors → Seedhi choti convex
Reasons:
- Upright / Erect image
- Wider field of view
CBSE 2025List four properties of images formed by convex mirrors:
- Virtual Image
- Upright / Erect Image
- Diminished Image
- On other side of mirror w.r.t. object (Between F & P)
Summary – Concave and Convex Mirror
Concave Mirror
Inverted ↓↓
Erect/Upright (Enlarged) → Seedhi + Badi image
Convex Mirror
Only erect and virtual image
Always diminished → Seedhi + Choti image
Sign Convention
- All distances are measured from Pole (P).
- +x axis → direction of incident light
- −x axis → opposite direction
- h = +ve → above Principal Axis
- h = −ve → below Principal Axis
Mirror Formula
1/f = 1/v + 1/u
f = focal length | u = object distance | v = image distance
Note: u → always −ve Convex → f → +ve
m = hi/ho = −v/u
hi = height of image | ho = height of object
Magnification (m) and Nature of Image :-
m = 2
hi = 2×ho ↑ twice, erect
m = 1/2
hi = ½×ho ↑ half, erect
m = −2
hi = −2×ho ↓ twice, inverted
m = −1/2
hi = −½×ho ↓ half, inverted
Spherical Lens :-
Convex Lens (Thick in middle)
Concave Lens (Thin in middle)
Optical Centre (O) → Ray through O goes undeviated
Principal Focus (F) and Focal Length (f) – Lens
Convex Lens (Converging)
Note: Two F — F₁ and F₂ due to two curved surfaces.
Concave Lens (Diverging)
Image Formation (Convex Lens) Rules
Parallel → F₂
Through O → undeviated
Through F₁ → parallel
Image Formation for Convex Lens
Object at ∞
- Image at F₂
- Real, Inverted, Highly Diminished
Object Beyond 2F₁
- Image between F₂ and 2F₂
- Real, Inverted, Diminished
Object at 2F₁
- Image at 2F₂
- Real, Inverted, Same Size
Object Between 2F₁ and F₁
- Image beyond 2F₂
- Real, Inverted, Magnified
Object at F₁
- Image at ∞
- Real, Inverted, Highly Magnified
Object Between F₁ and O
- Image on same side as object
- Virtual, Erect, Magnified
Convex lens → Real Inverted image; Erect when object between F₁ and O → most magnified image
Image Formation Summary (Convex Lens)
| Position of Object | Position of Image | Relative Size | Nature |
|---|---|---|---|
| At infinity | At focus F₂ | Highly diminished, point-sized | Real and inverted |
| Beyond 2F₁ | Between F₂ and 2F₂ | Diminished | Real and inverted |
| At 2F₁ | At 2F₂ | Same size | Real and inverted |
| Between F₁ and 2F₁ | Beyond 2F₂ | Enlarged | Real and inverted |
| At focus F₁ | At infinity | Infinitely large / highly enlarged | Real and inverted |
| Between F₁ and O | Same side as object | Enlarged | Virtual and erect |
Light - Lens and Refraction (Complete Notes)
Image Formation by Concave Lens
A concave lens always forms:
- Virtual Image
- Erect Image
- Diminished Image
Animated Concave Lens Diagram
Object at Infinity
- Image forms at Focus (F)
- Highly diminished
- Virtual and erect
Object at Finite Distance
- Image forms between F and Optical Centre
- Diminished
- Virtual
Summary of Convex and Concave Lens
| Lens Type | Image Nature | Size |
|---|---|---|
| Convex Lens | Real / Inverted (mostly) | Magnified or Diminished |
| Concave Lens | Virtual / Erect | Diminished |
Lens Formula
1/f = 1/v − 1/u
Where:
f = focal length
v = image distance
u = object distance
f = focal length
v = image distance
u = object distance
Magnification
m = v / u
Also:
m = height of image / height of object
Power of Lens
P = 1 / f
Unit = Dioptre (D)
Convex Lens → Positive Power
Concave Lens → Negative Power
Convex Lens → Positive Power
Concave Lens → Negative Power
Refraction of Light
Refraction is the bending of light when it travels from one medium to another medium due to change in speed of light.
Animated Refraction Diagram
When light goes from:
- Rarer → Denser → bends towards normal
- Denser → Rarer → bends away from normal
Refractive Index
n = c / v
c = Speed of light in vacuum
v = Speed of light in medium
v = Speed of light in medium
Snell's Law
sin i / sin r = constant
This constant is called refractive index.
Refraction Through Glass Slab
Animated Glass Slab Diagram
Important Points:
- Emergent ray is parallel to incident ray
- Angle of incidence = Angle of emergence
Case of No Refraction
1. Normal Incidence
i = 0°
r = 0°
2. No change in refractive index
(No medium change)
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