Light - Reflection
Introduction
Light is a form of energy that enables us to see objects. Reflection is the phenomenon where light bounces back from a surface. ICSE Class 9 covers reflection from plane mirrors and spherical mirrors.
Laws of Reflection
- The incident ray, reflected ray, and the normal at the point of incidence all lie in the same plane.
- The angle of incidence (i) equals the angle of reflection (r).
Terms:
- Incident ray: Light ray approaching the surface
- Reflected ray: Light ray leaving the surface
- Normal: Perpendicular line at the point of incidence
- Angle of incidence (i): Angle between incident ray and normal
- Angle of reflection (r): Angle between reflected ray and normal
Plane Mirror - Image Properties
| Property | Characteristic |
|---|---|
| Nature | Virtual and erect |
| Size | Same size as object |
| Distance | Image distance = Object distance |
| Lateral inversion | Left appears right and vice versa |
| Position | Behind the mirror |
Spherical Mirrors
A spherical mirror is a mirror with a curved reflecting surface that is part of a sphere.
Types
| Type | Reflecting Surface | Focal Length | Image Nature |
|---|---|---|---|
| Concave mirror | Inward (caves in) | Positive (f = R/2) | Real and virtual depending on distance |
| Convex mirror | Outward (bulges out) | Negative | Always virtual, diminished, erect |
Key Terms
- Pole (P): Centre of the mirror surface
- Centre of curvature (C): Centre of the sphere
- Radius of curvature (R): Distance from P to C
- Principal axis: Line joining P and C
- Focus (F): Point where parallel rays converge/diverges
- Focal length (f): Distance from P to F = R/2
Ray Diagrams
Concave Mirror (6 Cases)
| Object Position | Image Position | Nature | Size |
|---|---|---|---|
| At infinity | At F | Real, inverted | Highly diminished |
| Beyond C | Between C and F | Real, inverted | Diminished |
| At C | At C | Real, inverted | Same size |
| Between C and F | Beyond C | Real, inverted | Enlarged |
| At F | At infinity | Real, inverted | Highly enlarged |
| Between P and F | Behind mirror | Virtual, erect | Enlarged |
Convex Mirror (Always)
Image is always virtual, erect, and diminished, located between P and F behind the mirror.
Mirror Formula
1/f = 1/u + 1/v
Where:
- f = focal length
- u = object distance (always negative for real objects)
- v = image distance
Sign Convention:
- Distances measured from the pole
- Distances in front of mirror: negative
- Distances behind mirror: positive
- Heights above principal axis: positive
- Heights below principal axis: negative
Magnification
m = -v/u = h₁/h₀
Where m = magnification, h₁ = image height, h₀ = object height
| m value | Interpretation |
|---|---|
| m > 0 | Image is virtual and erect |
| m < 0 | Image is real and inverted |
| m | |
| m | |
| m |
Image is at 30 cm in front of mirror. m = -v/u = -(-30)/(-30) = -1 Image is real, inverted, and same size as object. </Solution> </ICSEExample>
Uses of Spherical Mirrors
| Mirror Type | Uses |
|---|---|
| Concave | Shaving mirrors, headlights, dental mirrors, solar furnaces, telescopes |
| Convex | Rear-view mirrors in vehicles, security mirrors in shops |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Confusing concave and convex | Concave: caves in (converging), Convex: bulges out (diverging) |
| Wrong sign convention | All distances from pole; in front = negative, behind = positive |
| f = R/2 formula sign | f = R/2 for concave; f = -R/2 for convex |
| Virtual images can be seen | Virtual images cannot be projected on a screen |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Ray diagrams for spherical mirrors | 4-5 marks | Very common |
| Mirror formula numericals | 4-5 marks | Very common |
| Plane mirror properties | 2-3 marks | Common |
| Uses of mirrors | 2-3 marks | Frequently asked |
Self-Test
Q1: State the two laws of reflection.
Q2: An object is placed 20 cm from a concave mirror of focal length 10 cm. Find the image position and magnification.
Q3: Why is a convex mirror used as a rear-view mirror in vehicles?
Q4: Define focal length and radius of curvature. What is the relationship between them?
Q5: An object 5 cm tall is placed 15 cm from a convex mirror of focal length 20 cm. Find the height of the image.
