Light: Reflection & Refraction

Class 10 Science • Chapter 09 • Detailed Notes

1. Reflection of Light

Reflection is the phenomenon of bouncing back of light into the same medium when it strikes a smooth, polished surface (mirror).

Key Terms: Concave Mirror and Convex Lens

Figure 1.0: Key Terms � Concave Mirror (Left) & Convex Lens (Right)

Laws of Reflection

Law 1: The angle of incidence (∠i) is always equal to the angle of reflection (∠r). [∠i = ∠r]
Law 2: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

Note: Both angles are measured from the Normal (perpendicular to the surface at point of incidence), NOT from the surface.

Lateral Inversion & Plane Mirror Images

Lateral Inversion: In a plane mirror, the left side of the object appears as the right side of the image. This is why "AMBULANCE" is written in reverse on ambulances � so it reads correctly in rear-view mirrors.

Properties of image in a plane mirror:

Virtual and Erect
Same size as object
As far behind mirror as object is in front
Laterally Inverted

Spherical Mirrors � Key Terms

            Pole (P): Midpoint of reflecting surface of mirror.
Centre of Curvature (C): Centre of the hollow sphere of which the mirror is a part.
                
Radius of Curvature (R): Radius of the sphere; distance from P to C.
Principal Axis: Line through P and C.
Principal Focus (F): Point where parallel rays (from infinity) after reflection
                    meet (Concave) or appear to diverge from (Convex).
Key Relation: R = 2f (Radius of Curvature = 2 × Focal Length)

        

            Uses of Mirrors:

            Concave Mirror: Shaving/make-up mirrors (virtual, magnified), dentist's mirror, solar
            furnaces, headlights/torches (reflectors), reflecting telescopes.

            Convex Mirror: Rear-view mirrors in vehicles (wide field of view, always erect image),
            security mirrors in shops.
        

New Cartesian Sign Convention

Figure 1.1: New Cartesian Sign Convention for Mirrors

            All distances measured from Pole (P).
Distances in direction of incident light → Positive (+).
Distances opposite to incident light → Negative (−).
                
Heights above principal axis → Positive (+).
Heights below principal axis → Negative (−).
For mirrors: object always placed to the left → u is always negative.

        

Rules for Ray Tracing (Mirrors)

            A ray parallel to Principal Axis → after reflection, passes through Principal Focus (F).
A ray passing through Principal Focus (F) → after reflection, becomes parallel to Principal
                    Axis.
A ray passing through Centre of Curvature (C) → retraces its path (Normal incidence).
A ray obliquely incident at Pole (P) → reflects with equal angle (∠i = ∠r).

        

Figure 1.1b: Rules for Ray Tracing (Mirrors)

Image Formation by Concave Mirror

Figure 1.2: Image Formation by Concave Mirror

Object Position	Image Position	Size	Nature
At Infinity	At Focus F	Highly Diminished	Real & Inverted
Beyond C	Between F and C	Diminished	Real & Inverted
At C	At C	Same Size	Real & Inverted
Between F and C	Beyond C	Enlarged	Real & Inverted
At Focus F	At Infinity	Highly Enlarged	Real & Inverted
Between F and P	Behind mirror	Enlarged	Virtual & Erect

Image Formation by Convex Mirror

Figure 1.3: Image Formation by Convex Mirror

CONVEX MIRROR KEY FACT Convex mirror always forms a Virtual, Erect, and Diminished image � regardless of object position. This is why they are used as rear-view mirrors.

MIRROR FORMULA $$ \frac{1}{v} + \frac{1}{u} = \frac{1}{f} \quad \text{also: } f = \frac{R}{2} $$ $$ m = -\frac{v}{u} = \frac{h'}{h} $$ If m is (−) → Image is Real & Inverted.
If m is (+) → Image is Virtual & Erect.

2. Refraction of Light

Refraction is the bending of light when it passes from one transparent medium to another, due to the change in its speed.

Laws of Refraction (Snell's Law)

            The incident ray, the refracted ray, and the normal at the point of incidence all lie in the
                    same plane.
Snell's Law: The ratio of sin of angle of incidence to sin of angle of refraction
                    is constant for a given pair of media:
                    $$ \frac{\sin i}{\sin r} = n_{21} = \text{constant} $$
                

        

            Refractive Index (n) � Absolute:
            $$ n_m = \frac{c}{v} = \frac{\text{Speed in vacuum}}{\text{Speed in medium}} $$
            Key values: Water = 1.33  |  Glass = 1.5  |  Diamond = 2.42

            Higher n → optically denser → light bends more → more "brilliant".
        

$Refraction: Rarer to Denser and Denser to Rarer$ Figure 1.4a: Light Bending � Rarer to Denser & Denser to Rarer

            Direction of Bending:

            • Rarer to Denser (e.g., air to glass): Speed decreases → bends
            towards normal (∠r < ∠i).

            • Denser to Rarer (e.g., glass to air): Speed increases → bends
            away from normal (∠r > ∠i).
        

Refraction through a Glass Slab & Lateral Displacement

$Refraction through Glass Slab$ Figure 1.4b: Refraction through a Glass Slab

When light passes through a glass slab (two parallel faces), the emergent ray is parallel to the incident ray but is shifted sideways. This perpendicular shift is called Lateral Displacement.

It increases with: (1) Greater thickness of slab (2) Higher refractive index (3) Larger angle of incidence.

3. Lenses

Lens	Type	Focal Length (f)	Power (P)	Use
Convex	Converging	Positive (+)	Positive (+)	Magnifying glass, camera, spectacles for hypermetropia
Concave	Diverging	Negative (−)	Negative (−)	Spectacles for myopia, peepholes

Rules for Ray Tracing (Lenses)

            Rule 1: A ray parallel to Principal Axis → after refraction, passes through
                    F2 (Convex) or appears to come from F2 (Concave).
Rule 2: A ray through Optical Centre (O) → goes undeviated
                    (straight through without bending).
Rule 3: A ray through F1 (Convex) or directed towards F1
                    (Concave) → emerges parallel to Principal Axis.

        

Figure 1.5a: Rules for Ray Tracing (Lenses)

Image Formation by Convex Lens

Figure 1.5b: Image Formation by Convex Lens

Object Position	Image Position	Size	Nature
At Infinity	At Focus F₂	Highly Diminished	Real & Inverted
Beyond 2F₁	Between F₂ and 2F₂	Diminished	Real & Inverted
At 2F₁	At 2F₂	Same Size	Real & Inverted
Between F₁ and 2F₁	Beyond 2F₂	Enlarged	Real & Inverted
At Focus F₁	At Infinity	Highly Enlarged	Real & Inverted
Between F₁ and O	Same side as object	Enlarged	Virtual & Erect

Image Formation by Concave Lens

Figure 1.6: Image Formation by Concave Lens

Object Position	Image Position	Size	Nature
At Infinity	At Focus F₁ (same side)	Highly Diminished	Virtual & Erect
Between Infinity & O	Between F₁ and O (same side)	Diminished	Virtual & Erect

LENS FORMULA $$ \frac{1}{v} - \frac{1}{u} = \frac{1}{f} $$ $$ m = \frac{v}{u} \quad (\text{Note: positive sign, unlike mirror}) $$ $$ P = \frac{1}{f(\text{in metres})} \text{ Dioptre (D)} $$ Power of combined lenses: P_net = P₁ + P₂ + P₃ (algebraic sum)

MIRROR vs LENS FORMULA � CRITICAL DIFFERENCE Mirror: 1/v + 1/u = 1/f | m = −v/u
Lens: 1/v − 1/u = 1/f | m = +v/u
The sign difference in both the formula and magnification is the most common exam trap!