Class 10 : Light - Reflection and Refraction - Notes

Light – Reflection and Refraction
Complete Class 10 CBSE Notes & Questions

1. Introduction to Light

• Light is a form of energy enabling vision; travels in straight lines in uniform media.
• Dual nature: Behaves as both wave and particle.
• Speed of light: 3 × 10⁸ m/s in air/vacuum.

2. Reflection of Light

A. Types of Reflection

• Regular: From smooth and shiny surfaces (mirrors).
• Diffuse: From rough surfaces (paper, wall).

B. Laws of Reflection

• Angle of incidence equals angle of reflection.
• Incident ray, reflected ray, and normal all lie in one plane.

C. Plane Mirror Features

Feature	Description
Nature of Image	Virtual, erect
Image Size	Equal to object
Image Distance	Same behind as object is in front
Lateral Inversion	Yes
Min. Mirror Height	Half the object's height for full image

Key: Mirror must be at least half your height to see your full image, regardless of distance from mirror!

3. Spherical Mirrors

A. Types & Structure

• Concave (Converging): Caves inward, focuses rays.
• Convex (Diverging): Bulges outward, spreads rays.

Term	Symbol	Meaning
Pole	P	Center of mirror’s face
Center of Curvature	C	Center of hypothetical sphere
Radius of Curvature	R	Distance PC
Principal Focus	F	Where reflected rays meet/diverge
Focal Length	f	PF (R = 2f)
Aperture	–	Size/diameter of reflecting surface

B. Image by Concave Mirror

Object Position	Image Position	Nature	Size
At infinity	At Focus (F)	Real, inverted	Point-sized
Beyond C	Between F & C	Real, inverted	Diminished
At C	At C	Real, inverted	Same
Between C, F	Beyond C	Real, inverted	Enlarged
At F	At infinity	Real, inverted	Highly enlarged
Between P, F	Behind mirror	Virtual, erect	Enlarged

C. Image by Convex Mirror

Position	Image Position	Nature	Size
At infinity	At Focus (behind)	Virtual, erect	Highly diminished
Anywhere in front	Between P & F (behind)	Virtual, erect	Diminished

Full image of a large object? Only a convex mirror can always show the entire image, regardless of object size or position; the image is diminished. A concave mirror does this only when object is at C (center of curvature).

D. Uses of Mirrors

Mirror Type	Uses
Concave	Headlights, solar cookers, shaving/dentist mirrors, telescopes
Convex	Vehicle rear-view, security mirrors (shops, ATMs), road corners

4. Sign Convention & Mirror Formulas

• Left of pole (object side): Negative distance
• Right of pole (behind mirror): Positive distance
• Height above axis: Positive, below axis: Negative

Mirror Formula: 1/v + 1/u = 1/f
Magnification: m = -v/u

5. Refraction of Light

• Refraction: Light bends at an interface due to speed change.
• Laws (Snell’s Law):
  • Incident, refracted ray, normal in same plane
  • sin i / sin r = n₂₁

Medium	Refractive Index
Air	1.00
Water	1.33
Kerosene	1.44
Glass	1.5
Diamond	2.42

• Enters denser: bends toward normal (speeds slows)
• Enters rarer: bends away (speeds up)
• No refraction for perpendicular (normal) incidence
• Lateral Displacement: Sideways shift in parallel-sided slabs
• Optical density ≠ Mass density: Optical density depends on how light interacts with atomic/molecular structure (e.g., electron density), while mass density depends on atomic mass and packing. Example: Kerosene (hydrocarbons) has tightly bound electrons → higher refractive index despite lower mass density.
• Kerosene is more optically dense than water (slows light more), but water is physically heavier. This shows that optical and mass density are independent properties.

6. Spherical Lenses

A. Types

Lens Type	Shape	Nature	Focal Length
Convex	Thicker at center	Converging	Positive
Concave	Thinner at center	Diverging	Negative

B. Image by Convex Lens

Object Position	Image Position	Nature	Size
At infinity	At F2	Real, inverted	Point
Beyond 2F1	Between F2 & 2F2	Real, inverted	Diminished
At 2F1	At 2F2	Real, inverted	Same
Between F1 & 2F1	Beyond 2F2	Real, inverted	Enlarged
At F1	At infinity	Real, inverted	Highly enlarged
Between F1 & O	Same side	Virtual, erect	Enlarged

Concave lenses: Always form virtual, erect, diminished images on same side as object.

Lens Formula: 1/v − 1/u = 1/f
Magnification: m = v/u

C. Power of Lens

• P = 1/f (f in meters)
• Unit: Dioptre (D); Convex: positive, Concave: negative
• Combined power: Add powers algebraically
Combined Power: $P=P_1+P_2+P_3+\dots$

7. Applications: Mirrors & Lenses

Device/Use	Type	Reason
Shaving/Dentist mirror	Concave	Magnifies, shows upright, close image
Car headlight/solar cooker	Concave	Focuses rays/energy
Rear-view/security mirror	Convex	Wide, erect view for safety
Magnifying glass	Convex lens	Magnifies small objects
Camera lens	Convex lens	Forms real, inverted image
Myopia correction	Concave lens	Diverges rays
Hypermetropia correction	Convex lens	Converges rays

8. More Key Points

• Critical angle & Total internal reflection: Occurs if angle exceeds critical (used in optical fibers, diamond sparkle).
• Optical vs mass density: High mass density does not always mean high refractive index.
• If half mirror/lens covered: Full image forms, but less bright.
• Full object image: Always with convex mirror; only at C for concave.
• Virtual image: Upright, not caught on screen.

9. Main Formulas (Quick List)

• Mirror: 1/v + 1/u = 1/f
• Lens: 1/v − 1/u = 1/f
• Magnification (mirror): m = -v/u
• Magnification (lens): m = v/u
• Power of lens: P = 1/f (in meters)
• R = 2f

10. Effect of Cutting Lenses and Immersing Them in Water

I. Cutting a Lens

A. Cutting Vertically (Along the Principal Axis – Top to Bottom)

• The lens is divided into two equal left and right parts.
• Each part becomes a plano-convex (or plano-concave) lens—one flat side, one curved side.
• Focal length becomes double the original lens.
• The image position stays the same, but the image becomes dimmer because less light enters.
• Both halves can still form the full image, but with reduced brightness.

B. Cutting Horizontally (Parallel to the Principal Axis – Upper and Lower Halves)

• The shape (curvature) of both surfaces remains the same in each half.
• Focal length remains unchanged.
• Image brightness decreases, but the image is formed at the same position as before.
• Only the upper or lower half of the lens contributes to image formation (less light, full image).

II. Effect of Immersing Lenses in Water

A. Change in Focal Length

• In air, lenses work strongly due to a large difference in refractive index (lens vs. air).
• In water, this difference reduces, so the focusing power of the lens decreases.
• Focal length increases; the lens becomes less powerful.
• Convex lens: Still converges rays, but less strongly.
• Concave lens: Still diverges rays, but less strongly.

B. Special Case – Similar Refractive Index (Lens & Medium)

• If the refractive index of lens and medium are nearly the same, the lens barely bends light.
• It may not form an image; focal length becomes very large or infinite (acts like plain glass).

C. Air Bubble in Water

• An air bubble in water acts like a concave (diverging) lens.
• This is because air is optically rarer than water, so light diverges when passing through the bubble.

11. Some Important Questions & Answers

What is light?

Light is energy that helps us see by traveling in straight lines in a uniform medium.

State the laws of reflection.

Angle of incidence equals angle of reflection. Incident ray, reflected ray, and normal lie in same plane.

What minimum height mirror is needed for a 1.5 m person to see full image?

0.75 m (half the person's height).

Which mirror always gives a full object image?

Convex mirror—forms diminished, complete image of any object size or position.

Why can't concave mirrors be used for vehicle rear-view?

They can produce inverted or magnified images, which are unsafe for driving.

Why does a pencil appear bent in water?

Due to refraction; light rays bend at water-air boundary because of speed change.

If half a lens is covered, will image be formed?

Yes, but image will be dimmer as fewer rays form it.

Two uses of convex lens?

Magnifying glass; correcting hypermetropia; camera lenses.

What is total internal reflection? Example?

When light is completely reflected within a denser medium (e.g., in optical fiber) if angle of incidence exceeds critical angle.

What happens if object distance increases in front of a convex lens?

Image moves closer to the focus and becomes smaller (size decreases).

How to calculate power of concave lens of focal length -0.5 m?

P = 1/f = 1/(-0.5) = -2 D (dioptres).

State one difference between reflection and refraction.

Reflection is bouncing back into same medium; refraction is bending while entering another medium.

A 2 m tall object is placed 4 m in front of a plane mirror. Distance between object and image?

8 m (4 m object-to-mirror + 4 m mirror-to-image).

When does concave mirror produce same-size image?

When object is placed at center of curvature (C).

Explain: "Convex mirrors are used in shops/security."

They show whole room, give wide field view; always an upright, reduced image so customers/staff can see much more area.

Why is coin at bottom of beaker of water seen raised?

Refraction bends rays; coin appears closer due to apparent depth.

Ray diagram for concave mirror when object between C and F?

Image is beyond C, real, inverted, enlarged. [Draw in exam for marks]

Does a convex mirror ever form a real image?

No, always virtual, diminished, erect images behind the mirror.

Under what conditions is the image by convex lens virtual?

When object is between focus (F) and the lens.

12. HOTS & Revision Questions

Why do convex mirrors always produce diminished images? How is this useful?

They spread reflected rays, making images smaller—useful for wide field in rear-view mirrors and security.

How does lens curvature relate to power?

More curvature = shorter focal length = higher power.

Light from glass (n=1.5) to water (n=1.33): Bends toward or away from normal?

Bends away from normal, as it enters a rarer medium.

Coin at bottom of water appears raised. Why?

Apparent depth effect from refraction—light rays bend away from normal as they emerge.

If glass slab is placed over printed text, why does it appear lifted?

Refraction and lateral displacement make text appear higher than its real position.

If an object is placed at 30 cm in front of a concave mirror with f = -10 cm, what's image position?

Using formula: 1/v + 1/(-30) = 1/(-10) ⇒ v = -15 cm (real, inverted, between F and C).

13. Revision Tips

• Draw neat ray diagrams for image-based questions.
• Always mention nature (real/virtual, erect/inverted) and size (enlarged/diminished) in answers.
• Use correct units in power and focal length calculations.
• Practice using formula with correct sign conventions.
• Remember object at C (concave mirror) = same-size real, inverted image; convex mirror = always small, upright image.

Memory aid: "Convex = Complete view; Concave = Can magnify/Concentrate!"