Class 10 science chapter 10 Wave and optics

1. Introduction to Wave and Light

Waves are disturbances that transmit energy through a medium (like water or air) or no medium (like light in space).
Light is an electromagnetic wave, meaning it can travel through a vacuum and doesn’t require a medium.

2. Denser Medium vs Rarer Medium

Denser medium: where light slows down (e.g., glass, water)
Rarer medium: where light speeds up (e.g., air, vacuum)
This change in speed causes refraction (bending) of light.

3. Refraction of Light

3.1 Causes of Refraction

Light travels slower in denser media (lower speed) → it bends toward the normal.
In rarer media, it bends away from the normal.

3.2 Refraction through a Glass Slab

A light ray entering the slab bends toward the normal.
Upon exiting, it bends away from the normal, emerging parallel to the incident ray but displaced sideways.

3.3 Terminology Related to Refraction

Incident Ray: Incoming light
Angle of Incidence (i): Between incident ray & normal
Refracted Ray: Ray inside new medium
Angle of Refraction (r): Between refracted ray & normal
Normal: Perpendicular at point of incidence
Emergent Ray: Exits the medium
Emergent Angle (e): Between emergent ray & normal

3.4 Laws of Refraction (Snell’s Laws)

Incident ray, refracted ray, and normal lie in the same plane.
Snell’s Law (Refraction)
sin i / sin r = constant = n

Where:

i = Angle of incidence
r = Angle of refraction
n = Refractive index of the medium

Refractive Index (n)

The refractive index is the ratio of the speed of light in air to its speed in a medium, or the ratio of the sine of angles of incidence and refraction:

n = sin i / sin r = v_air / v_medium

Where:

i = Angle of incidence
r = Angle of refraction
v_air = Speed of light in air
v_medium = Speed of light in the medium

Typical values:

n (water) ≈ 1.33
n (glass) ≈ 1.5

5. Consequences of Refraction

A) At Water–Air Interface

Apparent depth: Objects look shallower when underwater.
Stick-in-water effect: A stick appears bent at water’s surface.

B) Atmospheric Refraction

Sunrise/Sunset: Visible even when sun below horizon due to bending.
Twinkling of stars: Caused by air layers bending light randomly.
Looming: Distant objects appear floating or higher.
Distorted sunsets/moonrises: Appear flattened or stretched.
Daylight stars/planets: Rarely seen due to extreme refraction at horizon.
Improved visibility: Refraction over heated ground creates mirages.

6. Total Internal Reflection (TIR) & Critical Angle

Critical angle: Minimum angle of incidence in denser medium above which TIR occurs.
Condition for Total Internal Reflection
- : Light must travel from a denser to a rarer medium, and the angle of incidence (i) must be greater than the critical angle (θ_c).
- i > θ_c
Only under this condition will total internal reflection occur.
Critical angle formula:

Critical Angle Formula

sin θ_c = n₂ / n₁

Where:

θ_c = Critical angle
n₁ = Refractive index of denser medium
n₂ = Refractive index of rarer medium

This formula is valid only when light travels from a denser to a rarer medium.

A) Consequences of TIR

Sparkling diamonds: Light trapped inside, then ejected through facets.
Bright air-bubble surfaces: TIR inside the bubble.
Mirages: Light reflects within warm air layers, showing false water.

B) Applications of TIR

Optical fibers:
- Structure: Transparent core, cladding with lower refractive index.
- Working: Light reflects inside core with TIR, travels long distances.
- Uses: Telecommunication (internet, cable TV), endoscopy in medicine, surgical imaging.
- Endoscopy Breakdown:
  - Uses optical fibers to carry light into the body.
  - Allows doctors to see internal organs.
  - Enables minimally invasive (keyhole) surgery.

7. Dispersion of Light

Dispersion: White light decomposes into colors (ROYGBIV) when passing through a prism.
Cause: Each color has a different refractive index → bends differently.
Example: Rainbows—sunlight disperses and reflects inside raindrops forming colored arc.

8. Lenses & Image Formation

8.1 Lens Terminology

Principal Axis: Central straight line through lens
Optical Centre (O): Point where light passes undeviated
Centre of Curvature: Centre of the original sphere of which lens is part
Principal Focus (F): Point where parallel incident rays converge (convex) or appear to diverge from (concave)
Focal Length (f): Distance from O to F

8.2 Types of Lenses

Convex (Converging): Thicker in middle
Concave (Diverging): Thicker at edges

8.3 Light-Ray Rules

Parallel ray → passes through focus
Focus-ray → emerges parallel
Centre-ray → passes straight

8.4 Image Formation by Convex Lens

Object Position	Image Position & Nature
At Infinity	At F, real, inverted, highly diminished
Beyond 2F	Between F & 2F, real, inverted, diminished
At 2F	At 2F, real, inverted, same size
Between F & 2F	Beyond 2F, real, inverted, enlarged
At F	No image (rays parallel)
Between F & O	Virtual, upright, enlarged

8.5 Concave Lens

Always produces virtual, upright, reduced image between lens and focus.

8.6 Lens Power

Power of a Lens

P = ¹⁄_{f (m)}

Where:
P = Power of the lens (in diopters)
f = Focal length (in meters)

Positive → Convex lens
Negative → Concave lens

9. Human Eye: Structure & Function

Cornea: Transparent front surface; aids focusing
Pupil: Adjustable aperture controls light entry
Lens: Focuses light on retina
Ciliary Muscles: Adjust lens shape (accommodation)
Retina: Contains photoreceptors (rods/cones)
Optic Nerve: Transmits images to brain

9.1 Understanding Accommodation

Distant vision: Lens flattens using ciliary relaxation
Near vision: Lens thickens using ciliary contraction

9.2 Far Point & Near Point

Far point (D): Farthest distance seen clearly (∞ in normal eye)
Near point: Closest distance seen clearly (25 cm typical)

10. Defects of Vision & Corrections

A) Myopia (Near-sighted)

Light focuses before retina
Caused by elongated eyeball or strong lens
Corrected with concave lens

B) Hypermetropia (Far-sighted)

Light focuses behind retina
Caused by short eyeball or weak lens
Corrected with convex lens

11. Alternatives to Spectacles

a) Contact Lenses

Thin lenses on cornea for vision correction
Advantages: More natural vision
Use: Cleanliness essential to avoid infection

b) Laser Eye Surgery

Reshapes cornea permanently
LASIK: Cuts flap, reshapes deeper layer
PRK: Reshapes surface layer
Pros: No glasses needed
Cons: Costly, slight risks of dry eyes/irregular vision

12. Other Eye Conditions

a) Cataract

Cloudy lens leading to blurred vision
Treated via surgical lens replacement

b) Night Blindness

Difficulty seeing in dim light
Caused by deficiency in vitamin A
Treatment: Dietary improvement (carrots, leafy vegetables)

c) Colour Blindness

Genetic inability to distinguish colors (often red-green)
No cure, but lenses and apps can help

d) Corneal Injuries

Corneal ulcer: infection on the cornea
Corneal edema: swelling
Keratoconus: cornea shape change
Corneal transplant: replacing damaged cornea

Interesting Facts

The cornea has no blood vessels; it gets oxygen directly from air.
LASIK reshaping takes under 10 minutes per eye.
A rainbow is a full circle; we usually see only the top half.
Optical fibers can transmit light signals across the world with <1% loss per kilometer.

Quick Revision Summary

Refraction: Light bends entering a denser/rarer medium
TIR: Full reflection inside dense medium; lens basis of fiber optics
Dispersion: White light splits into colors
Lens rules produce varied image types; power derived from focal length
Eye: adjusts via accommodation, can be corrected by lenses or surgery
Includes cataract, night blindness, colour blindness, and cornea issues

Common Mistakes

Confusing emergence and incident angles
Misidentifying virtual vs real rays in lens diagrams
Labeling defects incorrectly (e.g. using concave for hypermetropia)
Forgetting negative vs positive lens power
Misunderstanding accommodation process

also check out :- chapter 1 , chapter 2 , chapter 3 , chapter 4 , chapter 5