Light and Sound

Subject: General Science — Physics | Topic: Light and Sound Exam: AP Group 2 (APPSC) | Sources: NCERT Class X, Lucent's GS

Introduction

Light and Sound together form one of the highest-yield physics topics for APPSC. Expect 2-3 questions covering mirror/lens types, eye defects, and sound properties. The mirror formula and lens types are perennial favourites.

Nature of Light

• Light is an electromagnetic wave travelling at 3 × 10⁸ m/s in vacuum (speed of light, c).
• Light travels in straight lines (rectilinear propagation) — causes shadows and eclipses.
• Light does NOT need a medium to travel (unlike sound).

Reflection of Light

Laws of Reflection:

1. Angle of incidence = Angle of reflection
2. Incident ray, reflected ray, and normal lie in the same plane

Mirror Formula: 1/v + 1/u = 1/f

Magnification: m = −v/u = h'/h

Refraction of Light

Refraction is the bending of light when passing from one medium to another due to change in speed.

Snell's Law: n₁ sin(i) = n₂ sin(r)

Refractive index n = speed of light in vacuum / speed in medium.

• Light bends toward normal when entering a denser medium, away when entering rarer medium.
• Total Internal Reflection (TIR): When light travels from denser to rarer medium at angle > critical angle, all light reflects back. Applications: optical fibres, mirages, diamond brilliance.

Lenses

Lens Formula: 1/v − 1/u = 1/f

Power of a lens: P = 1/f (in metres). SI unit: Dioptre (D). Convex: positive power. Concave: negative power.

Dispersion and Scattering

• White light splits into 7 colours through a prism: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
• Violet deviates most, red deviates least.
• Rainbow: Formed by dispersion, refraction, and TIR in water droplets.
• Sky appears blue: Rayleigh scattering of shorter wavelengths (blue light).
• Sun appears red at sunrise/sunset: Blue light scattered away over long atmospheric path.
• Tyndall effect: Scattering by colloidal particles (light beam visible in dusty room).

Human Eye and Defects

• The eye lens is a convex lens forming a real, inverted image on the retina.
• Accommodation: Eye lens changes focal length by adjusting curvature (ciliary muscles).
• Near point: 25 cm. Far point: Infinity (normal eye).

Sound

• Audible range: 20 Hz to 20,000 Hz. Below 20 Hz: infrasound. Above 20,000 Hz: ultrasound.
• Echo: Reflected sound heard distinctly. Minimum distance: 17.2 m from reflecting surface.
• SONAR: Uses ultrasound for underwater detection. Distance = (speed × time) / 2.
• Doppler Effect: Apparent frequency change with relative motion. Pitch increases when approaching, decreases when receding.
• Ultrasound applications: Medical imaging (sonography), cleaning, SONAR, crack detection.

v = f × λ (velocity = frequency × wavelength)

Likely Exam Questions

1. The speed of light in vacuum is: Ans: 3 × 10⁸ m/s

2. The refractive index of diamond is: Ans: 2.42

3. Myopia is corrected using: Ans: Concave lens

4. The SI unit of power of a lens is: Ans: Dioptre (D)

5. The colour that deviates most through a prism is: Ans: Violet

6. Sound travels fastest in: Ans: Solids (e.g., steel at ~5,000 m/s)

7. The human audible frequency range is: Ans: 20 Hz to 20,000 Hz

8. SONAR uses which type of sound waves? Ans: Ultrasound (frequency > 20,000 Hz)

9. The minimum distance for an echo to be heard is: Ans: 17.2 metres

10. The sky appears blue due to: Ans: Rayleigh scattering of shorter wavelengths (blue light)