Science: Light and Sound — Physics for PSC

Light and Sound are consistently tested in Kerala PSC Science sections. Questions focus on mirror/lens properties, optical phenomena, and sound-related concepts. Expect 2-3 questions per paper.

Nature of Light

Property	Details
Nature	Electromagnetic wave (also behaves as particle — photon); wave-particle duality
Speed in vacuum	3 x 10^8 m/s (approximately 3 lakh km/s)
Speed in medium	Slower than vacuum; slowest in denser medium
Does not require	Medium for propagation (travels through vacuum)
Discovered wave nature	Christian Huygens (wave theory); Thomas Young (double-slit experiment proved it)
Discovered particle nature	Albert Einstein (photoelectric effect, 1905)
Electromagnetic spectrum	Radio waves - Microwaves - Infrared - Visible light - UV - X-rays - Gamma rays (increasing frequency)

Reflection of Light

Law/Concept	Details
First Law	Incident ray, reflected ray, and normal all lie in the same plane
Second Law	Angle of incidence = Angle of reflection
Regular reflection	From smooth surfaces (mirror) — clear image
Diffuse/Irregular reflection	From rough surfaces — scattered light, no clear image

Mirrors

Types of Mirrors

Type	Shape	Uses
Plane Mirror	Flat	Dressing tables, periscopes; image is virtual, erect, same size, laterally inverted
Concave Mirror	Curves inward (converging)	Shaving mirrors, torch reflectors, headlights, dental mirrors, solar furnaces
Convex Mirror	Curves outward (diverging)	Rear-view mirrors in vehicles (wider field of view); image always virtual, erect, diminished

Concave Mirror — Image Formation

Object Position	Image Position	Nature of Image
At infinity	At Focus (F)	Real, inverted, point-sized
Beyond C (Centre of curvature)	Between F and C	Real, inverted, diminished
At C	At C	Real, inverted, same size
Between F and C	Beyond C	Real, inverted, magnified
At F	At infinity	Real, inverted, highly magnified
Between F and Pole	Behind mirror	Virtual, erect, magnified

Mirror Formula and Terms

Term	Symbol	Definition
Focal length	f	Distance from pole to focus (f = R/2)
Radius of curvature	R	Distance from pole to centre of curvature
Mirror formula	1/v + 1/u = 1/f	v = image distance, u = object distance
Magnification	m = -v/u	Negative = inverted; positive = erect

Refraction of Light

Concept	Details
Definition	Bending of light when it passes from one medium to another
Cause	Change in speed of light between media
Denser to rarer	Light bends away from normal (speed increases)
Rarer to denser	Light bends towards normal (speed decreases)
Snell’s Law	n1 sin(i) = n2 sin(r); where n = refractive index
Refractive index	Ratio of speed of light in vacuum to speed in medium (n = c/v)

Refractive Indices (approximate)

Medium	Refractive Index
Vacuum	1.00
Air	1.0003 (approximately 1)
Water	1.33
Glass	1.50 (varies by type)
Diamond	2.42

Phenomena Due to Refraction

Phenomenon	Explanation
Twinkling of stars	Atmospheric refraction — light passes through layers of different density
Apparent position of stars	Stars appear slightly higher than actual position due to atmospheric refraction
Early sunrise and delayed sunset	Sun visible about 2 minutes before actual sunrise due to atmospheric refraction
Pool appears shallow	Light bends away from normal as it exits water; bottom appears raised
Mirage	Total internal reflection of light in hot air layers near ground
Rainbow	Dispersion + internal reflection + refraction in water droplets
Broken pencil in water	Refraction at water-air interface

Total Internal Reflection

Concept	Details
Condition 1	Light must travel from denser to rarer medium
Condition 2	Angle of incidence must be greater than critical angle
Critical angle	Angle of incidence at which angle of refraction = 90 degrees
Applications	Optical fibre, diamond sparkle, mirage, prism binoculars
Diamond sparkle	High refractive index (2.42) means small critical angle (24.4 degrees) — more total internal reflection

Lenses

Type	Shape	Nature
Convex (Converging)	Thicker in middle	Converges light rays; used in magnifying glass, camera, human eye, microscope, telescope
Concave (Diverging)	Thinner in middle	Diverges light rays; used to correct myopia (short-sightedness); image always virtual, erect, diminished

Lens Formula

Formula	Details
Lens formula	1/v - 1/u = 1/f
Power of lens	P = 1/f (in metres); unit = Dioptre (D)
Convex lens power	Positive
Concave lens power	Negative
Magnification	m = v/u = h’/h

Dispersion of Light

Concept	Details
Definition	Splitting of white light into 7 colours by a prism
Colours (VIBGYOR)	Violet, Indigo, Blue, Green, Yellow, Orange, Red
Most deviated	Violet (shortest wavelength, highest frequency)
Least deviated	Red (longest wavelength, lowest frequency)
Rainbow	Natural dispersion; primary rainbow — red outside, violet inside; secondary — reversed
Recombination	Newton’s disc (spinning disc with 7 colours appears white)

Defects of Vision

Defect	Problem	Corrected By
Myopia (Short-sightedness)	Cannot see distant objects; image forms before retina	Concave lens
Hypermetropia (Long-sightedness)	Cannot see near objects; image forms behind retina	Convex lens
Presbyopia	Age-related; cannot see near objects	Bifocal lens
Astigmatism	Uneven cornea curvature; blurred vision	Cylindrical lens

Sound

Nature of Sound

Property	Details
Type	Mechanical wave (requires medium)
Nature	Longitudinal wave (compressions and rarefactions)
Cannot travel through	Vacuum
Speed in air (20 C)	343 m/s (approximately)
Speed order	Solid (fastest) then Liquid then Gas (slowest)
Speed in water	About 1500 m/s
Speed in steel	About 5000 m/s
Frequency range (human)	20 Hz to 20,000 Hz (audible range)

Properties of Sound Waves

Property	Details
Pitch	Related to frequency — higher frequency = higher pitch
Loudness	Related to amplitude — larger amplitude = louder sound
Quality/Timbre	Distinguishes same note played on different instruments; related to waveform
Intensity	Sound energy per unit area per unit time; unit = W/m^2
Loudness unit	Decibel (dB); threshold of hearing = 0 dB; normal conversation = 60 dB; pain threshold = 120 dB

Types of Sound by Frequency

Type	Frequency Range	Examples
Infrasonic	Below 20 Hz	Earthquakes, whale communication, volcanic eruptions
Audible (Sonic)	20 Hz to 20,000 Hz	Speech, music, everyday sounds
Ultrasonic	Above 20,000 Hz	Bat navigation (echolocation), SONAR, medical ultrasonography, cleaning jewellery

Echo

Concept	Details
Definition	Repetition of sound due to reflection from a surface
Minimum distance	17.2 metres (at 20 C in air) — for distinct echo; based on persistence of sound in ear (0.1 seconds)
Reverberation	Multiple reflections of sound in enclosed space; persistence of sound
Applications	Stethoscope, megaphone, ear trumpet use reflection of sound

Doppler Effect

Concept	Details
Definition	Apparent change in frequency of sound when source and observer are in relative motion
Source approaches	Frequency appears to increase (higher pitch); wavelength decreases
Source recedes	Frequency appears to decrease (lower pitch); wavelength increases
Everyday example	Ambulance siren — higher pitch approaching, lower pitch moving away
Applications	RADAR speed guns, SONAR, measuring star velocities (red shift/blue shift), medical Doppler ultrasound
Not applicable when	Source and observer both stationary relative to each other
Light analogy	Red shift (receding — lower frequency); Blue shift (approaching — higher frequency)

SONAR

Fact	Details
Full form	Sound Navigation and Ranging
Uses	Detecting underwater objects (submarines, depth of ocean, fish shoals)
Principle	Ultrasonic waves sent; echo received; distance = (speed x time) / 2
Invented	Paul Langevin (1917, during World War I)

Quick Recall — PSC Favourites

Question	Answer
Speed of light in vacuum?	3 x 10^8 m/s
Speed of sound in air?	343 m/s (approximately)
Sound travels fastest in?	Solids
Human audible range?	20 Hz to 20,000 Hz
Myopia corrected by?	Concave lens
Hypermetropia corrected by?	Convex lens
Rear-view mirror type?	Convex mirror
Shaving mirror type?	Concave mirror
Rainbow is caused by?	Dispersion + total internal reflection + refraction
VIBGYOR — most deviated colour?	Violet
Least deviated colour?	Red
Doppler effect for approaching source?	Higher pitch (increased frequency)
SONAR uses which waves?	Ultrasonic waves
Minimum distance for echo?	17.2 metres
Optical fibre works on?	Total internal reflection
Twinkling of stars due to?	Atmospheric refraction
Refractive index of diamond?	2.42
Unit of power of lens?	Dioptre (D)