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CDS Sound & Light

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This page covers CDS Sound & Light with complete concept notes, 4 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Sound & Light— Rules & Concept

Core ConceptRead this first — the foundation of the topic

Sound and Light are two fundamental forms of energy that behave as waves. Both topics are heavily tested in RRB Group D with 2-3 questions appearing in every paper. SOUND WAVES - Core Concept:

Sound is a mechanical wave that needs a medium to travel. It cannot travel in vacuum. Sound waves are longitudinal waves where particles vibrate parallel to the direction of wave motion. When you speak, your vocal cords vibrate air molecules, creating compressions and rarefactions that reach your ear.

Key RulesCore rules you must know cold
1

Speed in air = 340 m/s (at room temperature)

2

Speed increases with temperature and density of medium

3

Speed hierarchy: Solids > Liquids > Gases

4

Frequency determines pitch, amplitude determines loudness

5

Human hearing range: 20 Hz to 20,000 Hz

Formula BlockMemorise — at least one formula appears in every paper

for Sound:

Speed = Frequency × Wavelength (v = f × λ)
Time = Distance / Speed (for echo problems)
For echo: Total distance = 2 × distance to obstacle

LIGHT - Core Concept:

Light is electromagnetic radiation that can travel through vacuum. It behaves both as wave and particle. Light travels in straight lines and is the fastest thing in universe.

Key Properties of Light:

• Speed in vacuum = 3 × 10^8 m/s

• Shows reflection, refraction, dispersion

• White light contains 7 colors (VIBGYOR)

• Cannot bend around corners like sound

Formula Block for Light:

Speed of light = 3 × 10^8 m/s
Reflection: Angle of incidence = Angle of reflection
Mirror formula: 1/f = 1/u + 1/v
Magnification = v/u = height of image/height of object
Exam PatternsWhat examiners ask — read before attempting PYQs
RRB Group D consistently asks

Echo calculations, speed comparisons, mirror problems, color of light questions, and sound frequency problems. Most questions are formula-based with direct substitution. SHORTCUT #1 - Echo Time Trick: For echo problems, remember the magic number 17. If obstacle is 17 meters away, echo returns in 0.1 seconds

Use ratio

Distance = 17 × (Time in seconds × 10) meters.

Worked ExampleSolve this step-by-step before moving on
1
Step 1

Total distance traveled by sound = Speed × Time = 340 × 2 = 680 m

2
Step 2

Sound travels to building and back, so actual distance = 680/2 = 340 m

3
Step 3

Distance to building = 340 meters Shortcut check: 17 × (2 × 10) = 17 × 20 = 340 m ✓ Worked Example 2 - Mirror Problem: Object placed 20 cm from concave mirror of focal length 15 cm. Find image distance.

1
Step 1

Given u = -20 cm, f = -15 cm (negative for concave)

2
Step 2

Use 1/f = 1/u + 1/v

3
Step 3

1/(-15) = 1/(-20) + 1/v

4
Step 4

-1/15 = -1/20 + 1/v

5
Step 5

1/v = -1/15 + 1/20 = (-4 + 3)/60 = -1/60

6
Step 6

v = -60 cm (image formed 60 cm in front of mirror) SHORTCUT #2 - Speed Comparison Trick: Remember the ratio - Sound in steel : water : air = 17 : 5 : 1. If speed in air is 340 m/s, then in water it's 1700 m/s and in steel it's 5780 m/s. SHORTCUT #3 - Color Memory Trick: VIBGYOR sequence: Violet (shortest wavelength, highest frequency) to Red (longest wavelength, lowest frequency). In dispersion, violet bends most, red bends least.

Exam TrapsCommon mistakes students make — avoid these

Students forget to double the distance in echo problems. Sound travels to the obstacle AND back. Always remember: Total distance = 2 × distance to obstacle.

This mistake appears in 60% of wrong answers. Another frequent error is mixing up concave and convex mirror properties. Concave mirrors can form both real and virtual images, while convex mirrors ALWAYS form virtual, erect, and diminished images.

Key Points to Remember

  • Sound needs medium to travel, light can travel through vacuum
  • Speed of sound in air = 340 m/s, speed of light = 3 × 10^8 m/s
  • Echo distance formula: Distance = (Speed × Time)/2
  • Human hearing range: 20 Hz to 20,000 Hz
  • Mirror formula shortcut: 1/f = 1/u + 1/v
  • Sound speed hierarchy: Solids > Liquids > Gases
  • VIBGYOR: Violet has shortest wavelength, Red has longest
  • Concave mirrors converge light, convex mirrors diverge light
  • Echo time trick: 17 meters distance = 0.1 second echo time
  • Convex mirrors always form virtual, erect, diminished images

Exam-Specific Tips

  • Speed of sound in air at room temperature is 340 m/s
  • Speed of light in vacuum is 3 × 10^8 m/s
  • Human audible frequency range is 20 Hz to 20,000 Hz
  • Sound cannot travel in vacuum but light can
  • White light splits into 7 colors during dispersion: VIBGYOR
  • Angle of incidence equals angle of reflection in mirrors
  • Ultrasonic waves have frequency above 20,000 Hz
  • Infrasonic waves have frequency below 20 Hz
Practice MCQs

Sound & Light — Practice Questions

4graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

A concave mirror has a focal length of 15 cm. An object is placed at a distance of 30 cm from the mirror. What is the nature of the image formed?

Practice 2medium

A concave mirror has a focal length of 15 cm. An object is placed at a distance of 30 cm from the mirror. What is the nature and position of the image formed?

Practice 3medium

A concave mirror has a focal length of 15 cm. An object is placed 30 cm in front of it. What is the nature of the image formed?

Practice 4hard

A concave mirror has a focal length of 15 cm. An object is placed at a distance of 10 cm from the mirror. What is the nature and position of the image formed?

60-Second Revision — Sound & Light

  • Remember: Echo distance = (Speed × Time)/2, don't forget to divide by 2
  • Formula: Mirror equation 1/f = 1/u + 1/v with sign conventions
  • Speed: Sound 340 m/s in air, Light 3 × 10^8 m/s in vacuum
  • Trap: Convex mirrors ALWAYS form virtual images, never real
  • Shortcut: 17m distance gives 0.1s echo time for quick calculations
  • Memory: VIBGYOR sequence - Violet bends most in prism
  • Concept: Sound needs medium, light travels through vacuum
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