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.
Speed in air = 340 m/s (at room temperature)
Speed increases with temperature and density of medium
Speed hierarchy: Solids > Liquids > Gases
Frequency determines pitch, amplitude determines loudness
Human hearing range: 20 Hz to 20,000 Hz
for Sound:
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:
• Shows reflection, refraction, dispersion
• White light contains 7 colors (VIBGYOR)
• Cannot bend around corners like sound
Formula Block for Light:
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
Distance = 17 × (Time in seconds × 10) meters.
Total distance traveled by sound = Speed × Time = 340 × 2 = 680 m
Sound travels to building and back, so actual distance = 680/2 = 340 m
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.
Given u = -20 cm, f = -15 cm (negative for concave)
Use 1/f = 1/u + 1/v
1/(-15) = 1/(-20) + 1/v
-1/15 = -1/20 + 1/v
1/v = -1/15 + 1/20 = (-4 + 3)/60 = -1/60
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.
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.
12graded MCQs · easy to hard · full solution & trap analysis