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 Properties of Sound
• 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
Formula Block 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
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Exam Patterns
What 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 Example 1 - Echo Problem
1
Total distance traveled by sound = Speed × Time = 340 × 2 = 680 m
2
Sound travels to building and back, so actual distance = 680/2 = 340 m
3
Distance to building = 340 meters
Shortcut check: 17 × (2 × 10) = 17 × 20 = 340 m
✏️Worked Example 2 - Mirror Problem
1
Given u = -20 cm, f = -15 cm (negative for concave)
2
Use 1/f = 1/u + 1/v
3
1/(-15) = 1/(-20) + 1/v
4
-1/15 = -1/20 + 1/v
5
1/v = -1/15 + 1/20 = (-4 + 3)/60 = -1/60
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
⚠️#1 COMMON MISTAKE
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.
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