Light Sound and Waves

Reflection: light bounces off a surface; angle of incidence = angle of reflection. Refraction: light bends when passing between media of different densities (e.g. a pencil looking bent in water). Light bends TOWARDS the normal entering a denser medium, AWAY when entering a rarer one. Plane mirror forms a virtual, erect, same-size image (laterally inverted). Concave mirror (converging) is used in shaving mirrors, torches and headlights; convex mirror (diverging) gives a smaller, erect image and a wider field of view — used as vehicle rear-view mirrors. Memory aid: 'ConCAVE = CAVE inward = magnify; ConVEX = bulge out = wider view'.

Speed of light in vacuum = 3 x 10^8 m/s (the universe's speed limit; nothing travels faster). Light needs no medium and travels fastest in vacuum. Speed of sound in air at 20 C is about 343 m/s (commonly 330-340). Sound NEEDS a material medium — it cannot travel through vacuum, which is why space is silent. Sound travels FASTEST in solids, slower in liquids, slowest in gases (opposite of light). Memory aid: 'Light loves emptiness, Sound needs matter.' Wave equation: speed v = frequency (f) x wavelength (lambda). This relation applies to all waves.

An echo is reflected sound. To hear a distinct echo, the reflecting surface must be at least about 17 m away (so the sound takes at least 0.1 s to return). Frequency is measured in hertz (Hz). The audible range for humans is 20 Hz to 20,000 Hz. Below 20 Hz is infrasonic (e.g. produced by earthquakes, elephants); above 20,000 Hz is ultrasonic (e.g. used by bats, dolphins, and in sonography/SONAR). Pitch depends on frequency (higher frequency = higher pitch); loudness depends on amplitude. SONAR uses ultrasound to measure ocean depth and detect objects underwater.

Cover the answer, recall, then check. 12 cards on light, sound and waves for RPF SI.

Q1. What is the speed of light in vacuum?
A1. About 3 × 10⁸ m/s (3 lakh km/s). Light is the fastest known entity; it slows down in denser media like glass or water.

Q2. Relate wave speed, frequency and wavelength.
A2. v = f × λ (speed = frequency × wavelength). Frequency is measured in hertz (Hz), wavelength in metres.

Q3. Which mirror is used as a vehicle rear-view mirror and why?
A3. Convex mirror — it always forms a small, erect image and gives a wider field of view.

Q4. Which mirror is used as a dentist's/shaving mirror?
A4. Concave mirror — when the object is close (within focus), it gives an enlarged, erect, magnified image.

Q5. What causes dispersion of white light through a prism?
A5. Different colours bend by different amounts (violet most, red least), splitting white light into the spectrum VIBGYOR. Demonstrated by Isaac Newton.

Q6. Name the defect of vision corrected by a concave lens and one by a convex lens.
A6. Myopia (short-sightedness) is corrected by a concave (diverging) lens; hypermetropia (long-sightedness) by a convex (converging) lens.

Q7. What is the audible range of frequency for humans?
A7. 20 Hz to 20,000 Hz. Below 20 Hz is infrasonic; above 20,000 Hz is ultrasonic.

Q8. Can sound travel through vacuum? Why?
A8. No. Sound is a mechanical wave and needs a material medium (solid, liquid or gas) to travel; light can travel through vacuum but sound cannot.

Q9. In which medium does sound travel fastest?
A9. In solids (fastest), then liquids, then gases (slowest), because particles are more closely packed in solids. In air it is ≈ 343 m/s at room temperature.

Q10. What is an echo and the minimum distance needed to hear one?
A10. An echo is a reflected sound heard distinctly after the original. The reflecting surface must be at least ~17 m away (so the reflected sound returns after 0.1 s).

Q11. On what principle does SONAR work?
A11. Reflection of ultrasonic sound waves (echo). SONAR measures depth/distance of underwater objects by timing the returning echo.

Q12. Differentiate a transverse wave from a longitudinal wave.
A12. In a transverse wave particles vibrate perpendicular to wave direction (e.g. light, water ripples). In a longitudinal wave they vibrate parallel to it (e.g. sound), forming compressions and rarefactions.

Worked Example

Problem: A man standing 165 m in front of a tall cliff shouts and hears the echo of his voice after 1 second. Calculate the speed of sound in air.

Solution:
Step 1 — Understand the echo. The sound travels from the man to the cliff and reflects back to him, so it covers the distance twice.
Step 2 — Total distance travelled by the sound = 2 × 165 = 330 m.
Step 3 — This distance is covered in the echo time of 1 second.
Speed = Distance / Time = 330 m / 1 s = 330 m/s.

Answer: The speed of sound in air is 330 m/s.