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Light & Sound

General Science · Physics

Useful for: Railway RRB, OSSC RHT — Regular High School Teacher, OSEPA Upper Primary — Science, OSSC Science, OSSSC Govt TGT Science — CBZ, OSSSC Govt TGT Science — PCM, SSB Lecturer (Degree Colleges), SSB TGT CBZ

and 1 more exams

SSB TGT PCM

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Overview

Light travels as electromagnetic waves and can be reflected and refracted; sound is a mechanical wave that needs a medium.

Key points

Angle of incidence = angle of reflection.
Convex lens converges light; concave lens diverges it.
Power of a lens P = 1/f (f in metres); unit dioptre.
Sound needs a medium; speed in air ≈ 343 m/s, faster in solids.
Human hearing: 20 Hz to 20 kHz; above is ultrasound, below is infrasound.

Explanation

Mirror formula: 1/f = 1/v + 1/u.
Refraction causes bending of light from one medium to another.
An echo needs the reflector to be at least about 17 m away.
Dispersion splits white light into colours (rainbow).

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

A lens of focal length 0.5 m has power +2 D (convex).
Minimum echo distance ≈ 343 × 0.1 ÷ 2 ≈ 17 m.
Sound above 20 kHz is ultrasound (used in sonography).

Traps and advanced points

Trap: Thinking sound travels in vacuum.
Trap: Sign errors in lens power.
Trap: Mixing ultrasound and infrasound.

Quick revision

P = 1/f.
Sound needs a medium.
20 Hz–20 kHz audible.

Practice MCQs

Revision question: Which Newton law is associated with action and reaction?
1. Second law
2. Third law
3. First law
4. Law of gravitation only
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Answer: (b) Third law. Third law is associated with action and reaction.
Static-GK drill: AIDS is caused by which virus?
1. HBV
2. HIV
3. Influenza virus
4. Dengue bacterium
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Answer: (b) HIV. AIDS is caused by HIV.
Static-GK drill: How many chromosomes are normally present in a human somatic cell?
1. 44
2. 46
3. 48
4. 23
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Answer: (b) 46. How many chromosomes are normally present in a human somatic cell? — 46.
Static-GK drill: Sound cannot travel through which medium?
1. Air
2. Water
3. Steel
4. Vacuum
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Answer: (d) Vacuum. Sound cannot travel through Vacuum.
Static-GK drill: Vinegar contains mainly which acid?
1. Formic acid
2. Citric acid
3. Acetic acid
4. Sulfuric acid
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Answer: (c) Acetic acid. Vinegar contains mainly Acetic acid.
Which vitamin is also known as ascorbic acid?
1. Vitamin B12
2. Vitamin C
3. Vitamin A
4. Vitamin D
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Answer: (b) Vitamin C. Ascorbic acid is vitamin C; its deficiency causes scurvy.
Which option correctly answers this General Science check: SI unit of power?
1. nephron
2. rate of reaction
3. watt
4. sodium
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Answer: (c) watt. Power = work/time; 1 watt = 1 joule/second.
Which is the lightest gas?
1. Nitrogen
2. Oxygen
3. Hydrogen
4. Helium
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Answer: (c) Hydrogen. Hydrogen has the lowest density of all gases.
During a storm we see lightning before we hear thunder because —
1. thunder is produced later
2. light travels much faster than sound
3. sound travels faster than light
4. our ears are slower than eyes
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Answer: (b) light travels much faster than sound. Both are produced together; light (3 × 10⁸ m/s) reaches us far sooner than sound (about 340 m/s).
The angle of incidence equals the angle of —
1. reflection
2. deviation
3. dispersion
4. refraction
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Answer: (a) reflection. Law of reflection.