1. Sound – Basics
- Sound is a form of energy produced by vibrating bodies.
- It needs a material medium (solid, liquid, gas) to travel – cannot travel in vacuum.
- Sound travels fastest in solids, slowest in gases.
हिंदी: ध्वनि = कंपन से पैदा होने वाली ऊर्जा।
Sound के लिए माध्यम (medium) ज़रूरी – vacuum में नहीं फैलती।
1.1 Characteristics of Sound
- Loudness – how strong/loud the sound is (depends on amplitude).
- Pitch – how shrill or deep the sound is (depends on frequency).
- Quality – helps distinguish between sounds of same loudness & pitch (e.g., flute vs violin).
1.2 Echo (NDA favourite)
Echo – reflection of sound heard separately from original sound.
For an echo to be heard distinctly, sound must return after about 0.1 s.
Approx distance: about 17 m from reflecting surface (at 340 m/s).
हिंदी: Echo = गूँज – लौटकर आने वाली ध्वनि, जो मूल ध्वनि से अलग सुनाई दे।
2. Light – Basics
- Light travels in straight lines – rectilinear propagation of light.
- Speed in vacuum: ~3 × 108 m/s.
- Light is a form of energy that stimulates vision.
2.1 Reflection of Light
Reflection: bouncing back of light from a surface.
Laws:
- Angle of incidence = angle of reflection.
- Incident ray, reflected ray and normal lie in same plane.
हिंदी: Plane mirror में जो image दिखती है, वह उलटी–सीधी (left–right उलट) लेकिन समान आकार की होती है।
2.2 Refraction of Light
Refraction: bending of light when it passes from one medium to another (e.g. air to water).
Everyday examples:
- Stick appears bent in water.
- Bottom of a tank appears raised.
2.3 Lenses & Human Eye (short NDA view)
- Convex lens: thicker in middle; can converge light; used in magnifying glass, camera, eye, microscope.
- Concave lens: thinner in middle; diverges light; used in spectacles for short-sightedness.
- Human eye: natural optical instrument; image formed on retina; lens adjusts its focal length (accommodation).
2.4 Simple Optical Instruments
- Periscope: uses plane mirrors; used in submarines to see objects above water.
- Microscope: uses lenses to see very small objects.
- Telescope: used to observe distant objects like stars and planets.
3. Basic Electricity (high-yield summary)
- Electric current: flow of electric charge (electrons) per unit time.
- Measured in ampere (A); instrument – ammeter.
- Potential difference (voltage): work done to move unit charge; measured in volt (V); instrument – voltmeter.
- Ohm’s Law: V = I × R (NDA often asks this formula).
- Resistance: opposition to current; measured in ohm (Ω).
हिंदी:
Current = एक सेकंड में बहने वाला charge,
Voltage = दो बिंदुओं के बीच ऊर्जा का अंतर,
Resistance = current को oppose करने की क्षमता।
3.1 Cells, Batteries & Fuses
- Primary cell: cannot be recharged (dry cell).
- Secondary cell: can be recharged (lead-acid battery, Li-ion battery).
- Fuse: thin wire with low melting point; melts when current is high → protects circuit.
📘 Practice Questions (NDA Style)
1. Echo is due to:
a) Refraction of sound
b) Reflection of sound
c) Diffraction of sound
d) Interference of sound
2. Which quantity decides the pitch of a sound?
a) Amplitude
b) Frequency
c) Velocity
d) Wavelength
3. In a plane mirror, the image formed is:
a) Real and inverted
b) Virtual and inverted
c) Real and erect
d) Virtual and erect
4. Light bending when it goes from air to water is called:
a) Reflection
b) Refraction
c) Diffusion
d) Scattering
5. Ohm’s Law is:
a) V = I/R
b) V = I × R
c) I = R/V
d) R = VI
6. The device used to protect an electric circuit from overloading is:
a) Resistor
b) Fuse
c) Voltmeter
d) Galvanometer
a) Refraction of sound
b) Reflection of sound
c) Diffraction of sound
d) Interference of sound
2. Which quantity decides the pitch of a sound?
a) Amplitude
b) Frequency
c) Velocity
d) Wavelength
3. In a plane mirror, the image formed is:
a) Real and inverted
b) Virtual and inverted
c) Real and erect
d) Virtual and erect
4. Light bending when it goes from air to water is called:
a) Reflection
b) Refraction
c) Diffusion
d) Scattering
5. Ohm’s Law is:
a) V = I/R
b) V = I × R
c) I = R/V
d) R = VI
6. The device used to protect an electric circuit from overloading is:
a) Resistor
b) Fuse
c) Voltmeter
d) Galvanometer
Answers:
1 – b
2 – b
3 – d
4 – b
5 – b
6 – b
1 – b
2 – b
3 – d
4 – b
5 – b
6 – b
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।