Theory & Concepts

Master Physics (Laws of Motion, Light, Sound, Electricity) for SSC CGL

Get comprehensive theory, expert shortcuts, and hand-picked practice questions for Physics (Laws of Motion, Light, Sound, Electricity) specifically designed for the SSC CGL 2025-26 pattern.

25+

Exams Covered

100+

Study Modules

5K+

Practice Notes

SSC CGL RRB NTPC IBPS PO CAT

40 min readGK: ScienceWeightage: High

General Physics: Mechanics, Optics, Sound & Electricity

Physics is a highly conceptual component of the SSC CGL general awareness tier. Questions routinely test daily applications of Newton's laws, light phenomena (refraction, critical angle, total internal reflection), wave behaviors (sound speeds, echo conditions), and electric circuit equations. This interactive guide simplifies these topics with structured revision blocks, animations, and 20 handpicked practice MCQs.

Learning path

Newton's Laws & Mechanics
Geometrical Optics & Wave Theory
Sound characteristics & Electricity
20 Solved Practice Questions

1. Mechanics: Laws of Motion & Momentum

Mechanics forms the base of physics. In the CGL exam, questions are heavily oriented towards qualitative applications of Newton's Laws of Motion:

First Law (Law of Inertia): An object remains in its state of rest or uniform motion in a straight line unless acted upon by an external force.
- Inertia of Rest: When a bus starts suddenly, passengers fall backward.
- Inertia of Motion: When a running bus stops suddenly, passengers fall forward.
Second Law (Acceleration): The rate of change of momentum of an object is directly proportional to the applied force and takes place in the direction of the force.
Force = mass × acceleration (F = ma).
Example: A cricket fielder pulls his hands backward while catching a ball to increase the impact time, thereby reducing the force on his hands.
Third Law (Action & Reaction): To every action, there is an equal and opposite reaction.
- Recoil of a gun when a bullet is fired.
- The propulsion of rockets (thrust created by downward expanding gases pushes the rocket upward).
Conservation of Linear Momentum: In the absence of an external force, the total linear momentum of a system remains constant. It explains elastic and inelastic collisions.

2. Optics & Light Phenomena

Optics questions test the behaviors of light as it interacts with mirrors, lenses, and different media.

Interactive Ray Diagram Study Card

Click through the options below to visualize how the object position changes the reflected image in a concave mirror. This covers all critical CGL cases:

Interactive Concave Mirror Simulation

Select Object Position

Image Characteristics

Position

Between C and F

Nature

Real & Inverted

Size

Diminished

Rays from the object converge between C and F, forming a smaller, inverted real image.

Key Wave & Light Concepts

Refraction & Snell's Law: The bending of light as it passes from one medium to another. Refractive Index (n) is the ratio of speed of light in vacuum to speed in the medium (n = c/v).
Application: A coin inside a glass of water appears raised due to refraction.
Total Internal Reflection (TIR): When light traveling from a denser to a rarer medium strikes the boundary at an angle greater than the critical angle, it reflects back entirely into the denser medium.
- Sparkling of cut diamonds.
- Transmission of data through optical fibers.
- Formation of a mirage in deserts.
Scattering of Light: The redirection of light by microscopic particles. Rayleigh scattering states that scattering is inversely proportional to the fourth power of wavelength.
Application: The sky appears blue because short blue wavelengths are scattered more than red. Sunsets appear red as longer wavelengths penetrate the thicker atmosphere.

3. Wave Motion, Sound & Electricity

Acoustics & Sound Waves

Sound is a longitudinal mechanical wave requiring a material medium to propagate (cannot travel in a vacuum).

Speed of Sound: Heavily dependent on elasticity and density of the medium. The order of speed is: Solids > Liquids > Gases. Speed increases with an increase in humidity and temperature.
Characteristics:
- Pitch: Depends directly on frequency (high frequency = high shrill pitch).
- Loudness: Depends on amplitude (loudness is proportional to square of amplitude, measured in Decibels).
Echo Conditions: To hear a clear echo, the minimum distance between the sound source and reflecting barrier must be 17.2 meters (taking persistence of hearing as 0.1 seconds).

Electricity & Circuits

Ohm's Law: The current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, keeping temperature constant.
V = IR (where R is Resistance, measured in Ohms Ω).
Resistors in Combination:
- Series: R_total = R₁ + R₂ + R₃... (current remains same, voltage divides).
- Parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃... (voltage remains same, current divides).
Heating Effect (Joule's Law): Heat generated is directly proportional to the square of current, resistance, and time.
H = I²Rt.
Application: Electric fuses (made of tin-lead alloy with high resistance and low melting point) melt during overloads to break the circuit.

4. Practice Questions (20 Premium Solved Questions)

Question 01CGL Pattern

Newton's First Law of Motion, which defines the tendency of objects to resist any change in their state of rest or motion, is also known as:

A) Law of Inertia

B) Law of Momentum

C) Law of Impulse

D) Law of Acceleration

Correct answer: A) Law of Inertia

Step-by-step Solution

Step 1: Recall the definition of inertia: the inherent property of a body to resist change in its state of rest or uniform motion.

Step 2: Galileo first formulated this, and Newton incorporated it as his First Law.

Step 3: Hence, the First Law is widely known as the Law of Inertia.

Conclusion: Option A is correct.

Question 02CGL Pattern

When a moving bus stops suddenly, the passengers are thrown forward. This occurrence is explained by which of the following?

A) Inertia of rest

B) Inertia of motion

C) Newton's Third Law

D) Conservation of momentum

Correct answer: B) Inertia of motion

Step-by-step Solution

Step 1: Before brakes are applied, the bus and the passengers' bodies are moving together in the state of motion.

Step 2: When brakes stop the bus, the lower body of the passenger in contact with the seat stops, but the upper body tends to remain in motion due to inertia.

Step 3: This specific resistance is called the inertia of motion, throwing them forward.

Conclusion: Option B is correct.

Question 03CGL Pattern

A cricket player pulls his hands backward while catching a fast-moving cricket ball. This action is a practical application of which law?

A) Newton's First Law

B) Newton's Second Law

C) Newton's Third Law

D) Law of Gravitation

Correct answer: B) Newton's Second Law

Step-by-step Solution

Step 1: Newton's Second Law states Force is proportional to the rate of change of momentum (F = dp/dt).

Step 2: By pulling hands backward, the fielder increases the time (dt) over which the momentum is reduced to zero.

Step 3: Since time increases, the force experienced by his hands decreases significantly, preventing injury.

Conclusion: Option B is correct.

Question 04CGL Pattern

The working and propulsion of a rocket in space operates on the principle of:

A) Conservation of mass

B) Conservation of energy

C) Conservation of linear momentum

D) Kepler's Laws

Correct answer: C) Conservation of linear momentum

Step-by-step Solution

Step 1: Rocket propulsion ejects exhaust gases downward at very high velocity.

Step 2: To conserve the total linear momentum of the system (which must remain constant), the rocket experiences an equal momentum in the upward direction.

Step 3: This also aligns with Newton's Third Law (action-reaction).

Conclusion: Option C is correct.

Question 05CGL Pattern

At what position should an object be placed in front of a concave mirror so that a virtual, erect, and magnified image is formed behind the mirror?

A) At Focus (F)

B) Between Focus (F) and Center of Curvature (C)

C) Between Pole (P) and Focus (F)

D) Beyond Center of Curvature (C)

Correct answer: C) Between Pole (P) and Focus (F)

Step-by-step Solution

Step 1: A concave mirror generally forms real, inverted images for most object locations.

Step 2: The single exception is when the object is placed extremely close to the mirror surface, within the focal length.

Step 3: Placing it between the Pole (P) and Focus (F) causes reflected rays to diverge. Extrapolating them backwards forms an erect, magnified virtual image behind the mirror.

Conclusion: Option C is correct.

Question 06CGL Pattern

The optical illusion of a 'mirage' observed in hot deserts is primarily caused by which of the following phenomena?

A) Total Internal Reflection of light

B) Scattering of light

C) Diffraction of light

D) Double Refraction

Correct answer: A) Total Internal Reflection of light

Step-by-step Solution

Step 1: Hot ground heats the adjacent air, making it less dense (rarer medium) than the cooler air above (denser medium).

Step 2: Sky light passing downward bends away from the normal, eventually hitting hot layers at an angle exceeding the critical angle.

Step 3: This causes Total Internal Reflection (TIR), making the sky appear reflected on the ground like a pool of water.

Conclusion: Option A is correct.

Question 07CGL Pattern

The transmission of light signals through flexible glass fibers without loss of energy operates on the principle of:

A) Optical dispersion

B) Total Internal Reflection

C) Polarization

D) Refraction only

Correct answer: B) Total Internal Reflection

Step-by-step Solution

Step 1: Optical fibers have a high-refractive-index core surrounded by a lower-refractive-index cladding.

Step 2: Light signals entered at a specific angle undergo continuous, repeated Total Internal Reflection (TIR) along the core-cladding boundary.

Step 3: This keeps the light trapped, propagating it long distances with minimal attenuation.

Conclusion: Option B is correct.

Question 08CGL Pattern

Which of the following represents the correct mirror formula relating object distance (u), image distance (v), and focal length (f)?

A) 1/f = 1/v - 1/u

B) 1/f = 1/v + 1/u

C) f = u + v

D) 1/f = v/u

Correct answer: B) 1/f = 1/v + 1/u

Step-by-step Solution

Step 1: Recall mirror vs lens formulas.

Step 2: The mirror formula contains a positive sign: 1/f = 1/v + 1/u.

Step 3: The lens formula contains a negative sign: 1/f = 1/v - 1/u.

Conclusion: Option B is correct.

Question 09CGL Pattern

The SI unit of the refractive power of an optical lens is:

A) Decibel

B) Dioptre

C) Watt

D) Joule-meter

Correct answer: B) Dioptre

Step-by-step Solution

Step 1: Power of a lens (P) is defined as the reciprocal of its focal length (f) measured in meters (P = 1/f).

Step 2: The SI unit of power is m⁻¹, which is named Dioptre (denoted by D).

Conclusion: Option B is correct.

Question 10CGL Pattern

Why does the clear sky appear blue to an observer during the daytime?

A) Absorption of blue light by the ozone layer

B) Scattering of shorter wavelengths of light by air molecules

C) Refraction of sunlight through moisture

D) Reflection of ocean color by the clouds

Correct answer: B) Scattering of shorter wavelengths of light by air molecules

Step-by-step Solution

Step 1: Sunlight consists of a spectrum of colors. The atmosphere contains molecules smaller than light wavelengths.

Step 2: Rayleigh scattering states that shorter wavelengths (violet and blue) are scattered far more intensely than longer ones (red).

Step 3: Although violet is scattered more, our eyes are more sensitive to blue, making the sky look blue.

Conclusion: Option B is correct.

Question 11CGL Pattern

Sound waves are unable to propagate through which of the following environments?

A) Steel

B) Distilled water

C) Vacuum

D) Hydrogen gas

Correct answer: C) Vacuum

Step-by-step Solution

Step 1: Sound waves are mechanical longitudinal waves.

Step 2: They require a physical material medium (containing atoms/molecules to vibrate) for transmission.

Step 3: Since vacuum lacks matter, sound cannot travel through it.

Conclusion: Option C is correct.

Question 12CGL Pattern

The pitch of a sound wave, which determines whether a sound is flat/grave or sharp/shrill, is determined by its:

A) Amplitude

B) Velocity

C) Frequency

D) Wavelength only

Correct answer: C) Frequency

Step-by-step Solution

Step 1: Pitch is a physiological sensation of sound.

Step 2: It is directly proportional to the frequency of the sound wave. Higher frequency equals a more shrill sound (like a whistle or voice of women/children).

Step 3: Amplitude determines the loudness.

Conclusion: Option C is correct.

Question 13CGL Pattern

What is the minimum distance required between a source of sound and a reflecting wall to hear a distinct, clear echo?

A) 1.72 meters

B) 17.2 meters

C) 34.4 meters

D) 100 meters

Correct answer: B) 17.2 meters

Step-by-step Solution

Step 1: The human ear retains any sound sensation for about 0.1 seconds (persistence of hearing).

Step 2: Distance = speed × time. Taking speed of sound in air as 344 m/s, the total round trip distance must be at least 344 × 0.1 = 34.4 meters.

Step 3: The distance to the wall must be half of this round-trip distance, which is 17.2 meters.

Conclusion: Option B is correct.

Question 14CGL Pattern

The speed of sound in a medium depends on its physical properties. Which of the following shows the correct order of speed?

A) Gases > Liquids > Solids

B) Liquids > Solids > Gases

C) Solids > Liquids > Gases

D) Speed is same in all media

Correct answer: C) Solids > Liquids > Gases

Step-by-step Solution

Step 1: Speed of sound = √(Elasticity / Density).

Step 2: Solids have high elasticity (rigidity) compared to liquids and gases, which outweighs their higher density.

Step 3: Therefore, sound travels fastest in solids, moderately in liquids, and slowest in gases.

Conclusion: Option C is correct.

Question 15CGL Pattern

According to Ohm's Law, if the temperature and physical parameters of a conductor are kept constant, the electric current (I) is:

A) Inversely proportional to potential difference

B) Directly proportional to potential difference

C) Equal to the square of resistance

D) Independent of voltage

Correct answer: B) Directly proportional to potential difference

Step-by-step Solution

Step 1: Ohm's Law states V ∝ I.

Step 2: This means potential difference (V) is directly proportional to the current (I). Hence, current is directly proportional to potential difference.

Conclusion: Option B is correct.

Question 16CGL Pattern

If three resistors of values 2 Ω, 3 Ω, and 6 Ω are connected in a parallel configuration, what is their equivalent net resistance?

A) 11 Ω

B) 6 Ω

C) 1 Ω

D) 0.5 Ω

Correct answer: C) 1 Ω

Step-by-step Solution

Step 1: For parallel connection, 1/Rp = 1/R1 + 1/R2 + 1/R3.

Step 2: 1/Rp = 1/2 + 1/3 + 1/6.

Step 3: Find common denominator (6): 1/Rp = (3 + 2 + 1) / 6 = 6/6 = 1.

Step 4: Reciprocal of 1 is 1 Ω.

Conclusion: Option C is correct.

Question 17CGL Pattern

Which of the following mathematical equations represents Joule's Law of heating effect of electric current?

A) H = V/R

B) H = I²Rt

C) H = IR²t

D) H = I/V

Correct answer: B) H = I²Rt

Step-by-step Solution

Step 1: Joule's heating states that heat (H) is generated when current (I) flows through a resistance (R) for a duration (t).

Step 2: The formula is H = I²Rt (in Joules).

Conclusion: Option B is correct.

Question 18CGL Pattern

An electric safety fuse wire is placed in domestic circuits to protect appliances. It must have which characteristics?

A) Low resistance and high melting point

B) High resistance and low melting point

C) Low resistance and low melting point

D) High resistance and high melting point

Correct answer: B) High resistance and low melting point

Step-by-step Solution

Step 1: A fuse wire must heat up quickly when current exceeds the safe limit (requiring high resistance).

Step 2: It must melt quickly when hot to break the circuit and stop the current flow (requiring a low melting point).

Step 3: Usually made of an alloy of lead and tin.

Conclusion: Option B is correct.

Question 19CGL Pattern

If the distance between two electric point charges is doubled, the electrostatic force of attraction or repulsion between them becomes:

A) Doubled

B) Halved

C) Four times

D) One-fourth

Correct answer: D) One-fourth

Step-by-step Solution

Step 1: Coulomb's Law states Force is inversely proportional to the square of the distance (F ∝ 1/r²).

Step 2: If distance (r) is doubled to 2r, the force becomes proportional to 1/(2r)² = 1/(4r²).

Step 3: Thus, force is reduced to one-fourth of its original value.

Conclusion: Option D is correct.

Question 20CGL Pattern

The sound waves in the range of frequency below 20 Hz, which humans cannot hear, are termed as:

A) Ultrasonic waves

B) Infrasonic waves

C) Audible range

D) Supersonic waves

Correct answer: B) Infrasonic waves

Step-by-step Solution

Step 1: The human audible range is 20 Hz to 20,000 Hz.

Step 2: Frequencies below 20 Hz are infrasonic waves (produced by earthquakes, elephants).

Step 3: Frequencies above 20,000 Hz are ultrasonic waves (used by bats, Sonar).

Conclusion: Option B is correct.

Strategy errors to avoid

Mirror vs. Lens signs

Always check the sign in the formula. Mirrors use addition (1/v + 1/u = 1/f) while Lenses use subtraction (1/v - 1/u = 1/f). Confusing these signs is a common source of calculation errors.

Sound speed in different mediums

CGL often asks to order the speed of sound in steel, water, and air. Remember that sound travels faster in media with higher elasticity. Steel (solid) > Water (liquid) > Air (gas).

Master the next modules

Chemistry Concepts Biology Core Static GK Guide