UPCAT Physics — Electromagnetism, Mirrors & OpticsRevision Notes
Quick revision notes for Electromagnetism, Mirrors & Optics — the one-page refresher for UPCAT aspirants. Every item on this page has appeared in recent UPCAT Physics papers, so revising these is the shortest path to a confident performance in University of the Philippines's UPCAT 2026.
Exam context
The University of the Philippines College Admission Test is conducted by University of the Philippines and is scheduled for Mid-2026 (announced by UP Admissions). The Physics subtest is marked as "Core" in the official pattern, and Electromagnetism, Mirrors & Optics appears in position 6th of 6 in the UPCAT Physics review rotation. Passing mark: UPG ≤ 2.2 typical. Recent UPCAT 2026 papers have drawn roughly 20 questions from this subject.
Electromagnetism, Mirrors & Optics - Revision notes
This comprehensive revision guide covers three fundamental areas of physics that frequently appear in UPCAT and other college entrance exams: Electromagnetism (including circuits and Ohm's Law), Magnetism, and Optics (mirrors, lenses, and light behavior). These topics are interconnected and form the basis for understanding modern technology from electric circuits to optical instruments. Master these concepts through clear explanations, practical examples, and step-by-step problem-solving approaches.
Sections
Formulas
Example
If a 12V battery drives 2A through a resistor, then R = V/I = 12V/2A = 6Ω
Formula
V = IR
Variables
V = voltage (volts), I = current (amperes), R = resistance (ohms)
Application
Calculate any unknown electrical quantity when two are known
Example
A 60W light bulb at 120V draws I = P/V = 60W/120V = 0.5A
Formula
P = VI = I²R = V²/R
Variables
P = power (watts), V = voltage (volts), I = current (amperes), R = resistance (ohms)
Application
Calculate power consumption in electrical devices
Exam Tips
- Always check units in calculations - they must be consistent
- Use triangle method: cover unknown quantity in V=IR to find formula
- Remember that doubling voltage doubles current (if resistance constant)
- Power problems often require two-step calculations using Ohm's Law first
Key Points
- Electricity is the presence and flow of electric charge through conductors
- Current (I) is the rate at which electric charge flows, measured in amperes (A)
- Voltage (V) is the potential difference that drives current, measured in volts (V)
- Resistance (R) opposes current flow, measured in ohms (Ω)
- Ohm's Law relates these three quantities: V = IR
- Power consumption can be calculated using P = VI = I²R = V²/R
- Conductors (metals) allow easy current flow; insulators (rubber, plastic) resist it
Definitions
Term
Electric Current
Definition
The flow of electric charge through a conductor, measured as the amount of charge passing a point per unit time
Importance
Fundamental concept for understanding all electrical phenomena and circuit analysis
Term
Resistance
Definition
The opposition to electric current flow in a material, causing energy to be converted to heat
Importance
Determines how much current flows for a given voltage; key to circuit design
Term
Voltage
Definition
The electric potential difference between two points, providing the 'push' that drives current
Importance
The driving force in all electrical circuits; determines current flow with resistance
Section Title
Electricity and Ohm's Law
Common Mistakes
- Confusing current and voltage - current flows, voltage pushes
- Mixing up units: volts for voltage, amperes for current, ohms for resistance
- Forgetting that power increases with the square of current or voltage
- Not recognizing when to use different forms of power formula
Formulas
Example
Three 10Ω resistors in series: Rtotal = 10 + 10 + 10 = 30Ω
Formula
Rseries = R₁ + R₂ + R₃ + ...
Variables
R = individual resistances in ohms
Application
Calculate total resistance when resistors are connected in series
Example
Two 10Ω resistors in parallel: 1/Rtotal = 1/10 + 1/10 = 2/10, so Rtotal = 5Ω
Formula
1/Rparallel = 1/R₁ + 1/R₂ + 1/R₃ + ...
Variables
R = individual resistances in ohms
Application
Calculate total resistance when resistors are connected in parallel
Exam Tips
- Draw circuit diagrams to visualize series vs parallel connections
- For parallel resistance with two equal values: Rtotal = R/2
- Check your parallel calculations: total resistance must be smaller than any individual resistance
- Remember: series adds resistances, parallel reduces total resistance
Key Points
- Series circuits: components connected end-to-end, current is same throughout
- In series: total resistance = R₁ + R₂ + R₃ + ... (resistances add)
- In series: voltage divides among components, current stays constant
- Parallel circuits: components branch from common points, voltage is same across each
- In parallel: 1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + ... (reciprocals add)
- In parallel: current divides among branches, voltage stays constant
- Series circuits fail if one component breaks; parallel circuits continue working
Definitions
Term
Series Circuit
Definition
A circuit where components are connected in a single path, so current flows through each component sequentially
Importance
Understanding series helps explain why Christmas lights used to all go out when one bulb failed
Term
Parallel Circuit
Definition
A circuit where components are connected across common points, providing multiple paths for current
Importance
Explains how household electrical systems work - each appliance gets full voltage
Section Title
Series and Parallel Circuits
Common Mistakes
- Adding resistances directly in parallel (should use reciprocal formula)
- Thinking voltage is constant in series (it divides among components)
- Believing current is same in all parallel branches (it divides)
- Forgetting that parallel resistance is always less than smallest individual resistance
Exam Tips
- Remember the connection: electricity and magnetism are linked
- Know that electromagnets are stronger when current increases
- Understand that generators and motors work on electromagnetic principles
- Visualize magnetic field lines going from north to south pole
Key Points
- Magnetism is a force resulting from magnetic fields around magnets or moving charges
- Magnetic fields arise naturally (permanent magnets) or from electric current (electromagnets)
- Like poles repel, opposite poles attract
- Moving electric charges create magnetic fields
- Changing magnetic fields can induce electric current (electromagnetic induction)
- Electromagnets can be turned on/off and their strength controlled
- Earth itself acts as a giant magnet with north and south magnetic poles
Definitions
Term
Magnetism
Definition
A force that results from a magnetic field, arising either inherently in magnets or due to movement of electrically charged particles
Importance
Fundamental force that enables electric motors, generators, and many modern technologies
Term
Electromagnet
Definition
A magnet created by passing electric current through a conductor, which can be controlled and switched on or off
Importance
Key component in motors, speakers, MRI machines, and many electronic devices
Term
Magnetic Field
Definition
The region around a magnet where magnetic forces can be detected and measured
Importance
Understanding fields helps explain how magnets interact and how electromagnetic devices work
Section Title
Magnetism and Electromagnetism
Common Mistakes
- Confusing electric and magnetic fields (they're related but distinct)
- Thinking all metals are magnetic (only iron, nickel, cobalt are strongly magnetic)
- Forgetting that electromagnets need continuous current to maintain magnetism
- Not recognizing that moving charges always produce magnetic fields
Formulas
Example
Light hitting mirror at 30° from normal reflects at 30° on opposite side of normal
Formula
θᵢ = θᵣ
Variables
θᵢ = angle of incidence, θᵣ = angle of reflection (both measured from normal)
Application
Predicting direction of reflected light rays
Exam Tips
- Always draw the normal line first in reflection problems
- Remember: plane mirror images are VEST (Virtual, Erect, Same size, lateral inversion)
- Image distance behind mirror equals object distance in front
- For multiple mirror problems, trace light rays step by step
Key Points
- Law of Reflection: angle of incidence equals angle of reflection (θᵢ = θᵣ)
- Angles are measured from the normal (perpendicular line to surface)
- Plane mirror images are virtual, erect, same size, and same distance behind mirror
- Virtual images cannot be projected on screen - they appear to be behind mirror
- Mirror image is laterally inverted (left-right reversed)
- Multiple mirrors can create multiple images depending on angle between them
Definitions
Term
Virtual Image
Definition
An image that appears to be located behind a mirror or lens but cannot be projected on a screen
Importance
Distinguishes plane mirror images from real images formed by curved mirrors
Term
Normal
Definition
An imaginary line perpendicular to the surface at the point where light strikes
Importance
Reference line for measuring angles of incidence and reflection
Term
Lateral Inversion
Definition
The left-right reversal of images in plane mirrors
Importance
Explains why written text appears backwards in mirrors
Section Title
Reflection and Plane Mirrors
Common Mistakes
- Measuring angles from the mirror surface instead of the normal
- Thinking plane mirror images are real (they're always virtual)
- Believing image distance from mirror differs from object distance
- Confusing lateral inversion with upside-down inversion
Formulas
Example
Light from air (n=1) entering water (n=1.33) at 45° bends to sin⁻¹(sin45°/1.33) = 32°
Formula
n₁sinθ₁ = n₂sinθ₂
Variables
n = refractive index, θ = angle from normal, subscripts 1,2 refer to first and second media
Application
Calculate bending angle when light passes between different materials
Exam Tips
- Memorize common refractive indices: air (1.0), water (1.33), glass (~1.5)
- Draw diagrams showing normal and bend direction
- Remember: light slows down in higher index materials
- Practice calculating angles using inverse sine function
Key Points
- Refraction is bending of light when passing between different media
- Occurs because light speed changes in different materials
- Snell's Law: n₁sinθ₁ = n₂sinθ₂
- Refractive index (n) indicates how much light slows in a material
- Light bends toward normal when entering denser medium (higher n)
- Light bends away from normal when entering less dense medium (lower n)
- Total internal reflection occurs when light cannot exit denser medium
Definitions
Term
Refractive Index
Definition
A measure of how much light slows down in a material compared to vacuum, calculated as n = c/v
Importance
Determines how much light bends when entering different materials
Term
Total Internal Reflection
Definition
Complete reflection of light at the boundary when trying to pass from denser to less dense medium beyond critical angle
Importance
Principle behind fiber optics and explains why objects underwater appear differently
Section Title
Refraction and Snell's Law
Common Mistakes
- Using wrong angles in Snell's Law (must be from normal, not surface)
- Forgetting that higher refractive index means light bends toward normal
- Mixing up which medium is denser when predicting bend direction
- Not recognizing when total internal reflection occurs
Exam Tips
- Learn the concave mirror image chart by object position
- Remember: convex mirrors always produce small, virtual, erect images
- Real images are always inverted; virtual images are always erect
- Practice ray diagrams to visualize image formation
Key Points
- Concave mirrors: inside curved surface reflects (converging mirror)
- Convex mirrors: outside curved surface reflects (diverging mirror)
- Concave mirrors can form both real and virtual images depending on object position
- Convex mirrors always form virtual, erect, diminished images
- Real images can be projected on screen; virtual images cannot
- Focal point is where parallel rays converge (concave) or appear to diverge from (convex)
- Image characteristics depend on object position relative to focal point and center of curvature
Definitions
Term
Concave Mirror
Definition
A curved mirror where the inside spherical surface is the reflecting surface, causing parallel rays to converge
Importance
Used in telescopes, headlights, and makeup mirrors to focus or magnify
Term
Convex Mirror
Definition
A curved mirror where the outside spherical surface reflects, causing parallel rays to diverge
Importance
Used in car side mirrors and security mirrors to provide wide field of view
Term
Real Image
Definition
An image formed when light rays actually converge and can be projected on a screen
Importance
Distinguishes images that can be captured on film/sensors from virtual ones
Section Title
Curved Mirrors - Concave and Convex
Common Mistakes
- Confusing which surface reflects in concave vs convex mirrors
- Thinking convex mirrors can form real images (they can't)
- Mixing up magnification and image size relationships
- Not remembering that real images are inverted
Exam Tips
- Remember: converging lenses are like concave mirrors in image formation
- Diverging lenses are like convex mirrors - always virtual, erect, smaller
- Know practical applications: camera lenses, eyeglasses, magnifiers
- Understand that lens power depends on curvature and material
Key Points
- Lenses are transparent materials with curved surfaces that refract light
- Converging (convex) lenses: thicker at center, bring parallel rays to focus
- Diverging (concave) lenses: thinner at center, spread parallel rays apart
- Converging lenses can form real or virtual images depending on object position
- Diverging lenses always form virtual, erect, diminished images
- Used in eyeglasses, cameras, microscopes, telescopes, and magnifying glasses
- Lens behavior similar to mirrors but light passes through rather than reflects
Definitions
Term
Converging Lens
Definition
A lens that is thicker at the center than at the edges, causing parallel light rays to meet at a focal point
Importance
Essential component in cameras, telescopes, and correcting farsightedness
Term
Diverging Lens
Definition
A lens that is thinner at the center than at the edges, causing parallel light rays to spread apart
Importance
Used to correct nearsightedness and in some optical instruments for beam expansion
Term
Focal Length
Definition
The distance from the lens center to the focal point where parallel rays converge or appear to diverge from
Importance
Determines the lens power and magnification capabilities
Section Title
Lenses - Converging and Diverging
Common Mistakes
- Confusing lens shapes with mirror types (convex lens vs convex mirror behave differently)
- Thinking diverging lenses can produce real images (they cannot)
- Mixing up which lens type corrects which vision problem
- Not understanding that lenses work by refraction, mirrors by reflection
Connections
- Electricity and magnetism are fundamentally connected - moving charges create magnetic fields, changing magnetic fields induce electric currents
- Ohm's Law applies to both DC and AC circuits, making it essential for all electrical calculations
- Reflection and refraction principles apply to all types of mirrors and lenses, just with different geometries
- Both mirrors and lenses can form real or virtual images depending on their type and object position
- Electromagnetic radiation (light) exhibits both wave and particle properties, affecting how it interacts with optical devices
- Series and parallel circuit principles apply to complex electrical systems in homes, cars, and electronic devices
- Refractive index determines not just light bending but also the critical angle for total internal reflection used in fiber optics
Exam Strategy
Focus on understanding concepts rather than memorizing formulas - know when to apply Ohm's Law, how to identify series vs parallel circuits, and how to predict image characteristics for mirrors and lenses. Practice calculating electrical quantities step-by-step, draw diagrams for optics problems, and memorize key values like common refractive indices. For circuits, always check that your resistance calculations make sense (series increases total resistance, parallel decreases it). For optics, remember the patterns: plane mirrors always give same characteristics, convex mirrors always give smaller virtual images, and lens/mirror behavior follows predictable rules based on object position. Work through numerical problems systematically and double-check units in your answers.
Quick Review Questions
A 12V battery is connected to a 4Ω resistor. What current flows through the circuit?
Using Ohm's Law: I = V/R = 12V/4Ω = 3A. The current flowing through the circuit is 3 amperes.
Three 6Ω resistors are connected in parallel. What is the total resistance?
For parallel resistors: 1/Rtotal = 1/6 + 1/6 + 1/6 = 3/6 = 1/2, so Rtotal = 2Ω. Parallel resistance is always less than the smallest individual resistance.
What type of image does a plane mirror always produce?
Plane mirrors always produce virtual images (cannot be projected), erect (upright), same size as object, and located the same distance behind the mirror as the object is in front.
Light travels from air (n=1.0) into glass (n=1.5) at 60° from normal. What is the refraction angle?
Using Snell's Law: n₁sinθ₁ = n₂sinθ₂, so (1.0)(sin60°) = (1.5)(sinθ₂). Solving: sinθ₂ = sin60°/1.5 = 0.577, so θ₂ = 35.3°.
What type of image does a convex mirror always produce?
Convex mirrors always produce virtual images (behind mirror), erect (upright), and smaller than the object, regardless of object position. This is why they're used in car mirrors.
If voltage doubles in a circuit with constant resistance, what happens to power?
Power = V²/R. If voltage doubles, power becomes (2V)²/R = 4V²/R, which is 4 times the original power.
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