UPCAT Physics — Electromagnetism, Mirrors & OpticsCheat Sheet
A printable cheat sheet for Electromagnetism, Mirrors & Optics, built for UPCAT reviewers who want one go-to reference in the final stretch. Covers formulas, key definitions, common question types, and the University of the Philippines-specific twists you will see on UPCAT day.
Exam context
On the UPCAT 2026, the Physics subtest carries a "Core" weight in University of the Philippines's pattern. Electromagnetism, Mirrors & Optics lands at position 6th out of 6 in the standard review order. Target score is UPG ≤ 2.2 typical, and roughly 20 items come from Physics on a typical UPCAT paper.
Electromagnetism, Mirrors & Optics - Cheat sheet
Your last-minute revision companion for mastering electricity, magnetism, and optics before your UPCAT Physics exam.
Sections
Formulas
Formula
V = IR (Ohm's Law)
Meaning
V = voltage (volts), I = current (amperes), R = resistance (ohms)
Watch Out
Make sure units are consistent - volts, amperes, and ohms only
When To Use
When finding any unknown electrical quantity in a simple circuit
Formula
P = VI = I²R = V²/R
Meaning
P = power (watts), V = voltage, I = current, R = resistance
Watch Out
Choose the right form based on what variables you know
When To Use
When calculating electrical power consumption
Formula
Q = It
Meaning
Q = charge (coulombs), I = current (amperes), t = time (seconds)
Watch Out
Time must be in seconds, not minutes or hours
When To Use
When calculating total charge flow over time
Common Values
Value
1.6 × 10⁻¹⁹ C
Symbol
e
Quantity
Electron charge
Value
220 V
Symbol
V
Quantity
Typical household voltage
Value
20 mA
Symbol
I
Quantity
Current in typical LED
Section Title
Basic Electricity
Important Facts
- Current flows from positive to negative terminal (conventional current)
- Electrons actually flow from negative to positive
- AC current changes direction rapidly back and forth
- DC current flows in one direction only
- Electric field lines point from positive to negative charges
Key Definitions
Term
Electric current
Example
Water flowing through a pipe
Definition
The rate of flow of electric charge through a conductor
Term
Voltage
Example
Pressure difference that pushes water through a pipe
Definition
The electrical potential difference that drives current flow
Term
Resistance
Example
Narrow pipe restricting water flow
Definition
Opposition to the flow of electric current
Term
Conductor
Example
Copper wire
Definition
Material that allows easy flow of electric current
Term
Insulator
Example
Rubber or plastic covering on wires
Definition
Material that prevents flow of electric current
Diagrams To Know
- Series circuit diagram with resistors in line
- Parallel circuit diagram with resistors branching
- Electric field lines around positive and negative charges
- Simple battery circuit with switch and bulb
Reactions Or Equations
Note
Used to calculate electrical energy consumption
Equation
Energy = Power × Time (E = Pt)
Conditions
Power in watts, time in seconds
Formulas
Formula
Rs = R₁ + R₂ + R₃ + ... (Series)
Meaning
Rs = total resistance, R₁,R₂,R₃ = individual resistances
Watch Out
Series means current flows through each resistor sequentially
When To Use
When resistors are connected one after another in series
Formula
1/Rp = 1/R₁ + 1/R₂ + 1/R₃ + ... (Parallel)
Meaning
Rp = total resistance, R₁,R₂,R₃ = individual resistances
Watch Out
Parallel total resistance is always less than the smallest individual resistance
When To Use
When resistors branch out from the same two points
Formula
Vs = V₁ + V₂ + V₃ + ... (Series voltage)
Meaning
Vs = source voltage, V₁,V₂,V₃ = voltage across each resistor
Watch Out
Voltage divides proportionally to resistance values
When To Use
In series circuits where voltage is divided
Formula
Is = I₁ + I₂ + I₃ + ... (Parallel current)
Meaning
Is = source current, I₁,I₂,I₃ = current through each branch
Watch Out
Current divides inversely proportional to resistance
When To Use
In parallel circuits where current divides
Section Title
Circuit Analysis
Important Facts
- In series: same current everywhere, voltage divides
- In parallel: same voltage everywhere, current divides
- Total power is sum of individual powers in both series and parallel
- Kirchhoff's voltage law: sum of voltages around closed loop = 0
- Kirchhoff's current law: current in = current out at any junction
Key Definitions
Term
Series circuit
Example
Christmas lights that all go out when one burns out
Definition
Circuit where components are connected in a single path
Term
Parallel circuit
Example
House wiring where each appliance can work independently
Definition
Circuit where components branch out from common connection points
Term
Short circuit
Example
Wire directly connecting battery terminals
Definition
Low resistance path that bypasses the intended circuit
Diagrams To Know
- Series circuit with ammeter and voltmeter placement
- Parallel circuit with multiple branches
- Circuit diagrams using standard electrical symbols
Formulas
Formula
F = BIL (Force on current-carrying wire)
Meaning
F = force (N), B = magnetic field (T), I = current (A), L = length (m)
Watch Out
Wire must be perpendicular to magnetic field for maximum force
When To Use
When calculating force on wire in magnetic field
Formula
F = qvB (Force on moving charge)
Meaning
F = force (N), q = charge (C), v = velocity (m/s), B = magnetic field (T)
Watch Out
Charge must be moving perpendicular to field for maximum force
When To Use
When calculating force on charged particle in magnetic field
Common Values
Value
5 × 10⁻⁵ T
Symbol
B
Quantity
Earth's magnetic field
Section Title
Magnetism
Important Facts
- Magnetic field lines go from North to South pole outside magnet
- Like poles repel, unlike poles attract
- Moving electric charges create magnetic fields
- Changing magnetic fields induce electric currents
- Earth acts like giant magnet with magnetic North and South poles
Key Definitions
Term
Magnetic field
Example
Iron filings pattern around bar magnet
Definition
Region around magnet where magnetic force can be detected
Term
Electromagnet
Example
Crane magnet used to lift cars in junkyard
Definition
Magnet created by electric current flowing through coil of wire
Term
Electromagnetic induction
Example
Generator producing electricity by rotating coil in magnetic field
Definition
Generation of electric current by changing magnetic field
Diagrams To Know
- Bar magnet with field lines
- Electromagnet - wire coil around iron core
- Right-hand rule for current and magnetic field direction
- Compass needle alignment in magnetic field
Formulas
Formula
θᵢ = θᵣ (Law of Reflection)
Meaning
θᵢ = angle of incidence, θᵣ = angle of reflection (both measured from normal)
Watch Out
Angles are measured from the normal (perpendicular), not from surface
When To Use
For all reflection problems with mirrors
Formula
1/f = 1/dₒ + 1/dᵢ (Mirror equation)
Meaning
f = focal length, dₒ = object distance, dᵢ = image distance
Watch Out
Sign conventions: positive for real, negative for virtual
When To Use
For curved mirror problems to find image location
Formula
m = -dᵢ/dₒ = hᵢ/hₒ (Magnification)
Meaning
m = magnification, dᵢ = image distance, dₒ = object distance, hᵢ = image height, hₒ = object height
Watch Out
Negative magnification means inverted image
When To Use
To find size and orientation of mirror images
Section Title
Reflection and Mirrors
Important Facts
- Plane mirrors always form virtual, erect, same-size images
- Image distance equals object distance for plane mirrors
- Concave mirrors can form both real and virtual images
- Convex mirrors always form virtual, erect, reduced images
- Focal length is half the radius of curvature for spherical mirrors
Key Definitions
Term
Plane mirror
Example
Bathroom mirror
Definition
Flat mirror that produces virtual, erect, same-size images
Term
Concave mirror
Example
Makeup mirror, car headlight reflector
Definition
Curved mirror with reflecting surface on inside, converges light
Term
Convex mirror
Example
Car side mirror, security mirror in stores
Definition
Curved mirror with reflecting surface on outside, diverges light
Term
Virtual image
Example
Image you see in bathroom mirror
Definition
Image that cannot be projected on screen, appears to be behind mirror
Term
Real image
Example
Image on movie screen
Definition
Image that can be projected on screen, formed by actual light convergence
Diagrams To Know
- Ray diagram for plane mirror showing image formation
- Ray diagram for concave mirror - object beyond focal point
- Ray diagram for concave mirror - object within focal point
- Ray diagram for convex mirror
- Principal rays for curved mirrors
Formulas
Formula
n₁sinθ₁ = n₂sinθ₂ (Snell's Law)
Meaning
n₁,n₂ = refractive indices, θ₁,θ₂ = angles of incidence and refraction
Watch Out
Angles measured from normal to surface, not from surface itself
When To Use
When light passes from one medium to another
Formula
1/f = 1/dₒ + 1/dᵢ (Thin lens equation)
Meaning
f = focal length, dₒ = object distance, dᵢ = image distance
Watch Out
Sign conventions differ from mirrors - check carefully
When To Use
For lens problems to find image location
Formula
n = c/v
Meaning
n = refractive index, c = speed of light in vacuum, v = speed in medium
Watch Out
Refractive index is always ≥ 1
When To Use
To calculate refractive index of materials
Common Values
Value
1.00
Symbol
n
Quantity
Refractive index of air
Value
1.33
Symbol
n
Quantity
Refractive index of water
Value
1.5
Symbol
n
Quantity
Refractive index of glass
Value
3 × 10⁸ m/s
Symbol
c
Quantity
Speed of light
Section Title
Refraction and Lenses
Important Facts
- Light bends toward normal when entering denser medium
- Light bends away from normal when entering less dense medium
- Total internal reflection occurs when light tries to exit to less dense medium at large angle
- Converging lenses can form both real and virtual images
- Diverging lenses always form virtual, erect, reduced images
Key Definitions
Term
Refraction
Example
Straw appearing bent in glass of water
Definition
Bending of light when it passes from one medium to another
Term
Converging lens
Example
Magnifying glass, camera lens
Definition
Thicker in center, brings parallel rays to focus point
Term
Diverging lens
Example
Corrective lens for nearsightedness
Definition
Thinner in center, spreads parallel rays apart
Term
Refractive index
Example
Diamond has high refractive index, causes sparkle
Definition
Measure of how much light slows down in a material
Diagrams To Know
- Ray diagram showing refraction at interface
- Ray diagram for converging lens - object beyond 2F
- Ray diagram for converging lens - object within F
- Ray diagram for diverging lens
- Critical angle and total internal reflection
Must Remember
- Ohm's Law: V = IR - memorize this completely
- Series: current same, voltage divides; Parallel: voltage same, current divides
- Law of reflection: angle in = angle out (from normal)
- Snell's Law: n₁sinθ₁ = n₂sinθ₂
- Plane mirrors: virtual, erect, same size, same distance
- Convex mirrors always give virtual, erect, smaller images
- Refractive index of air = 1, water = 1.33, glass = 1.5
- Power = VI = I²R = V²/R
- Magnetic field lines go North to South outside magnet
- Critical angle for total internal reflection when going from dense to less dense medium
Last Minute Tips
- Draw ray diagrams for mirror/lens problems - they prevent sign errors
- For circuit problems, identify series vs parallel first, then apply appropriate formulas
- Remember angle measurements are from the normal, not the surface
- In Snell's law problems, identify which medium is denser (higher n)
- For electromagnetic induction, remember: changing magnetic field creates electric current
Comparison Tables
Rows
Values
- Same everywhere
- Divides among branches
Property
Current
Values
- Divides among components
- Same across all branches
Property
Voltage
Values
- Sum of individual resistances
- Less than smallest resistance
Property
Total Resistance
Values
- Breaks entire circuit
- Other components continue working
Property
Component Failure
Columns
- Property
- Series Circuit
- Parallel Circuit
Table Title
Series vs Parallel Circuits
Rows
Values
- Curves inward (cave-like)
- Curves outward (dome-like)
Property
Shape
Values
- Real or virtual
- Always virtual
Property
Image Type
Values
- Varies with distance
- Always smaller
Property
Image Size
Values
- Makeup mirrors, headlights
- Car mirrors, security mirrors
Property
Common Use
Columns
- Property
- Concave Mirror
- Convex Mirror
Table Title
Concave vs Convex Mirrors
Rows
Values
- Thicker in center
- Thinner in center
Property
Shape
Values
- Brings rays together
- Spreads rays apart
Property
Effect on light
Values
- Real or virtual
- Always virtual
Property
Image formation
Values
- Magnifying glass, cameras
- Correcting nearsightedness
Property
Common use
Columns
- Property
- Converging Lens
- Diverging Lens
Table Title
Converging vs Diverging Lenses
Ready to practise for the UPCAT 2026?
Super Tutor's AI review plan adapts to your weak areas and builds a weekly practice schedule around your target UPCAT exam date.