UPCAT Physics — Electromagnetism, Mirrors & OpticsDetailed Explanation
This is the "office hours" version of Electromagnetism, Mirrors & Optics for the UPCAT 2026. No shortcuts, no hand-waving — just a full unpacking of why University of the Philippines cares about each concept and how the Physics section items tend to play out on exam day. Read this once, then hit the practice questions with real understanding.
Exam context
For the University of the Philippines College Admission Test, University of the Philippines tests Physics under a "Core" label, with Electromagnetism, Mirrors & Optics in the 6th slot across 6 chapters. UPCAT candidates must clear the UPG ≤ 2.2 typical cut on the 2026 paper, which draws about 20 Physics questions. Date to watch: Mid-2026 (announced by UP Admissions).
Electromagnetism, Mirrors & Optics - Detailed explanation
Physics is the study of matter, energy, and their interactions in the universe. This chapter focuses on three interconnected areas that are fundamental to understanding light, electricity, and magnetism. Electromagnetism explains how electric and magnetic forces work together, while mirrors and optics help us understand how light behaves when it reflects and refracts. These concepts are not only important for your UPCAT exam but also explain many phenomena you observe daily - from how your smartphone screen works to why you see rainbows after rain. Understanding these principles will help you solve problems involving electrical circuits, magnetic fields, and optical instruments like cameras and eyeglasses.
Concepts
Electrical Circuits and Ohm's Law
Electricity is the flow of electric charge through materials. Think of it like water flowing through pipes - the water represents electric charge, and the pipes represent wires. Ohm's Law is the fundamental relationship that governs this flow: V = IR, where V is voltage (the electrical 'pressure'), I is current (the rate of charge flow), and R is resistance (opposition to flow). Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). This law helps us calculate how much current will flow through a circuit or how much voltage is needed for a specific current.
Examples
The current flowing through the bulb is 1.2 amperes. This means 1.2 coulombs of charge pass through the bulb every second.
Scenario
A light bulb with resistance 10 Ω is connected to a 12V battery
Solution
Using V = IR, we get I = V/R = 12V ÷ 10Ω = 1.2A
A 10-volt source is needed to push 2 amperes of current through a 5-ohm resistance.
Scenario
Calculate voltage needed for 2A current through a 5Ω resistor
Solution
Using V = IR, we get V = 2A × 5Ω = 10V
Applications
- Designing electrical circuits in homes and buildings
- Calculating power consumption of appliances
- Troubleshooting electrical problems
- Sizing electrical components like fuses and wires
- Understanding how electronic devices work
Misconceptions
- Confusing voltage with current - voltage is electrical pressure, current is flow rate
- Thinking resistance is always constant - it can change with temperature
- Forgetting units - always include V, A, and Ω in calculations
Related Concepts
- Series and Parallel Circuits
- Electrical Power
- Magnetism from Electric Current
Common Exam Questions
Example
Given V = 24V and R = 8Ω, find I. Answer: I = V/R = 24/8 = 3A
Approach
Identify given values, determine what to find, apply V = IR formula
Question Type
Direct calculation using Ohm's Law
Example
Find power consumed by 6Ω resistor with 2A current. P = I²R = (2)² × 6 = 24W
Approach
Use P = VI or P = I²R or P = V²/R combinations with Ohm's Law
Question Type
Power calculations
Key Points To Remember
- Ohm's Law: V = IR - voltage equals current times resistance
- Voltage (V) is electrical pressure measured in volts
- Current (I) is charge flow rate measured in amperes
- Resistance (R) opposes current flow, measured in ohms
- Higher resistance means less current for same voltage
- Higher voltage means more current for same resistance
Series and Parallel Circuits
Electrical components can be connected in two main ways: series and parallel. In series circuits, components are connected end-to-end like links in a chain, so current flows through each component one after another. The current is the same throughout, but voltage is divided among components. In parallel circuits, components branch out from a common point and reconnect, like tributaries of a river. Here, voltage is the same across each branch, but current divides among the branches. Understanding these configurations is crucial for analyzing complex circuits.
Examples
In series, resistances add up. The same 2A current flows through all resistors, but each has different voltage drops.
Scenario
Three resistors (2Ω, 3Ω, 5Ω) connected in series with 20V source
Solution
RT = 2 + 3 + 5 = 10Ω, I = V/RT = 20V/10Ω = 2A for entire circuit
Parallel resistance is always less than the smallest individual resistance. Total current IT = 12V/2.4Ω = 5A.
Scenario
Two resistors (4Ω and 6Ω) in parallel with 12V source
Solution
1/RT = 1/4 + 1/6 = 3/12 + 2/12 = 5/12, so RT = 12/5 = 2.4Ω
Applications
- House wiring uses parallel circuits so appliances work independently
- Christmas lights in series - if one bulb burns out, all go out
- Car headlights in parallel - if one fails, the other still works
- Battery packs combine cells in series for higher voltage or parallel for longer life
Misconceptions
- Thinking parallel resistance is sum of individual resistances - it's actually less
- Confusing which quantity is same in series vs parallel arrangements
- Forgetting that parallel total resistance is always smaller than smallest individual resistance
Related Concepts
- Ohm's Law
- Electrical Power Distribution
- Circuit Analysis
Common Exam Questions
Example
For series: just add resistances. For parallel: use reciprocal formula
Approach
Identify series or parallel, apply appropriate formula
Question Type
Calculate total resistance
Example
In parallel, voltage is same across branches. In series, current is same throughout
Approach
Use Ohm's Law with appropriate total or individual resistances
Question Type
Find current or voltage in branches
Key Points To Remember
- Series: Components connected end-to-end, same current, voltage divides
- Series total resistance: RT = R1 + R2 + R3 + ...
- Parallel: Components connected in branches, same voltage, current divides
- Parallel total resistance: 1/RT = 1/R1 + 1/R2 + 1/R3 + ...
- Series circuits: if one component fails, entire circuit stops
- Parallel circuits: if one branch fails, other branches continue working
Magnetism and Electromagnets
Magnetism is a force that can attract or repel certain materials, especially iron, nickel, and cobalt. Natural magnets have north and south poles, and like poles repel while unlike poles attract. What makes magnetism fascinating is its connection to electricity - moving electric charges create magnetic fields, and changing magnetic fields can create electric currents. This relationship is called electromagnetism. Electromagnets are temporary magnets created by passing electric current through coils of wire, often wrapped around iron cores. They can be turned on and off, and their strength can be controlled by changing the current.
Examples
The iron core concentrates and strengthens the magnetic field created by the current in the wire coil. More turns of wire and higher current make a stronger electromagnet.
Scenario
Creating an electromagnet by wrapping wire around an iron nail
Solution
When current flows through the coiled wire, it creates a magnetic field that magnetizes the iron nail
This demonstrates that electricity and magnetism are related. The direction of deflection follows the right-hand rule.
Scenario
A compass needle deflects near a current-carrying wire
Solution
The electric current creates a magnetic field that interacts with the compass needle's magnetic field
Applications
- Electric motors use electromagnets to convert electrical energy to mechanical motion
- Generators use moving magnets to convert mechanical energy to electrical energy
- MRI machines use powerful electromagnets for medical imaging
- Speakers and headphones use electromagnets to produce sound
- Magnetic levitation trains use electromagnets for propulsion and suspension
Misconceptions
- Thinking magnetic fields only exist around permanent magnets - they exist around any current
- Confusing magnetic field direction with force direction
- Believing electromagnets are always stronger than permanent magnets
Related Concepts
- Electric Current
- Electromagnetic Induction
- Motors and Generators
Common Exam Questions
Example
Current flowing north creates magnetic field circling clockwise when viewed from above
Approach
Use right-hand rule - thumb points in current direction, fingers curl in field direction
Question Type
Identify magnetic field direction
Example
Doubling current or turns doubles magnetic field strength
Approach
Consider current magnitude, number of coil turns, and core material
Question Type
Factors affecting electromagnet strength
Key Points To Remember
- Magnetism is a force that affects ferromagnetic materials
- Magnetic fields exist around magnets and current-carrying wires
- Like magnetic poles repel, unlike poles attract
- Moving electric charges create magnetic fields
- Changing magnetic fields induce electric currents
- Electromagnets can be controlled by electric current
Reflection and Plane Mirrors
When light hits a surface, it can bounce back - this is called reflection. The law of reflection states that the angle of incidence (incoming light) equals the angle of reflection (outgoing light), both measured from a line perpendicular to the surface called the normal. Plane mirrors are flat reflecting surfaces that create virtual images. These images appear to be behind the mirror at the same distance as the object is in front, but they cannot be projected on a screen. The image is laterally inverted (left appears right), the same size as the object, and upright.
Examples
The image is laterally inverted - if you raise your right hand, the image appears to raise its left hand. The image is the same size and upright.
Scenario
Looking at yourself in a bathroom mirror
Solution
You see a virtual image that appears to be the same distance behind the mirror as you are in front
This demonstrates the law of reflection. The total angle between incident and reflected rays is 60°.
Scenario
Light ray hitting a mirror at 30° from the normal
Solution
The reflected ray will make a 30° angle with the normal on the opposite side
Applications
- Bathroom mirrors for grooming and checking appearance
- Car side mirrors and rearview mirrors for safety
- Periscopes in submarines use multiple plane mirrors
- Kaleidoscopes create patterns using multiple plane mirrors
- Security mirrors in stores to monitor customers
Misconceptions
- Thinking angles are measured from the mirror surface instead of the normal
- Believing plane mirror images are real because they look clear
- Confusing lateral inversion with upside-down inversion
Related Concepts
- Light Properties
- Curved Mirrors
- Refraction
Common Exam Questions
Example
If incident ray is 45° from normal, reflected ray is also 45° from normal
Approach
Identify incident angle, apply θi = θr, draw ray diagram
Question Type
Apply law of reflection
Example
Image of 'AMBULANCE' written backwards on emergency vehicles
Approach
Remember VEST: Virtual, Erect, Same size, Turned (laterally inverted)
Question Type
Describe plane mirror image characteristics
Key Points To Remember
- Law of reflection: angle of incidence = angle of reflection
- Angles are measured from the normal (perpendicular line)
- Plane mirror images are virtual, erect, and same size
- Image distance behind mirror equals object distance in front
- Images are laterally inverted (left-right flipped)
- Virtual images cannot be projected on screens
Curved Mirrors - Concave and Convex
Curved mirrors have spherical surfaces that can focus or spread light rays. Concave mirrors curve inward (like the inside of a spoon) and can form both real and virtual images depending on object position. They converge parallel light rays to a focal point. Convex mirrors curve outward (like the outside of a spoon) and always form virtual, upright, and smaller images. They diverge parallel light rays. The focal length is the distance from the mirror to the focal point, and it's half the radius of curvature of the mirror.
Examples
This is how reflecting telescopes work - distant objects form small, real images that can be magnified by eyepieces.
Scenario
Object placed beyond the center of curvature of a concave mirror
Solution
Image formed is real, inverted, and smaller than the object
This gives a wider field of view but makes objects appear farther away - hence the warning 'objects in mirror are closer than they appear.'
Scenario
Using a convex mirror as a car side mirror
Solution
Always produces upright, virtual images smaller than the actual objects
Applications
- Concave mirrors in flashlights and car headlights to focus light
- Concave mirrors in telescopes to collect and focus starlight
- Convex mirrors in stores and parking lots for security surveillance
- Concave mirrors in solar concentrators to focus sunlight for heating
- Makeup mirrors use concave mirrors for magnification
Misconceptions
- Thinking convex mirrors can form real images - they cannot
- Confusing concave with convex - remember concave 'caves in'
- Believing all curved mirrors magnify - convex mirrors make images smaller
Related Concepts
- Plane Mirrors
- Lenses
- Optical Instruments
Common Exam Questions
Example
Object at focus of concave mirror produces no image (rays become parallel)
Approach
Use ray diagrams or remember the patterns for different object positions
Question Type
Determine image characteristics based on object position
Example
If mirror forms real image, it must be concave
Approach
Remember concave can form real images, convex cannot
Question Type
Distinguish between concave and convex mirror properties
Key Points To Remember
- Concave mirrors converge light (curve inward)
- Convex mirrors diverge light (curve outward)
- Focal length f = R/2 (R is radius of curvature)
- Convex mirrors always produce virtual, upright, smaller images
- Concave mirrors can produce real or virtual images depending on object position
- Real images can be projected on screens, virtual images cannot
Refraction and Snell's Law
Refraction occurs when light changes direction as it passes from one medium to another with different optical density. This happens because light travels at different speeds in different materials. Snell's Law quantifies this relationship: n₁sinθ₁ = n₂sinθ₂, where n is the refractive index (a measure of how much the material slows down light) and θ is the angle from the normal. When light goes from a less dense to more dense medium, it bends toward the normal. When going from more dense to less dense, it bends away from the normal.
Examples
The light bends away from the normal as it goes from denser water to less dense air, making the pencil appear displaced.
Scenario
A pencil appears bent when placed in a glass of water
Solution
Light from the submerged part refracts as it exits water (n=1.33) into air (n=1.0)
The light ray bends toward the normal when entering the denser glass medium.
Scenario
Light ray going from air into glass at 45° angle
Solution
Using Snell's Law with nair=1.0, nglass=1.5: sinθglass = (1.0 × sin45°)/1.5 = 0.471, so θglass = 28.1°
Applications
- Eyeglasses and contact lenses correct vision by refracting light
- Camera lenses focus light by refraction to form clear images
- Fiber optic cables use total internal reflection for data transmission
- Prisms separate white light into colors by different refractions
- Mirages occur due to refraction in layers of air at different temperatures
Misconceptions
- Thinking light always bends the same amount regardless of materials
- Confusing the direction of bending - remember denser medium bends toward normal
- Forgetting that angles are measured from the normal, not the surface
Related Concepts
- Light Properties
- Lenses
- Total Internal Reflection
Common Exam Questions
Example
Given incident angle and both refractive indices, calculate refracted angle
Approach
Identify the two mediums, their refractive indices, and given angle
Question Type
Apply Snell's Law calculations
Example
Light going from air (n=1) to water (n=1.33) bends toward normal
Approach
Compare refractive indices - higher n means denser medium
Question Type
Predict direction of bending
Key Points To Remember
- Refraction is bending of light when changing mediums
- Snell's Law: n₁sinθ₁ = n₂sinθ₂
- Refractive index n = speed of light in vacuum / speed in medium
- Light bends toward normal when entering denser medium
- Light bends away from normal when entering less dense medium
- Higher refractive index means light travels slower in that medium
Lenses - Converging and Diverging
Lenses are transparent objects that refract light to form images. Converging (convex) lenses are thicker at the center and bring parallel light rays together at a focal point. They can form both real and virtual images depending on object position. Diverging (concave) lenses are thinner at the center and spread parallel light rays apart, always forming virtual, upright, and smaller images. The behavior of lenses is similar to curved mirrors but involves refraction instead of reflection.
Examples
The eye sees the virtual image as larger than the actual object, making text appear bigger and easier to read.
Scenario
Using a magnifying glass (converging lens) to read small text
Solution
Object placed closer than focal length produces virtual, upright, magnified image
The converging lens refracts light from distant objects to form a sharp image on the camera's sensor or film.
Scenario
Camera lens focusing distant scenery onto film
Solution
Distant objects form real, inverted, smaller images at the focal length
Applications
- Eyeglasses correct nearsightedness with diverging lenses
- Eyeglasses correct farsightedness with converging lenses
- Camera lenses focus light to create photographs
- Microscopes use multiple lenses to magnify tiny objects
- Telescopes use lenses to observe distant celestial objects
Misconceptions
- Thinking all lenses magnify - diverging lenses make images smaller
- Confusing converging with diverging lens shapes
- Believing lenses work the same way as mirrors - they use refraction, not reflection
Related Concepts
- Refraction
- Optical Instruments
- Vision Correction
Common Exam Questions
Example
Person who can't see distant objects clearly needs diverging lens
Approach
Nearsighted needs diverging lens, farsighted needs converging lens
Question Type
Determine lens type needed for vision correction
Example
Given focal length and object distance, find image distance
Approach
Use 1/f = 1/do + 1/di, solve for unknown quantity
Question Type
Calculate image position using lens equation
Key Points To Remember
- Converging lenses are thicker at center, focus parallel rays
- Diverging lenses are thinner at center, spread parallel rays
- Converging lenses can form real or virtual images
- Diverging lenses always form virtual, upright, smaller images
- Focal length is positive for converging, negative for diverging lenses
- Lens equation: 1/f = 1/do + 1/di (f=focal length, do=object distance, di=image distance)
Practice Problems
This problem combines Ohm's Law with power calculations. Remember that power can be calculated using P = VI, P = I²R, or P = V²/R depending on what values are given.
Problem
A 15V battery is connected to a circuit with total resistance of 5Ω. Calculate the current flowing through the circuit and the power consumed.
Solution
Using Ohm's Law: I = V/R = 15V/5Ω = 3A. Power P = VI = 15V × 3A = 45W or P = I²R = (3A)² × 5Ω = 45W
In parallel circuits, reciprocals of resistances add up. The total resistance is always less than the smallest individual resistance. Each branch gets the full 24V.
Problem
Three resistors of 4Ω, 6Ω, and 12Ω are connected in parallel. Find the total resistance and the total current if connected to a 24V source.
Solution
1/RT = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12 = 1/2, so RT = 2Ω. Total current IT = V/RT = 24V/2Ω = 12A
Light bends toward the normal when entering a denser medium. The refracted angle is always smaller than the incident angle in this case.
Problem
A light ray traveling in air hits a glass surface at an angle of 60° from the normal. If the refractive index of glass is 1.5, find the angle of refraction.
Solution
Using Snell's Law: n₁sinθ₁ = n₂sinθ₂. (1.0)(sin60°) = (1.5)(sinθ₂). sinθ₂ = sin60°/1.5 = 0.866/1.5 = 0.577, so θ₂ = 35.3°
The object should be placed 30 cm from the mirror. The image will be at di = 60 cm, real, inverted, and twice the size.
Problem
A concave mirror has a focal length of 20 cm. Where should an object be placed to get a real image that is twice the size of the object?
Solution
For magnification m = -2 (negative because real image is inverted), and using m = -di/do, we get di = -2do. Using 1/f = 1/do + 1/di: 1/20 = 1/do + 1/(-2do) = 1/do - 1/(2do) = 1/(2do), so do = 30 cm
Exam Preparation Tips
- Master Ohm's Law (V=IR) and be comfortable rearranging it to solve for any variable
- For series circuits: resistances add, current is same everywhere, voltage divides
- For parallel circuits: reciprocals of resistances add, voltage is same across branches, current divides
- Remember VEST for plane mirrors: Virtual, Erect, Same size, Turned (laterally inverted)
- Concave mirrors converge light and can form real images; convex mirrors diverge light and only form virtual images
- For refraction problems, always identify both materials and their refractive indices before applying Snell's Law
- Draw ray diagrams for mirror and lens problems to visualize image formation
- Practice unit conversions and keep track of units in calculations
- Learn the typical refractive indices: air/vacuum = 1.0, water = 1.33, glass ≈ 1.5
- Understand the difference between real images (can be projected) and virtual images (cannot be projected)
- Memorize common formulas: P = VI = I²R = V²/R for electrical power
- For electromagnets, remember that magnetic field strength depends on current and number of coil turns
In summary
Understanding electromagnetism, mirrors, and optics provides you with powerful tools to explain many phenomena in the physical world. From the simple circuits that power your devices to the complex optics in cameras and telescopes, these principles are everywhere around us. The key to mastering this chapter is recognizing the patterns: Ohm's Law governs all electrical circuits, the law of reflection explains how mirrors work, and Snell's Law describes how light bends. Practice applying these fundamental principles to different scenarios, and remember that drawing diagrams often makes complex problems much clearer. These concepts frequently appear in UPCAT and other entrance exams, so focus on understanding the underlying physics rather than just memorizing formulas. With solid understanding of these topics, you'll be well-prepared to tackle more advanced physics concepts and real-world applications.
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