Light, Sound, and Electricity
This domain tests reflection and refraction, image formation by mirrors and lenses, the properties of sound, and basic electric circuits using Ohm's law.
Core rules
- Light travels in straight lines in a uniform medium. Reflection follows the law that angle of incidence equals angle of reflection. Refraction is bending due to a change in speed when light enters another medium.
- A convex lens converges light; a concave lens diverges it. A concave mirror can form real or virtual images depending on object position; a plane mirror forms a virtual, upright, laterally inverted image of the same size.
- Sound needs a material medium and cannot travel through a vacuum. Frequency determines pitch; amplitude determines loudness.
- Electric current is the rate of charge flow, measured in amperes. Ohm's law states V = IR for a conductor at constant temperature. In series, current is the same and voltage divides; in parallel, voltage is the same and current divides.
Worked example
A resistor of 4 Ω is connected across a 12 V battery. Find the current. Use V = IR, so I = V/R = 12/4 = 3 A. Check the units: volts divided by ohms gives amperes. If the resistor were replaced by one of 6 Ω, the current would fall to 2 A because current is inversely proportional to resistance at constant voltage.
Traps that cost marks
Do not say current is used up in a series circuit; it is the same at every point. Do not state that sound travels faster in a vacuum; it cannot travel there at all. Distinguish real images, which can be caught on a screen, from virtual images. When using Ohm's law, keep voltage, current, and resistance in consistent units.
Key points
- Reflection: angle of incidence equals angle of reflection.
- Sound requires a medium; frequency controls pitch, amplitude controls loudness.
- Ohm's law: V = IR.
- Series current is constant; parallel voltage is constant.