UPCAT Physics — Fluids, Waves & LightFlash Cards

A sound wave has a frequency of 440 Hz and travels at 343 m/s. Calculate its wavelength.

Formula: v = λf, so λ = v/f Step 1: λ = 343 m/s ÷ 440 Hz Step 2: λ = 0.78 m Answer: The wavelength is 0.78 m (or 78 cm)

What is the difference between luminous and illuminated bodies? Give examples.

Luminous bodies produce and emit their own light (e.g., sun, light bulb, candle, firefly). Illuminated bodies can only be seen when light from another source strikes them (e.g., moon, book, table, yourself). We see illuminated objects because they reflect light from luminous sources.

Light travels at 3.0 × 10⁸ m/s and has a wavelength of 600 nm. Find its frequency.

Formula: v = λf, so f = v/λ Step 1: Convert nm to m: 600 nm = 600 × 10⁻⁹ m Step 2: f = (3.0 × 10⁸ m/s) ÷ (600 × 10⁻⁹ m) Step 3: f = 5.0 × 10¹⁴ Hz Answer: The frequency is 5.0 × 10¹⁴ Hz

Why can you hear someone calling your name from around a corner, but you cannot see them?

This demonstrates wave diffraction. Sound waves have longer wavelengths (meters to centimeters) compared to light waves (nanometers). Longer wavelengths can bend around obstacles more effectively. Sound diffracts around corners and buildings, while light's short wavelength cannot bend around large obstacles like walls.

What happens to wave speed when a wave moves from air into water?

Wave speed changes because the medium changes density. For sound waves: speed increases in water (about 1500 m/s) compared to air (343 m/s) because water is denser and more elastic. For light waves: speed decreases in water due to higher refractive index. This speed change causes refraction (bending) of the wave.

A lightning bolt creates a sound wave with frequency 34 Hz and wavelength 10.0 m. Calculate the wave speed.

Formula: v = λf Step 1: v = (10.0 m)(34 Hz) Step 2: v = 340 m/s Answer: The wave speed is 340 m/s Check: 340 m/s ÷ 10.0 m = 34 Hz ✓

Explain the difference between transparent, translucent, and opaque materials.

Transparent: Light passes through clearly, objects visible (glass, clear water, air). Translucent: Light passes through but is scattered, objects not clearly visible (frosted glass, tissue paper, clouds). Opaque: No light passes through (wood, metal, concrete). These properties depend on how the material's structure interacts with light waves.

When should you use the wave equation v = λf? Give a practical example.

Use when you know any two of: wave speed (v), wavelength (λ), or frequency (f) to find the third. Example: Radio station broadcasts at 101.5 MHz. Radio waves travel at light speed (3×10⁸ m/s). Find wavelength: λ = v/f = (3×10⁸)/(101.5×10⁶) = 2.96 m. This determines antenna length needed.

A water wave has wavelength 2.0 m and frequency 0.5 Hz. Find its speed.

Formula: v = λf Step 1: v = (2.0 m)(0.5 Hz) Step 2: v = 1.0 m/s Answer: The wave speed is 1.0 m/s

How do transverse and compressional waves differ in particle motion?

Transverse waves: Particles move perpendicular (at right angles) to wave direction. Examples: light waves, water waves, waves on a string. Compressional waves: Particles move parallel (same direction) as wave travels through compressions and rarefactions. Examples: sound waves, earthquake P-waves. Both transport energy without transporting matter.

State the relationship between wave energy and amplitude.

Wave energy is proportional to the square of amplitude (E ∝ A²). This means: doubling amplitude quadruples the energy. Example: A tsunami with twice the wave height carries four times more destructive energy. Amplitude determines how much energy the wave transports, while frequency and wavelength determine other wave properties.

Why does light from a star take years to reach Earth, but we see it instantly when it arrives?

Light travels at constant speed (3×10⁸ m/s) but covers enormous distances in space. Example: Light from nearest star (4.3 light-years away) travels for 4.3 years to reach us. Once it enters our atmosphere and hits our eyes, we detect it immediately because the final journey is very short. The delay is travel time, not detection time.

Calculate the time for light to travel from the Sun to Earth (150 million km).

Formula: time = distance/speed Step 1: Convert km to m: 150 × 10⁶ km = 1.5 × 10¹¹ m Step 2: time = (1.5 × 10¹¹ m)/(3.0 × 10⁸ m/s) Step 3: time = 500 seconds = 8.33 minutes Answer: Light takes about 8.3 minutes to travel from Sun to Earth

What causes wave interference and what are its types?

Interference occurs when two or more waves meet and combine. Constructive interference: Waves add up (peaks + peaks), creating larger amplitude. Destructive interference: Waves cancel out (peaks + troughs), reducing amplitude. Examples: Noise-canceling headphones use destructive interference; stereo speakers use constructive interference for better sound.

How do rod cells and cone cells in the eye differ in function?

Rod cells: Detect light intensity, work in dim light, responsible for night vision, cannot detect color (black/white vision). Cone cells: Detect different colors, work in bright light, responsible for color vision, three types (red, green, blue sensitive). Both convert light into electrical signals sent to the brain for processing.

A wave has frequency 2.0 Hz and speed 8.0 m/s. Find its wavelength.

Formula: v = λf, so λ = v/f Step 1: λ = 8.0 m/s ÷ 2.0 Hz Step 2: λ = 4.0 m Answer: The wavelength is 4.0 m

Why do objects appear different colors?

Object color depends on which light wavelengths it reflects or absorbs. A red apple absorbs blue and green light but reflects red light to our eyes. A white object reflects all colors equally. A black object absorbs all colors. The color we see is the reflected light, not the absorbed light. This is why objects can look different under colored lights.

List the electromagnetic spectrum in order of increasing frequency.

From lowest to highest frequency: Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays. Remember: As frequency increases, wavelength decreases. All travel at light speed (3×10⁸ m/s) in vacuum. The Sun emits mainly infrared, visible light, and ultraviolet radiation.

What determines the loudness and pitch of sound waves?

Loudness: Determined by wave amplitude (energy). Larger amplitude = louder sound. Measured in decibels (dB). Pitch: Determined by wave frequency. Higher frequency = higher pitch. Example: A guitar string vibrating 440 Hz produces the note A. Tightening the string increases frequency and pitch; plucking harder increases amplitude and loudness.

A radio wave has wavelength 3.0 m. Calculate its frequency.

Formula: v = λf, so f = v/λ Step 1: For radio waves, v = 3.0 × 10⁸ m/s Step 2: f = (3.0 × 10⁸ m/s) ÷ (3.0 m) Step 3: f = 1.0 × 10⁸ Hz = 100 MHz Answer: The frequency is 100 MHz