Year 9 Physics Revision: Waves, Light & Heat

Wavelength can be calculated using the formula \( \text{Wavelength} = \frac{\text{Speed}}{\text{Frequency}} \). Given the speed of sound in air is 340 m/s and the frequency is 680 Hz, we can substitute these values into the formula: \( \text{Wavelength} = \frac{340 \, \text{m/s}}{680 \, \text{Hz}} = 0.5 \, \text{m} \). This indicates that the distance between successive crests of the sound wave is 0.5 meters.

As you move from radio waves to gamma rays in the electromagnetic spectrum, both frequency and energy increase. This is because gamma rays have much shorter wavelengths than radio waves. According to the relationship between frequency (f), wavelength (λ), and energy (E) in electromagnetic waves, as frequency increases, energy also increases, while wavelength decreases. Thus, the transition from radio waves to gamma rays is characterized by higher frequency and energy, accompanied by a shorter wavelength.

When light travels from a medium with a higher refractive index (glass) to one with a lower refractive index (air), it speeds up. This increase in speed causes the light to bend away from the normal line, which is an imaginary line perpendicular to the surface at the point of incidence. The bending occurs due to the change in the speed of light in different media, following Snell's Law, which states that the angle of incidence is related to the angle of refraction based on the refractive indices of the two media.

Conduction involves the transfer of heat through direct contact between particles, making it most effective in solids where particles are closely packed. Convection, however, primarily occurs in fluids (liquids and gases) due to the movement of warmer, less dense areas rising and cooler, denser areas sinking. Radiation stands out as it does not require a medium for heat transfer, allowing it to travel through a vacuum using electromagnetic waves. Additionally, dark, dull surfaces are more effective at absorbing and emitting infrared radiation compared to light, shiny surfaces, enhancing their thermal interactions.

In a convection current, warm fluid rises because heating causes the fluid's molecules to move faster and spread apart, making it less dense. This reduced density allows the warmer fluid to rise through the cooler, denser fluid surrounding it. Conversely, cooler fluid is denser due to slower-moving molecules that are closer together, causing it to sink. This cycle creates a continuous movement of fluid, generating convection currents.

Sound travels fastest through solids due to the dense arrangement of particles. In solids, particles are closely packed, enabling rapid transmission of vibrational energy through direct collisions. This efficient transfer contrasts with liquids and gases, where particles are more spread out, leading to slower sound propagation. The speed of sound is influenced by the medium's density and elasticity, both of which are higher in solids than in other states of matter, resulting in faster sound travel.

To determine the frequency of a wave, the relationship between wave speed, wavelength, and frequency is used, expressed as \( \text{Frequency} = \frac{\text{Wave Speed}}{\text{Wavelength}} \). In this case, \( \frac{24 \, \text{m/s}}{3 \, \text{m}} = 8 \, \text{Hz} \), confirming the student's claim. Additionally, in sound waves, the pitch is directly related to frequency; higher frequencies correspond to higher pitches. Therefore, the student is correct in both the frequency value and the assertion that frequency determines pitch.