Question

waves and matter
check your understanding - question 2 of 3 attempt 1 of 2
which three statements about waves are true?
a. light waves travel fastest through empty space.
b. mechanical waves travel fastest through empty space.
c. when a wave passes into a new medium and changes speed, its path bends.
d. sound waves travel faster through steel than through air.
e. electromagnetic waves require a medium.

Explanation:

• Option A: Light is an electromagnetic wave. Electromagnetic waves travel fastest in a vacuum (empty space) as there are no particles to interfere with their propagation. The speed of light in a vacuum is \(c = 3\times10^{8}\space m/s\).
• Option C: This is the phenomenon of refraction. When a wave (e.g., light wave, sound wave) enters a new medium with a different refractive index (for light) or a different elastic and inertial property (for sound), the change in speed causes the wave to bend. The law of refraction (Snell's law for light) mathematically describes this behavior \(\frac{\sin\theta_1}{\sin\theta_2}=\frac{v_1}{v_2}\), where \(v_1\) and \(v_2\) are the speeds of the wave in the two media and \(\theta_1\) and \(\theta_2\) are the angles of incidence and refraction respectively.
• Option D: Sound is a mechanical wave. Mechanical waves rely on the vibration of particles in a medium. In solids (like steel), the particles are closely packed. The speed of sound \(v=\sqrt{\frac{B}{

ho}}\) (for longitudinal waves, where \(B\) is the bulk modulus and \(
ho\) is the density). Solids generally have higher bulk modulus compared to gases (air). For example, the speed of sound in air is approximately \(v_{air}\approx343\space m/s\) at room - temperature, while in steel it is around \(v_{steel}\approx 5960\space m/s\).

• Option B: Mechanical waves (e.g., sound waves, water waves) require a medium. In empty space (vacuum), there are no particles for mechanical waves to oscillate, so they cannot travel.
• Option E: Electromagnetic waves (e.g., light, radio waves) do not require a medium. They can travel through a vacuum as they are composed of oscillating electric and magnetic fields that regenerate each other.

Answer:

A. Light waves travel fastest through empty space.
C. When a wave passes into a new medium and changes speed, its path bends.
D. Sound waves travel faster through steel than through air.