The waves which cannot travel without a medium are sound waves.

Sound waves are mechanical waves that require a medium, such as air, water, or solid objects, to travel through. When an object vibrates, it creates a disturbance in the surrounding medium, which then propagates as a sound wave. The particles of the medium vibrate back and forth, transferring energy from one particle to another, and the wave travels through the medium.

On the other hand, X-rays, radio waves, and light waves are examples of electromagnetic waves, which can travel through a vacuum or empty space without the need for a medium. They are created by the oscillation of electric and magnetic fields and do not require particles to propagate.

When a wave propagating through a medium encounters a change in medium, several properties of the wave can be affected. However, one property that remains the same is the frequency.

Frequency is the number of complete oscillations or cycles of a wave that occur in a given time period. It is measured in hertz (Hz) and is inversely proportional to the wavelength of a wave. The frequency of a wave remains constant when it encounters a change in medium because it depends on the source of the wave and not the medium through which it is traveling.

Explanation:

When a wave encounters a change in medium, its speed, amplitude, and wavelength may change, but the frequency remains the same. This is known as the principle of conservation of frequency.

1. Speed:
The speed of a wave is the distance it travels in a given time period. It is determined by the properties of the medium through which the wave is traveling. When a wave enters a new medium, its speed may change due to differences in the properties of the medium, such as density or elasticity.

2. Amplitude:
The amplitude of a wave is the maximum displacement of particles in the medium from their equilibrium position. It represents the energy carried by the wave. When a wave encounters a change in medium, its amplitude may change due to reflection, refraction, or absorption of the wave energy by the new medium.

3. Wavelength:
The wavelength of a wave is the distance between two consecutive points in the wave that are in phase. It is inversely proportional to the frequency of the wave. When a wave enters a new medium, its wavelength may change due to differences in the speed of the wave in the new medium.

However, the frequency of a wave remains constant because it is determined by the source of the wave, such as a vibrating object or an electromagnetic source. The frequency does not depend on the properties of the medium through which the wave is traveling.

In conclusion, when a wave encounters a change in medium, the speed, amplitude, and wavelength of the wave may change, but the frequency remains the same. This principle is important in understanding the behavior of waves when they interact with different materials and mediums.

3.option4
4.option3