Unit 12: Waves – Long Questions & Answers

As waves travel through a medium, they cause the particles within it to vibrate. These vibrations allow energy to be transferred from one particle to the next. However, the particles themselves do not move along with the wave; they simply oscillate around their original positions.

Waves can transfer energy from one place to another without transferring matter.

Examples from daily life:

• Water Ripples: When a twig is dropped into still water, ripples carry energy outward, yet the water and the twig remain mostly in place.
• Sound Waves: When a guitar string is plucked, air particles vibrate in place to pass the energy forward to a listener's ear without the air itself moving permanently from the source.
• Spring Vibrations: In a stretched spring, compressions and rarefactions travel through the coils, transmitting energy without the entire spring moving forward.

The primary difference between these two types of waves is their dependence on a medium for travel:

• Mechanical Waves: Mechanical waves are waves that require a material medium such as air, water, or a solid material to travel. These waves cannot pass through vacuum because they rely on the vibration of particles in the medium to transfer energy. For example, sound waves, water waves, and waves on a string.
• Electromagnetic Waves: Electromagnetic waves are different from mechanical waves because they do not require a material medium to travel. These waves can move through empty space (vacuum), that is why sunlight and other types of radiation from the Sun are able to reach the Earth through space. For example, radiowaves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

Transverse Waves: In transverse waves, the particles of the medium vibrate at right angles to the direction in which the wave is travelling.

How they are produced: A simple way to observe a transverse wave is by holding one end of a rope and moving it up and down. While the wave travels horizontally along the rope, the rope segments move vertically.

Key Regions:

• Crest: The crest is the region of a transverse wave, above the mean position. It is the upper part of a wave.
• Trough: The trough is the region of a transverse wave, below the mean position. It is the lower part of a wave.

For example, water waves, electromagnetic radiation (such as light, radiowaves, and X-rays).

Longitudinal Waves: In longitudinal waves, the particles of the medium vibrate parallel to the direction in which the wave is travelling.

How they are produced: These waves are produced when a disturbance pushes particles together and then allows them to spread apart. For instance, when someone claps his hands, the air molecules near the hands are first pushed together.

Key Regions:

• Compression: A compression is a region in a longitudinal wave where the particles of the medium are close together, resulting in high pressure.
• Rarefaction: A rarefaction is a region in a longitudinal wave where the particles are spread apart, resulting in low pressure.

For example, sound wave.

Reflection: Reflection is the bouncing back of waves into the same medium after striking the surface of another medium.

Explanation: This behaviour can be clearly observed using a ripple tank. In this setup, when straight water waves generated by a vibrating bar move across the tank and strike a barrier, they reflect from the surface. The property is governed by the law of reflection, which states that: The angle at which the waves approach the barrier is called the angle of incidence, and the angle at which they reflect is called the angle of reflection. These two angles are always equal. This is called law of reflection.

Angle of incidence = Angle of reflection

Examples: This property is common to all types of waves, including water waves, sound waves (such as echoes), and light waves (as seen in mirrors).

Refraction: When a wave passes from one medium into another at an angle, its wavelength and speed change, causing the wave to change direction. This process is called refraction.

Explanation: Refraction occurs when waves pass from one medium into another and change direction due to change in speed. As the water waves cross into this shallow region, they slow down and bend, demonstrating the refraction of waves. Although the speed and wavelength of the waves change during this process, their frequency remains the same.

Examples: This change in wave behaviour is illustrated in Figure, where wave fronts are shown bending as they pass from deep water into shallow water.

Diffraction: The spreading of waves when they pass through a slit or move around an obstacle is called diffraction.

Explanation: This behaviour can be easily observed in water waves when they encounter openings of different sizes. The amount of diffraction depends on the size of the slit relative to the wavelength:

• Case 1: When the size of slit is wider than the wavelength, the waves pass mostly straight through with only slight bending at the edges.
• Case 2: If the slit size is nearly equal to the wavelength, the waves spread out more and appear almost circular beyond the slit.
• Case 3: When the size of slit is smaller than the wavelength, the waves show maximum diffraction, forming strong circular patterns.

Example: We can hear a person behind a wall due to diffraction of sound waves. Diffraction is the bending and spreading of waves around obstacles. Sound waves have relatively long wavelengths, so they can bend around walls and reach our ears even without a direct path.

Diffraction is the spreading of waves when they pass through a slit or around an obstacle. (For detailed explanation, see Q.12.4.)

The amount of diffraction depends on the size of the slit relative to the wavelength:

• When the size of slit is wider than the wavelength, the waves pass mostly straight through with only slight bending at the edges.
• If the slit size is nearly equal to the wavelength, the waves spread out more and appear almost circular beyond the slit.
• When the size of slit is smaller than the wavelength, the waves show maximum diffraction, forming strong circular patterns.

This shows that the amount of diffraction increases when the size of slit is closer to or smaller than the wavelength of the waves.