Introduction to Sound

 Sound plays an important role in our life. It is through sound we know that a period in school is over or if someone is approaching you by just listening to the footsteps. Vibrating objects produce sound. Vibration is the to and fro or back and forth movement of an object. Sound needs a medium to travel. Hence, it cannot travel in a vacuum.

Introduction to waves

• The sound is produced by vibrating objects.
• They travel from one place to another in the form of waves. Hence, the name sound waves.

Wave and particle motion of waves

• Mechanical waves are waves that travel through a material medium.
• It is of two types: depending on the direction of motion of the particle of the medium and the wave propagation:

*transverse
*longitudinal

Transverse waves

• Particle motion is to perpendicular the direction of wave motion.
• This type of wave is a mechanical wave called a transverse wave. E.g.: Light, or even a Mexican wave in a stadium.

Longitudinal waves

• When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.
• It is also a mechanical wave.
• Example: a slinky.

Sound Properties

Introduction to sound waves

– Sound needs a medium to propagate. The matter or material through which sounds propagates is called a medium.
– Sound cannot travel in a vacuum. The moon does not have an atmosphere, hence, you can hear on the moon.

Sounds by Humans

How do humans produce sound?

• The sound produced in the voice box called the larynx located at the upper end of the windpipe.
• 2 vocal cords get stretched across in the voice box. Has a slit, through which air is forced out by the lungs.
• Muscles attached to vocal cords make it tight or loose.

Hearing

Human ear

• Outer ear = pinna: collects sound from the surroundings.
• Sound passes through a tube called an auditory canal.
• The eardrum (tympanic membrane) → vibrates when the sound incident.
• Vibrations are sent to the inner ear, from there it goes to the brain as signals via the auditory nerve.

Amplitude, Time Period, and Frequency

Amplitude, frequency, and time period of vibrations

• The magnitude of disturbance in the medium on either side of the mean value is called Amplitude(A). Larger the amplitude, the louder the sound.
• The number of oscillations per second is called frequency. Expressed in Hertz (Hz).
• Time is taken for one complete oscillation to travel across a point. T = 1/f. (Seconds)

Loudness and Pitch

• The volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.
• The number of oscillations per unit time. Directly proportional to frequency.

Audible and inaudible sounds

• Audible range = 20Hz to 20kHz known as the Sonic range.
• Below 20 Hz (inaudible) → infrasonic range
• Above 20 kHz (inaudible) → Ultrasonic range

Noise Pollution

Noise and music

– Sounds with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears while noise is not.
– Unpleasant sounds are called noise.

Noise pollution and measures to control it

– Presence of excessive unwanted noise in our surroundings is called noise pollution.
– Can cause hearing impairment, sleeplessness, and also hypertension.
– Must be minimized by reducing noisy operations and honking in residential areas. Planting trees along roads also cut down the noise.

 

Introduction to Light

The world as we know is largely known through senses and the sense of sight is one of the most important senses. It is only when the light from an object enters our eyes that we see it. The light might have been either emitted by the object or reflected by it. Some of the important terminologies related to light are:

Light is the energy that enables us to see. Light is emitted from a source such as the Sun. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight.

How do we see objects?

Our eyes alone do not allow us to see. Light from a source falls on an object and then bounces off onto our eyes and that is how we perceive it.

Laws of Reflection

Reflection

Light gets reflected off surfaces. Any surface that is really well polished or shiny acts as a mirror. The phenomenon of light bouncing off surfaces is called reflection.

Laws of reflection

• The ray of light that strikes a reflecting surface is called the incident ray and the ray that gets reflected back is called the reflected ray.
• The imaginary line that is perpendicular to the reflecting surface is called the normal.
• The angle of incidence ∠i is the angle between the incident ray and the normal. The angle of reflection ∠r is the angle between the reflected ray and the normal.
• Laws of Reflection: Angle of incidence ∠i is always equal to the Angle of reflection ∠r (∠i=∠r). The angle of incidence, the angle of reflection, and the normal all lie on the same plane.

Ray diagram

The path taken by a narrow beam of light is depicted as a ray diagram.

Regular and Diffused Reflection

Regular and diffused reflection

Reflection from a smooth surface like a mirror that follows the laws of reflection is called as regular reflection.

Reflection over a surface that has irregularities where it does not follow laws of reflection is called as irregular or diffused reflection

Image formation in a plane mirror

• The image formed by a plane mirror is always virtual, erect and object and image are equidistant from the mirror.
• The image formed in a plane mirror undergoes lateral inversion.

Difference between the real and virtual image

A real image is formed by the actual convergence of light rays. In optics, a virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be located at the point of apparent divergence. Because the rays never really converge, a virtual image cannot be projected onto a screen.

Multiple reflections

• Two mirrors inclined to each other give multiple images, due to multiple reflections.
• When an object is kept between two parallel plane mirrors, infinite images are formed.

Calculating the total number of images

Number of images formed by plane mirrors is given by n=360∘θ−1.

Dispersion

Dispersion of light

• The splitting of light into its component colours is called as dispersion.
• Example: Rainbow

Human Eye

Human Eye

The human eye is a spherically shaped sense organ(diameter about 2.3 cm) that gives the sense of sight and helps us perceive colours and depth.

Functions of different organs in the human eye

• Light enters the eye through a thin membrane that forms a transparent bulge on the front of the eye, called as the cornea.
• Iris is a dark muscular diaphragm that controls the size of the pupil.
• The pupil is the small opening in the centre of the iris.
• The lens which is behind the pupil helps to adjust the focal length required to clearly focus on objects at various distances.
• The retina is the screen where the images get formed. It is a delicate membrane with a large number of photosensitive cells called rods and cones.
• Cones are sensitive to bright light and rods are sensitive to dim light.
• The optic nerve transmits the electrical signals from the eye to the brain.

Visual Defects

Visual defects

• Visual defects include the inability to see near objects, or the inability to see far objects.
• In old people the lens becomes cloudy, a condition called cataract that impairs vision.

Seeing Sans Eyes

Braille system

• Braille is a system to read and write for visually impaired people.
• Consists of 63 dot patterns or characters.
• These patterns are embossed in braille sheets that can be recognized on touch.
• The dots are raised in order to make it easier to touch.