Unit 4: Vision, sound and radiation

The Human Eye and Vision

Working of a human eye

The human eye works very much like a digital camera, capturing light and converting it into electrical signals for the brain.

1. Cornea: The transparent outer layer at the front. It does most of the focusing (refraction) of light.
2. Iris & Pupil: The iris is the coloured part (e.g., blue, brown). It is a muscle that controls the size of the pupil (the black hole in the center).
   • In bright light, the iris closes the pupil to reduce the amount of light entering.
   • In dim light, the iris opens the pupil to let more light in.
3. Lens: A flexible, transparent structure located behind the iris. Its job is fine-tuning the focus. Muscles attached to the lens (ciliary muscles) can change its shape:
   • To see distant objects: The lens becomes thinner and flatter.
   • To see nearby objects: The lens becomes thicker and more curved (this is called accommodation).
4. Retina: The light-sensitive layer at the back of the eye, like the sensor in a camera. It contains millions of photoreceptor cells:
   • Rods: Highly sensitive to light, used for night vision and detecting motion. They do not see colour.
   • Cones: Less sensitive, but used for seeing colour and fine detail. They work best in bright light.
5. Optic Nerve: The photoreceptors convert the light image into electrical signals, which are sent to the brain via the optic nerve. The brain then interprets these signals as "vision."

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Vision Defects and Concept of Colour

Vision Defects

A "normal" eye can focus light from distant objects perfectly onto the retina when the lens is relaxed. Common defects occur when this focusing is incorrect.

• Myopia (Nearsightedness):
  • Symptom: Can see nearby objects clearly, but distant objects are blurry.
  • Cause: The eye is too long, or the lens/cornea is too curved. Light from distant objects is focused *in front of* the retina.
  • Correction: A concave (diverging) lens is used in glasses to spread the light out slightly before it enters the eye, pushing the focal point back onto the retina.
• Hypermetropia (Farsightedness):
  • Symptom: Can see distant objects clearly, but has to strain to see nearby objects (or they are blurry).
  • Cause: The eye is too short, or the lens/cornea is not curved enough. Light from nearby objects is focused *behind* the retina.
  • Correction: A convex (converging) lens is used to bend the light more, pulling the focal point forward onto the retina.
• Presbyopia: This is "age-related farsightedness." As people age, the lens loses its flexibility and cannot become thick enough to focus on close objects. The correction is the same as hypermetropia (a convex lens), often in the form of "reading glasses."

Concept of Colour

"White" light (like from the sun) is actually a mixture of all visible colours. This can be seen in a rainbow or when light passes through a prism, splitting into a spectrum (Red, Orange, Yellow, Green, Blue, Violet).

An object's colour is determined by the wavelengths of light it reflects.

• A red apple appears red because its surface absorbs all colours *except* red, which it reflects to your eye.
• A white sheet of paper reflects *all* colours.
• A black object absorbs *all* colours (which is why it gets hotter in the sun).

In the retina, we have three types of cone cells, each most sensitive to a different wavelength:
(L) Red, (M) Green, and (S) Blue.
Your brain creates all the colours you see by mixing and comparing the strength of the signals from these three cone types. (This is the "RGB model" used in TVs and monitors).

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Radiation and Rays

Radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:

• Electromagnetic (EM) Radiation: Energy traveling as waves of oscillating electric and magnetic fields. This is the EM spectrum, which includes (in order of increasing energy/frequency):
  Radio waves → Microwaves → Infrared → Visible Light → Ultraviolet (UV) → X-rays → Gamma rays
• Particle Radiation: Energy traveling as high-speed subatomic particles (e.g., alpha particles, beta particles, neutrons).

Ionizing vs. Non-Ionizing Radiation:

• Non-Ionizing: Lower energy (Radio, Microwave, IR, Visible). Has enough energy to move or vibrate atoms, but not enough to "ionize" them (knock an electron off).
• Ionizing: Higher energy (UV, X-rays, Gamma rays, and particle radiation). Has enough energy to strip electrons from atoms, creating ions. This is what makes it dangerous to living tissue, as it can break DNA molecules.

The Radiation Poisoning

Radiation poisoning (or Acute Radiation Syndrome - ARS) is an acute illness caused by exposure to a large dose of ionizing radiation over a short period of time.

• Cause: The radiation damages or kills cells in the body, especially cells that divide rapidly (like in the bone marrow, stomach lining, and skin).
• Symptoms: Depends on the dose, but typically starts with nausea and vomiting within hours. This may be followed by a period of feeling well, which is then followed by severe illness (infection, bleeding, hair loss) as the bone marrow fails. Very high doses cause neurological failure and rapid death.
• Source: Nuclear accidents (e.g., Chernobyl), nuclear weapons, or severe accidents with medical or industrial radiation sources.

Medical Applications of Radiation

We can use radiation in controlled ways for great medical benefit, mostly in two categories:

1. Diagnostics (Imaging): Using radiation to "see" inside the body.
   • X-rays: X-rays pass through soft tissue (muscle, fat) but are blocked (absorbed) by dense tissue (like bone). A detector on the other side creates a "shadow" image, showing bones clearly.
   • CT Scans: A "Computed Tomography" scan is like a 3D X-ray. An X-ray machine spins around the body, taking many cross-section "slices," which a computer then assembles into a detailed 3D image. (See Unit 5)
   • Nuclear Medicine: A patient ingests or is injected with a radioactive "tracer." The tracer is designed to collect in a specific organ (e.g., the thyroid). A "gamma camera" then detects the gamma rays coming *out* of the patient, showing how the organ is functioning.
2. Therapy (Treatment):
   • Radiotherapy (Cancer Treatment): Uses very high-energy X-rays or gamma rays in a focused beam to kill cancer cells. The beam is aimed at the tumor from many different angles, so the tumor gets a massive, lethal dose, while the surrounding healthy tissue only gets a small, survivable dose from each pass.

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The Ear and Hearing

The Ear (Working)

The ear is a complex organ that converts sound waves (pressure waves in the air) into electrical signals for the brain.

1. Outer Ear: The visible part (pinna) acts like a funnel, collecting sound waves and guiding them into the ear canal.
2. Middle Ear: The sound waves hit the eardrum (tympanic membrane), causing it to vibrate. These vibrations are passed along and amplified by three tiny bones (the ossicles): the malleus (hammer), incus (anvil), and stapes (stirrup).
3. Inner Ear: The stapes pushes on a membrane called the oval window, sending the vibrations into the cochlea. The cochlea is a spiral-shaped, fluid-filled tube. Inside it, tiny hair cells are "tuned" to different frequencies. High-frequency sounds vibrate the hair cells near the entrance, while low-frequency sounds vibrate the ones deep inside. When a hair cell vibrates, it sends an electrical signal to the brain via the auditory nerve.

Hearing Spectra

The hearing spectrum (or audible range) is the range of sound frequencies that can be heard by a species.

• Humans: 20 Hz to 20,000 Hz (or 20 kHz). We are most sensitive in the 2,000 - 5,000 Hz range (human speech).
• Infrasound: Frequencies *below* 20 Hz. We can't hear it, but animals like elephants use it to communicate.
• Ultrasound: Frequencies *above* 20 kHz. We can't hear it, but dogs, bats, and dolphins use it for echolocation and communication. (See Unit 5 for medical use).

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Sound and Acoustics

Sound Waves and Hearing

Sound is a mechanical wave, which means it needs a medium (like air, water, or a solid) to travel. It cannot travel through a vacuum. It is a longitudinal wave, meaning the particles of the medium vibrate *parallel* to the direction of the wave's travel (creating compressions and rarefactions).

Hearing is the brain's perception and interpretation of these vibrations, as detected by the ear.

Sound Intensity (I)

Sound intensity is the power of a sound wave per unit area. It is an objective, physical measurement of the energy the wave carries.

I = Power / Area (Units: Watts/m²)

The human ear can detect a *vast* range of intensities, from the "threshold of hearing" (I₀ = 10⁻¹² W/m²) to the "threshold of pain" (I ≈ 1 W/m²).

The Decibel Scale (dB)

Because the range of intensities is so huge, we use a logarithmic scale called the decibel scale to measure sound level (β). This scale is much more manageable and better matches our perception of "loudness."

β (in dB) = 10 × log₁₀ ( I / I₀ )

• `I` is the intensity of the sound.
• `I₀ = 10⁻¹² W/m²` is the "threshold of hearing," the reference level.

Sound Source	Sound Level (dB)
Threshold of Hearing (I₀)	0 dB
Quiet whisper	~30 dB
Normal conversation	~60 dB
Heavy city traffic	~85 dB
Rock concert / Jet engine	~120 dB (Threshold of Pain)

Noise Pollution

Noise is defined as any unwanted or unpleasant sound, especially one that is loud or disruptive. Noise pollution is the presence of such sound at levels that are harmful to human and animal life.

Effects:

• Health: Hearing loss (damage to the hair cells), high blood pressure, stress, sleep disturbances, heart disease.
• Environment: Disrupts animal communication and navigation (especially in the ocean).

Any sound above 85 dB can cause permanent hearing damage with prolonged exposure.

Acoustics of loading and reverberation

Acoustics is the science of sound, particularly how it behaves in an enclosed space (like a concert hall or classroom).

• Acoustics of "loading": This is likely a typo for "auditoriums" or "buildings". Good acoustic design aims to make sound clear and pleasant.
  • Reflection: Hard, flat surfaces (like concrete walls) reflect sound and create echoes.
  • Absorption: Soft, porous surfaces (like curtains, carpets, acoustic foam) awsund, reducing echoes.
  • Diffusion: Irregular, convex surfaces scatter sound in many directions, preventing "flat" echoes and making the sound feel "live."
• Reverberation: This is the persistence of sound in a room after the original sound source has stopped. It's the combined effect of thousands of echoes.
  • Reverberation Time (RT60): The time it takes for the sound level to drop by 60 dB.
  • Too much reverberation (long RT60): (e.g., in a stone cathedral). Sound becomes "muddy" and speech is hard to understand.
  • Too little reverberation (short RT60): (e.g., in an anechoic chamber or a room with heavy curtains). The sound is "dead" and flat.
  • Good design: A concert hall for an orchestra needs a "warm" reverberation (RT60 ≈ 2s). A lecture hall needs a "clear" sound (RT60 < 1s).