● Transverse Electromagnetic Wave: Light is a transverse wave, but crucially, it is an electromagnetic wave. This means it is composed of mutually perpendicular oscillating electric and magnetic fields that propagate together. Unlike mechanical waves, these fields do not require a material medium to propagate.

● No Medium Required for Travel: The most significant difference between light and sound is that light can travel through a vacuum. This is why light from the Sun and distant stars reaches Earth, traversing vast empty stretches of space.

● Speed of Light in Vacuum (c): Light travels at an astounding speed in a vacuum, which is a universal constant: c≈3.00×108 meters per second (m/s) This is approximately 300,000 kilometers per second. This speed is the ultimate speed limit in the universe, according to Einstein's theory of relativity.

● Speed of Light in Media: When light passes through a material medium (like air, water, or glass), it slows down. The denser the medium (optically denser, not necessarily physically denser), the slower light travels. This change in speed is what causes refraction.

Visible light, the light our eyes can detect, is only a tiny sliver of a much larger continuum of electromagnetic waves, collectively known as the Electromagnetic Spectrum. All electromagnetic waves travel at the speed of light in a vacuum and differ only in their wavelength and frequency. As wavelength decreases, frequency increases (and vice versa), and the energy carried by the wave increases.

Let's explore the different regions of the EM spectrum, from longest wavelength/lowest frequency to shortest wavelength/highest frequency:
1. Radio Waves:
○ Properties: Longest wavelengths (meters to kilometers), lowest frequencies. Produced by oscillating electric currents.
○ Uses: Broadcast radio and television signals, wireless communication (Wi-Fi, Bluetooth), remote controls, MRI (Magnetic Resonance Imaging) in medicine.
○ Dangers: Generally considered non-ionizing and safe at typical exposure levels for humans.
2. Microwaves:
○ Properties: Shorter wavelengths (centimeters to meters) than radio waves.
○ Uses: Microwave ovens (cause water molecules in food to vibrate and heat up), radar (for detecting objects and measuring speed), satellite communication, mobile phones, GPS.
○ Dangers: Can cause internal heating of body tissues if exposed to high intensities (e.g., from a leaking microwave oven or strong radar emitter).
3. Infrared (IR) Radiation:
○ Properties: Longer wavelengths than visible light, often associated with heat. All objects emit infrared radiation, depending on their temperature.
○ Uses: Remote controls (for TVs, air conditioners), thermal imaging cameras (for night vision, identifying heat leaks in buildings, medical diagnostics), optical fibers (for data transmission), heat lamps, toasters.
○ Dangers: High intensity IR can cause skin burns or heat damage to the eyes.
4. Visible Light:
○ Properties: The narrow band of the EM spectrum that the human eye can detect. It ranges from red (longest wavelength, lowest frequency within visible light) to violet (shortest wavelength, highest frequency within visible light). This is the light that allows us to see colors.
○ Uses: Vision, illumination, photography, lasers, optical microscopes, fiber optics.
○ Dangers: Staring directly at very bright light sources (like the sun or powerful lasers) can cause severe and permanent eye damage.
5. Ultraviolet (UV) Radiation:
○ Properties: Shorter wavelengths and higher frequencies than visible light. Carries more energy than visible light.
○ Uses: Sunbeds (for tanning), sterilization (killing germs in water purifiers or medical equipment), detecting fake currency, forensic analysis, curing certain dental fillings. Also essential for Vitamin D production in the skin.
○ Dangers: UV radiation is ionizing (can remove electrons from atoms). Excessive exposure can lead to sunburn, premature skin aging, increased risk of skin cancer (melanoma), and eye damage (e.g., cataracts). The Earth's ozone layer largely protects us from the most harmful UV-C radiation.
6. X-rays:
○ Properties: Very short wavelengths, very high frequencies. High energy, highly penetrating.
○ Uses: Medical imaging (radiography for bone fractures, dental X-rays, mammograms), security scanning (airport luggage), industrial inspection, astronomy (to study hot gas in space).
○ Dangers: X-rays are a form of ionizing radiation and can cause cellular damage, potentially leading to cancer or genetic mutations. Exposure is minimized and carefully controlled in medical and security applications.
7. Gamma Rays (γ-rays):
○ Properties: Shortest wavelengths, highest frequencies. Highest energy in the EM spectrum. Produced by radioactive decay and nuclear reactions.
○ Uses: Radiotherapy (to kill cancer cells), sterilization of medical equipment and food products, industrial gauges, astronomy (studying energetic phenomena in the universe).
○ Dangers: Extremely penetrating and highly damaging ionizing radiation. Can cause severe cellular damage, acute radiation sickness, and a very high risk of cancer. Strict safety protocols are necessary when handling gamma ray sources.

Reflection is the phenomenon where light bounces off a surface and changes direction without passing through the surface.

● Laws of Reflection: These two laws govern how light reflects off a smooth surface:
○ The angle of incidence equals the angle of reflection (∠i=∠r).
■ To understand this, we use the concept of the normal, which is an imaginary line drawn perpendicular (at a 90-degree angle) to the reflecting surface at the point where the incident ray strikes.
■ The incident ray is the incoming light ray.
■ The reflected ray is the light ray bouncing off the surface.
■ 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.
○ The incident ray, the reflected ray, and the normal all lie in the same plane. This means they can all be drawn flat on a single piece of paper, not in three-dimensional space with one sticking out.

Refraction is the phenomenon of light bending as it passes from one transparent medium into another, due to a change in its speed.

● Laws of Refraction (Qualitative Understanding):
○ The incident ray, the refracted ray, and the normal all lie in the same plane. Similar to reflection, these three lines exist within a single flat plane.
○ When light passes from a less optically dense medium (a medium where light travels faster, e.g., air) to a more optically dense medium (a medium where light travels slower, e.g., water or glass), the light ray bends towards the normal. This makes the angle of refraction smaller than the angle of incidence.
○ When light passes from a more optically dense medium to a less optically dense medium, the light ray bends away from the normal. This makes the angle of refraction larger than the angle of incidence.
○ No Bending for Normal Incidence: If a light ray enters a new medium perpendicular to the surface (i.e., along the normal, with an angle of incidence of 0∘), it will not change direction (it won't bend), but its speed will still change.

Lenses are carefully shaped transparent materials (usually glass or plastic) that use the principle of refraction to converge or diverge light rays, thereby forming images. They are fundamental components in eyeglasses, cameras, telescopes, microscopes, and projectors.

● Key Terminology for Lenses:
1. Principal Axis: An imaginary straight line passing symmetrically through the optical center of the lens.
2. Optical Center (O): The central point of the lens through which light rays pass without deviation (without changing direction).
3. Principal Focus (F or focal point): For a converging lens, this is the point on the principal axis where parallel rays of light converge after passing through the lens. For a diverging lens, it's the point from which parallel rays of light appear to diverge after passing through the lens. Every lens has two principal foci, one on each side.
4. Focal Length (f): The distance between the optical center of the lens and its principal focus. It determines the 'power' of the lens (how strongly it converges or diverges light).