Light Physics: Electromagnetic Waves, Mirrors, Lenses

Posted on Aug 29, 2025 in Physics

Electromagnetic Spectrum: Seven Types

The electromagnetic spectrum encompasses various forms of energy, each with unique properties and applications:

  • Gamma-rays: Highly energetic form of electromagnetic radiation, often associated with nuclear processes.
  • X-rays: Used for medical imaging (e.g., seeing bones) and security scans.
  • Ultraviolet (UV) Radiation: Can cause sunburn, skin cancer, and is used in nail curing lamps.
  • Visible Light: The portion of the electromagnetic spectrum that the human eye can perceive, allowing us to see colors.
  • Infrared (IR) Radiation: Used in night vision technology, thermal imaging, and detecting forgeries in art.
  • Microwaves: Used for cooking food (microwave ovens) and in telecommunications, including satellite communication.
  • Radio Waves: A type of electromagnetic radiation existing in the lowest frequency, longest wavelength part of the electromagnetic spectrum, used for broadcasting and communication.

Ray Model of Light and Wave Properties

The ray model of light simplifies light’s behavior by assuming it travels in straight lines. This model is useful for predicting the location, size, and shape of shadows.

Key Wave Properties:

  • Wavelength: The distance from one crest to the next (or trough to trough) in a wave.
  • Amplitude: The distance from the centerline to the crest or trough of a wave, indicating its intensity.
  • Frequency: The number of complete wavelengths that pass a given point in one second, measured in Hertz (Hz).

Particle Model of Light: Photons

The particle model of light explains that light also exhibits particle-like properties. Albert Einstein proposed that light acts as a particle, called a photon, when an object absorbs its energy. Each photon carries a precise amount of energy. Photons carry more energy as the frequency of electromagnetic radiation increases and its wavelength decreases. This phenomenon is evident, for example, when electrons are emitted from a metal surface when struck by blue light (photoelectric effect).

Light Interaction: Reflection, Transmission, Refraction

When light encounters a surface or a new material, it can interact in several ways:

  • Reflection: Light “bounces off” a surface and changes direction.
  • Transmission: Light passes through a material. The material light passes through is called a medium.
  • Refraction: When light enters a different medium, it changes direction due to a change in speed. This bending of light is called refraction.

Types of Materials Based on Light Transmission:

  • Translucent Materials: These materials allow most light to pass through, but the light is scattered in many directions. Objects viewed through them appear blurry.
    • Examples: Frosted plastic, waxed paper.
  • Transparent Materials: These materials transmit almost all light, allowing objects to be seen clearly through them.
    • Examples: Clear glass, plastic, water, and air.

Laws of Reflection and Mirror Types

Laws of Reflection:

When light reflects off a surface, it follows specific laws:

  • The angle of reflection (r) is equal to the angle of incidence (i).
  • The incident ray, the reflected ray, and the normal (a line perpendicular to the surface) all lie on the same plane.
  • The reflected ray and the incident ray are on opposite sides of the normal.

Key Terminology for Reflection:

  • Incident Ray: The light ray traveling toward the reflecting surface.
  • Reflected Ray: = The light ray that has bounced off the reflecting surface.
  • Angle of Incidence: The angle between the incident ray and the normal.
  • Angle of Reflection: The angle between the reflected ray and the normal.
  • Normal: A line perpendicular to a surface, such as a mirror.
  • Plane: A flat surface.

Reflections in Mirrors: Real and Virtual Images

Light produces different images when it reflects off mirrors of various shapes. Images can be classified as real or virtual:

  • Real Images: Form when reflected rays actually meet. These images are visible in front of the mirror and can be projected onto a screen.
  • Virtual Images: Form when reflected rays do not actually meet; only their extended paths appear to converge. These images appear to be behind the mirror and cannot be projected.

Types of Mirrors:

  • Concave Mirror: A mirror in which the reflecting surface is curved inwards.
    • Example: Shaving mirrors (magnify objects, “zoom in”).
  • Convex Mirror: A mirror with an outward-curving reflecting surface, providing a wider field of view.
    • Example: Store security cameras, passenger-side car mirrors.
  • Plane Mirror: A flat reflecting surface.
    • Example: Regular household mirrors.

Refraction, Lenses, and Human Vision

Understanding Refraction:

Refraction occurs when light changes direction as it moves from one medium to another. This phenomenon happens because light travels at different speeds in different media. For instance, when light travels from a less dense medium (like air) to a more dense medium (like water), the light ray bends toward the normal.

Types of Lenses:

  • Converging Lens (Convex Lens):

    A converging lens brings parallel light rays toward a common focal point. It typically has one or two convex (outward-curving) surfaces and is thicker in the center than at the edges.

  • Diverging Lens (Concave Lens):

    A diverging lens causes parallel light rays to spread away from a common point. It typically has one or two concave (inward-curving) surfaces and is thinner in the center than at the edges.

How the Human Eye Works:

The human eye is a complex optical system that uses refraction to form images:

  1. Light travels in a straight line from an object or source to the eye.
  2. It first passes through the cornea, the transparent outer layer at the front of the eye, which acts as the primary lens, causing light rays to converge.
  3. The light then passes through the pupil and is further focused by the eye’s internal lens, which adjusts its shape to focus on objects at different distances (especially close objects).
  4. The focused light strikes the retina at the back of the eye, where an inverted image is formed.
  5. Specialized cells in the retina convert light into nerve impulses, which are then sent to the brain via the optic nerve.
  6. The brain interprets these impulses as upright, clear sight.