Understanding Light and Vision: Properties and Phenomena

Posted on Dec 18, 2024 in Physics

Light: Electromagnetic Waves

Light is a form of energy that propagates through space without the need for a material medium. It consists of electric and magnetic fields that are perpendicular to each other and to the direction of propagation.

Key Features of Light

  • It does not need a material medium for propagation.
  • Electric (E) and magnetic (B) fields vary sinusoidally with position (x) and time (t).
  • It is caused by accelerating electric charges, which lose energy that propagates as electromagnetic waves.
  • At any point in space where there is a changing magnetic field, an electric field exists, and vice-versa.
  • For a given value of x, the ratio of the magnitude of the electric field to the magnetic field is equal to the speed of wave propagation.
  • The wave propagation speed (v) is calculated by c = 1 / √(ε0 * μ0), where ε0 is the permittivity of free space and μ0 is the permeability of free space.
  • The propagation speed is also given by v = λ / T, where λ is the wavelength and T is the period.

Electromagnetic Spectrum

The electromagnetic spectrum encompasses a wide range of frequencies, including: radio waves (1010 Hz), microwaves (1012 Hz), infrared (IR) (1014 Hz), visible light (1015 Hz), ultraviolet (UV) (1017 Hz), X-rays (1019 Hz), and gamma rays (1022 Hz). The speed of light in a vacuum is approximately 3 x 108 m/s.

Rectilinear Propagation of Light

Both light sources and illuminated objects emit light in all directions, and this light propagates in straight lines. This principle explains phenomena such as shadows, penumbras, and eclipses. A shadow is an unlit region that appears behind an opaque body illuminated by a point source. If the source is not a point source, a penumbra (partially lit region) will also be observed.

Optical Phenomena

  1. Diffraction: The deviation from rectilinear propagation of waves when they pass through an opening comparable in size to their wavelength.
  2. Reflection: The change in the direction of wave propagation when it strikes the boundary between two different media.
  3. Refraction: The change in the direction of wave propagation when it passes from one medium to another.
  4. Dispersion: The phenomenon that occurs when a beam of light composed of different wavelengths is incident on a refracting material. This is due to the dependence of the refractive index on the wavelength.
  5. Absorption: The phenomenon by which a body retains part of the light energy it receives when illuminated.
  6. Interference: The superposition of two or more wave motions propagating through the same medium at a point in space. There are two types:
    • Constructive Interference: The resulting intensity is a reinforcement of the individual intensities, occurring when the waves are in phase.
    • Destructive Interference: The resulting disturbance is less than the original, occurring when the waves are out of phase.

Defects of Vision

Presbyopia

Presbyopia is an age-related defect characterized by a reduction in the eye’s ability to accommodate, due to fatigue of the ciliary muscles. It does not affect distance vision because the lens is not compressed. A common sign of presbyopia is the need to hold reading material at arm’s length and tilt the head back. It is corrected using converging bifocal lenses, which provide two focal points: one for near vision and one for far vision.

Myopia (Nearsightedness)

In myopia, the eye has an excessive convergence, causing light rays from a distant object to converge in front of the retina. This results in a blurred image on the retina. Myopic individuals have a near point that is closer than normal, allowing them to see near objects clearly but distant objects poorly. Correction involves the use of diverging lenses.

Hyperopia (Farsightedness)

Hyperopia is the opposite of myopia. Light rays from a near object converge behind the retina, also resulting in a blurred image. The lens has less curvature than normal, making it better suited for focusing on distant objects. Correction involves the use of converging lenses to bring the focal point closer to the lens.

Astigmatism

Astigmatism is caused by an irregularity in the curvature of the cornea, leading to an inability to clearly see two perpendicular lines in the same plane. The cornea receives partial images at different distances. It is corrected using cylindrical lenses that bring the focal point to the same location for different object planes.

Theories of Light

  • Corpuscular Theory (Newton): Light is composed of small particles (corpuscles) that travel in straight lines and in different directions. The sensation of light is produced when these corpuscles collide with our eyes.
  • Wave Theory: Light is a disturbance that propagates from the light source to the observer.

Ray Tracing

Mirrors

  1. Rays that strike the mirror parallel to the optical axis are reflected through the focal point.
  2. A ray passing through the center of curvature is reflected back along its original path.
  3. A ray passing through the focal point is reflected parallel to the optical axis.

Lenses

  1. Rays that strike the lens parallel to the optical axis are refracted and pass through the second focal point (F2).
  2. Rays that pass through the optical center of the lens do not undergo any deviation.
  3. Rays passing through the first focal point (F1) emerge from the lens parallel to the optical axis.