Introduction to Light and Optics

What is Light?

• Light is a transverse electromagnetic wave that can be seen by the human eye
• Light can travel through a vacuum
• The speed of light is 3 x 10^8 m/s
• Light is energy that activates the human eye and brain, providing sight.

The Field of Optics

• The science that studies light and its interaction with matter
• Helps us understand the nature of colors
• Allows us to design optical devices like microscopes, telescopes, lenses, and cameras
• Used in developments like lasers and optical fibers

The Dual Nature of Light

• Light has two different natures, acting like a particle and a wave.
• Particle Theory (Corpuscular Theory of Light)
  • States that light is composed of tiny particles called photons, which are emitted in all directions from a source.
• Wave Theory
  • Proposed by Christiaan Huygens, stating that light is emitted in all directions as a series of waves in a medium called luminiferous ether
• Electromagnetic Wave Theory
  • James Clerk Maxwell discovered that self-propagating electromagnetic waves travel through space at a constant speed, equal to the previously measured speed of light.
  • Maxwell concluded that light is a form of electromagnetic radiation.

Multiple Reflection

• A reflection occurs when an object is placed in front of a mirror.
• With two mirrors, a single source of light can be reflected multiple times, until the intensity of the light becomes low.
• The number of images of an object placed between two mirrors can be calculated by:
  • Number of Images = 360° / angle between mirrors) - 1
• As the angle between the mirrors decreases, the number of images increases.

Some Definitions of Reflection

• The Angle of Incidence: the angle between an incident ray on a surface and the line perpendicular to the surface at the point of incidence (called the normal).
• The Angle of Reflection: the angle between a reflected ray and the normal drawn at the point of incidence to a reflecting surface.

Refraction

• The change in direction of a wave passing from one medium to another
• The bending of a wave when it enters a medium where its speed is different.

Wave Optics

• The wave nature of light is needed to explain various phenomena like interference, diffraction, and polarization.

Interference

• Light waves interfere with each other much like mechanical waves do
• Interference occurs when the electric and magnetic fields of two or more light waves interact, adding together to give a new wave.

Conditions for Interference

• For sustained interference between two sources of light, two conditions must be met:
  • The sources must be coherent, meaning they maintain a constant phase with respect to each other.
  • The waves must have identical wavelengths

Coherent Sources

• Two sources are said to be coherent if they emit light waves of the same frequency, amplitude, and always in phase with each other.

Producing Coherent Sources

• Light from a monochromatic source is allowed to pass through a narrow slit.
• The light from the single slit is allowed to fall on a screen containing two narrow slits.
• Lasers are commonly used as coherent sources, producing intense, coherent, monochromatic beams that can be used to illuminate multiple slits directly.

Constructive Interference

• Occurs when two light waves collide peak-to-peak and trough-to-trough.
• The resulting light wave is stronger than either of its two components.

Destructive Interference

• Occurs when the waves are meeting peak-to-trough and trough-to-peak.
• The two waves cancel each other out, resulting in zero.

Young's Double Slit Experiment

• Thomas Young first demonstrated interference in light waves from two sources in 1801.
• Light is incident on a screen with a narrow slit (So).
• Light waves emerging from this slit arrive at a second screen that contains two narrow, parallel slits (S1 and S2).