Ray Optics Or Geometrical Optics

A branch of optics that describes light propagation in terms of rays is called ray optics.

• Light is an electromagnetic radiation.
• The visible wavelength range is approximately 400 nm to 750 nm.
• The speed of light in vacuum: c ≈ 3 × 10⁸ m/s
• When the wavelength of light is very small compared to the size of objects and apertures, light can be treated as travelling along straight lines (rays).
• Using this ray model, we study: reflection, refraction, dispersion, and image formation by optical instruments, including the human eye.

Law 1: Rectilinear Propagation

• In a homogeneous and isotropic medium, light travels in a straight line.

Law 2: Independence of Light Rays

• When two or more rays cross each other, they continue along their own paths without disturbing one another.

Law 3: Laws of Reflection

1. The incident ray, reflected ray, and the normal at the point of incidence all lie in the same plane.
2. The angle of reflection = angle of incidence.

Law 4: Laws of Refraction (Snell's Law)

1. The incident ray, refracted ray, and normal at the point of incidence all lie in the same plane.
2. For a given pair of media:
   n₁ sin⁡θ₁ = n₂ sin⁡θ₂

where n₁, n₂​ are the refractive indices of medium 1 and medium 2, and θ₁, θ₂​ are the angles of incidence and refraction with respect to the normal.

Problem: A spectacle glass slab has a thickness of 2 mm and a refractive index of n = 1.5. Find the time taken by light to cross it.

Solution:

Step 1: Find the speed of light in glass using n=c/vn = c/vn=c/v:

Step 2: Find time using time = distance/speed:

Answer: Time taken ≈ 10 ps (picoseconds)