====== Light ======
**Light** (আলো) is an electromagnetic wave (তড়িচ্চুম্বকিয় তরঙ্গ). You have studied some properties of waves and you will study electricity and magnetism in [[:courses:phy102|PHY 102]].

Propagation (সঞ্চারন) of light is best described by its **wave** (তরঙ্গ) nature, but the emission (নিঃসরন) and absorption (শোশন) of light is best described by its **particle** (কনা) nature. //Quantum electrodynamics// (কোয়ান্টাম তড়িৎগতিবিদ্যা) explains both the wave and particle natures using a single coherent theory.

Electromagnetic waves are waves on the electromagnetic field and, hence, must always be created by accelerating electric charges. An electric charge at rest (or moving with constant velocity) always has an electric (or electromagnetic) field around it extending to infinity. When the charge is accelerated, a wave is created on the field that propagates at the speed of light ($c=3\times 10^8$ m/s).

The leading edge of a wave is called a **wave front** (তরঙ্গমুখ). An imaginary line along the direction of propagation of a wave is called a **ray** (রশ্মি). Wave fronts are related to the wave nature and rays to the particle nature.

**Optics** (আলোকবিদ্যা) is the branch of physics that deals with the nature and propagation of light. **Geometric optics** describes everything in terms of rays. **Physical optics** describes things in terms of wave fronts. We will focus only on geometric optics.

===== Reflection and refraction =====
When light enters one medium from another medium, one part of it is **reflected** (প্রতিফলন) back from the interface of the two media and one part is **refracted** (প্রতিসরন) or transmitted through the interface.

If the interface is smooth, the **angle of incidence** (আপতন কোন, $\theta_a$) (angle of the incoming ray with the **normal** to the interface area) is always equal to the **angle of reflection** (প্রতিফলন কোন, $\theta_r$), i. e. $\theta_r = \theta_a$. This kind of reflection is called **specular reflection**. If the surface is not smooth, the rays are reflected back in arbitrary directions and the phenomenon is called **diffuse reflection** (বিক্ষিপ্ত প্রতিফলন). We will focus only on specular reflection.

The angles of incidence ($\theta_a$), reflection ($\theta_r$) and refraction ($\theta_b$) are conventionally measured from the normal to the interface surface. The normal and the rays must be in the same plane.

The angle of reflection is always equal to the angle of incidence if the interface is smooth. But the angle of refraction also depends on the **refractive indices** (প্রতিসরনাঙ্ক) of the two media given by **Snell’s law**: $n_a \sin\theta_a = n_b \sin\theta_b$ where $n_a$ is the refractive index of the medium from which light is leaving and $n_b$ is that of the medium in which light is entering. Willebrord Snell was a Dutch scientist; note that both telescope and microscope were invented in The Netherlands during the 16th century.

Snell’s law also shows that when light enters from a low-$n$ medium to a high-$n$ medium ($n_b>n_a$), it bends toward the normal. But when $n_b<n_a$, light bends away from the normal. A ray along the normal will not bend at all no matter what the refractive indices are. Refractive index of air is 1.003 which can be considered 1.0 for many practical purposes. Index of glass can be from 1.5 to 2.0.

Refractive index $n=c/v$ where $c$ is the speed of light in vacuum and $v$ is its speed in some other medium. Obviously, $n$ is always greater than 1.

Wave speed is proportional to the wavelength: $v=f\lambda$. When light wave enters one medium from another, speed and wavelength change, but their ratio $v/\lambda=f$ remains the same; frequency cannot change because of the law of conservation of energy ($E=hf=$ constant). Therefore $f=v/\lambda=c/\lambda_0$ and $\lambda=\lambda_0/n$. If light enters into a higher-$n$ medium ($n_b>n_a$), both its speed and wavelength decrease, or the wave is squeezed. The wave would stretch in the opposite case.

Path of refracted rays are reversible.

  - Why does a pencil half-immersed in water appears to be bent?
  - Why does the setting sun appears to be flattened vertically?

===== Dispersion =====

===== Polarization =====

===== Scattering =====

===== Huygen's principle =====