Speed of Light & Refractive Index

Speed of Light & Refractive Index

Light and Its Nature

Light as a Form of Energy

• Light is a form of energy that is propagated as electromagnetic waves.

• In the spectrum of electromagnetic waves, it lies between the ultraviolet and infrared regions and has a wavelength between 3900 Å and 7800 Å.

Wave Nature of Light

• Electromagnetic waves are transverse; hence, light is a transverse wave.

• All the electromagnetic waves, including light, do not require a medium to propagate.

• The wave nature of light explains the following phenomena:

  • Rectilinear propagation

  • Reflection

  • Refraction

  • Interference

  • Diffraction

  • Polarisation of light

Particle Nature of Light

• The phenomena like the photoelectric effect and Compton effect are not explained on the basis of the wave nature of light. These phenomena are explained on the basis of the quantum theory of light, or the particle nature of light, as proposed by Einstein.

• In quantum theory, light is regarded as a packet or bundle of energy called a photon. The photon is associated with an energy E=h⋅vE = h \cdot vE=h⋅v.

• Clearly, light behaves both as a wave and as a particle. Thus, light has a dual nature.

Speed of Light

• Measurement of Speed of Light

• The speed of light was first measured by Roemer in 1678 AD.

• The speed of light is maximum in vacuum and air, approximately 3×108 m/s3 \times 10^8 \, m/s3×108m/s, and is independent of density, temperature, and pressure.

Refractive Index

• Definition of Refractive Index (R.I.)

• The refractive index of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.

Speed of Light in Different Mediums

The speed of light is different in different media. The velocity of light is larger in a medium that has a small refractive index.

Light Travel Time

• Light takes 8 minutes 19 seconds (499 seconds) to reach from the Sun to the Earth.

• The light reflected from the moon takes 1.28 seconds to reach the Earth.

Types of Bodies

Luminous Bodies

• Those objects that emit light by themselves are called luminous bodies.

Examples: Sun, stars, electric bulb, etc.

Non-Luminous Bodies

• Those objects that do not emit light by themselves but are visible by the light falling on them emitted by self-luminous bodies are called non-luminous bodies.

Classification of Materials

Transparent

• The substances that allow most of the incident light to pass through them are called transparent.

Example: Glass, water.

Translucent

• The substances that allow a part of the incident light to pass through them are called translucent.

Example: Oiled paper.

Opaque

• The substances that do not allow the incident light to pass through them are called opaque bodies.

Example: Mirror, metal, wood, etc.

Shadow

A shadow is the dark region on a screen when an opaque object is placed in the path of light. This is due to the rectilinear propagation of light.

• Point Source of Light: In the case of a point source, the shadow has sharply defined outlines.

• Extended Source of Light: In the case of an extended source of light, the shadow has two distinct regions:

  • Umbra: The region in which no light reaches, resulting in complete darkness.

  • Penumbra: The region in which partial light reaches.

Reflection of Light

Light moving in one medium, when it falls at the surface of another medium, part of the light returns back to the same medium. This phenomenon of light returning to the first medium at the interface of two media is known as the reflection of light.

• Regular Reflection: If the reflecting surface is smooth, the reflection is regular.

• Irregular Reflection: In the case of a rough surface, the reflection is irregular.

Final Thoughts

Light is a form of energy that travels as electromagnetic waves, with wavelengths between 3900 Å and 7800 Å. It behaves both as a wave and a particle, a concept known as its dual nature. The wave nature explains phenomena like reflection, refraction, and interference, while the particle nature accounts for effects like the photoelectric effect.

The speed of light in a vacuum is about 3×108 m/s3 \ × 10^8 \, m/s3×108 m/s, and it varies in different media depending on their refractive index. Light reaches us from the Sun in 8 minutes 19 seconds and from the Moon in 1.28 seconds.

The behavior of light also gives rise to shadows, with regions of complete darkness (umbra) and partial darkness (penumbra). Understanding these properties helps us comprehend the world and technologies that depend on light.