Way opyics

• Sir Isaac Newton proposed a corpuscular theory of light, stating that light consists of extremely light and tiny particles known as corpuscles
• Corpuscular theory explains reflection and refraction of light
• Corpuscular theory fails to explain interference, diffraction, and polarization of light
• Newton's theory could not explain why the velocity of light was lesser in denser medium compared to vacuum
• Christopher Huygens proposed the wave theory of light in the early 18th century as a challenge to Newton’s corpuscular theory
• According to Huygens’s theory, light consists of waves that travel through a very dilute and highly elastic material medium present everywhere in space called ether
• The density of the ether medium is very low, and the modulus of elasticity is very high, resulting in a very large speed of light
• Huygens’s wave theory explained phenomena like reflection, refraction, interference, and diffraction of light
• Huygens’s wave theory failed to explain polarization, as he assumed that light waves, which are longitudinal in nature, are mechanical disturbances
• Huygens’s wave theory also failed to explain black body radiation, the photoelectric effect, and the Compton effect
• The hypothetical medium ether was never discovered, and now we know light can propagate in a vacuum
• Huygen’s principle states that each point on a wavefront is a source of secondary wavelets or disturbances
• Disturbances from secondary sources spread in all directions in the same way as from the primary source
• Secondary sources create wavelets similar to the primary source
• A common tangent on the wavelets in the forward direction gives the new wavefront at any instant of time
• The sum of spherical wavelets forms the wavefront
• Huygen’s principle is used to analyze wave propagation in diffraction and reflection
• The Huygens wave principle proved the concept of reflection of light
• The Huygens wave principle also approved the concept of refraction of light
• The Huygens wave principle proved the concept of interference of light
• The Huygens wave principle proved the concept of diffraction of light
• Huygen's principle failed to prove:

  • The concept of polarization of light
  • Emission of light
  • Absorption of light
  • The photoelectric effect
• All the rays are always perpendicular to the wavefront
• Time taken by a wave from one wavefront to another wavefront is always constant, even in different mediums where distance and velocity can change
• All points on a wavefront act like secondary sources known as secondary wavelets
• Wavefront is the locus of all points in the same phase
• Spherical Wavefront:

  • Point source radiating energy causes particles around it to oscillate, creating waves that travel in all directions and form a spherical wavefront
• Planar Wavefront:

  • If the source is at infinity, waves are parallel (e.g., light rays from the sun on earth), forming a planar wavefront
• Cylindrical Wavefront:

  • Formed when the light source is linear, with all points equidistant from the source and found on the surface of a cylinder
• Differences in amplitude and intensity between three types of wavefronts:
• Spherical wavefront:

  • Amplitude (A) proportional to 1/r
  • Intensity (I) proportional to 1/r^2
• Planar wavefront:

  • Amplitude (A) constant (r^0)
  • Intensity (I) constant (r^0)
• Cylindrical wavefront:

  • Amplitude (A) proportional to 1/√r
  • Intensity (I) proportional to 1/r
• Young's double slit experiment uses two coherent sources of light placed at a small distance apart
• The distance between the two slits is usually only a few orders of magnitude greater than the wavelength of light
• The experiment helped in understanding the wave theory of light
• A screen or photodetector is placed at a large distance, 'D', away from the slits
• The wave theory of light is explained with the help of a diagram
• For maximum intensity or bright fringe to be formed at point P, the path difference is given by Δz = nλ (where n = 0, ±1, ±2, . . . .)
• The distance of the nth bright fringe from the centre is xn = nλD/d