As already mentioned, optics in physics is a branch dealing with the properties, significance, and behaviour of light in different conditions. Light is an electromagnetic wave. Thus, other electromagnetic waves like X-rays also exhibit similar properties. The study helps develop several optical instruments like cameras, periscopes, microscopes and telescopes. Therefore, we can say that we use optics and optical instruments in our daily life from cameras to microscopes in laboratories. The eye lenses and the contact glasses all follow the principles of optics. Optical science is important in astronomy, engineering, photography, medicines, etc. The optics have two broad divisions: Ray optics and wave optics. Let’s study optics and their types in brief.

Types of Optics

There are two broad categories of optics, i.e. wave optics and ray optics. 

Ray Optics: Ray optics is the propagation of light in rays in straight lines. Ray optics involves the study of images formed with the help of plane mirrors, spherical mirrors, lenses (convex and concave), and prisms. The light ray travels in a straight line in a homogenous medium, whereas on changing the medium, it either bends towards or away from normal. 

Wave Optics or Wave theory of light: This theory came into light in the 17th century by two scientists, Christian Huygens and Robert Hooke. The Huygens believed that light was a wave that vibrates up and down perpendicularly in the direction of the propagation of the wave. However, Sir Isaac Newton was in support of the particle nature of light. The Huygens theory is now the backbone of modern optics in physics. 

The postulates of the wave optics theory by Huygens are as follows:

• Every point on the primary wavefront (the given wavefront) behaves as a new source of a new disturbance. These are called secondary wavelets. These secondary wavelets will travel with the velocity of light in all the directions of the medium
• If a surface touches these secondary wavelets tangentially (in the forward direction) at any instant, then it gives a new wavefront at that particular instant (the secondary wavefronts)
• Huygens proposed travelling of light in the form of wavefronts in their particular medium
• The light source emits energy in the form of waves in the medium
• For the need of a medium for its propagation, an ‘ether’ (a universal hypothetical medium) was proposed 
•  As already discussed, the points on the wavefront acting as a new centre of disturbance shall emit their waves (spherical). These are the secondary wavelets
• If seen from a farther distance, these wavefronts look like a plane 

Advantages of the Huygens Theory of Wave Optics

The theory explains the different phenomena of light i.e. reflection, refraction, diffraction and interference.

Disadvantages of the Huygens Theory of Wave Optics

The theory is very important in optics and physics however there are some demerits also. It was unable to differentiate between longitudinal and transverse light waves. It was also unable to explain the polarisation of light.

Phenomenon of Lights

Reflection of Light: 

If a light ray or a group of rays strikes a reflecting surface and bounces back to its original medium. Then, it is the reflection of light. 

Laws of Reflection of Light: The laws of reflection of light are as follows-

1. a) The angle of incidence (angle formed by incident ray and normal) is always equal to the angle of reflection (angle formed by reflected ray and normal)
2. b) The normal (imaginary line perpendicular to the boundary at the incident point), the incident ray (ray striking the reflecting surface), and the reflected ray (ray that bounces back to its medium) all lie on the same plane. 

Refraction of light: 

Refraction of light is the bending of a light ray (either towards or away from normal) while passing from one medium to another. 

Laws of Refraction: 

1. a) The normal (imaginary line perpendicular to the boundary at the incident point), the incident ray (ray striking the refracting surface) and the refracted ray (ray that enters another medium) all lie on the same plane. 
2. b) The ratio of sine angle of incidence to the sine angle of refraction is constant, This is known as Snell’s law. 

Conclusion 

Optics in physics is a branch dealing with the properties, significance, and behaviour of light in different conditions. The study helps develop several optical instruments like cameras, periscopes, microscopes and telescopes. Thus, we can say that we use optics and optical instruments in our daily life. There are two broad categories of optics, i.e. wave optics and ray optics. Ray optics is the propagation of light in the form of rays in straight lines. The wave optics or the wave theory of the Huygens states light was a wave that vibrates up and down perpendicularly in the direction of the propagation of the wave.