The UPSC mains GS-3 paper syllabus has various subjects like Economics, Science and Technology. Science and technology in UPSC is mostly concerned with current Affairs. but it is necessary to understand the basics of science.


  • What is Optics
  • Microscope
  • Telescope


  • Optics is a branch of physics that deals with the determination of behaviour and the properties of light, along with its interactions with the matter and also with the instruments which are used to detect it.
  • Optics, in a simple manner, is used to describe the behaviour of visible light, infrared light, and ultraviolet. Imaging is done with the help of a system called an image forming an optical system.
  • Ray optics is also called geometrical optics. It is a branch of science that describes light propagation in terms of “rays”.

Light And Its Optical Properties

  • Light is a form of energy that is in the form of an electromagnetic wave and is almost everywhere around us. The visible light has wavelengths measuring between 400–700 nanometres. The Sun is the primary source of light by which plants utilize this to produce their energy.
  • In physics, the term light also refers to electromagnetic radiation of different kinds of wavelengths, whether it is visible to the naked eye or not. Hence, by this, the gamma rays, microwaves,  X-rays, and radio waves are also types of light.

Light exhibits various properties which are given below:

  • Reflection - Reflection is one of the primary properties of light. Reflection is nothing but the images you see in the mirrors. Reflection is defined as the change in direction of light at an interface in-between two different media so that the wave-front returns into a medium from which it was originated. The typical examples for reflection of light include sound waves and water waves.
  • Speed of light - The rate at which the light travels in free space is called the Speed of light. For example, the light travels 30% slower in the water when compared to vacuum.
  • Refraction - The bending of light when it passes from one medium to another is called Refraction. This property of refraction is used in a number of devices like microscopes, magnifying lenses, corrective lenses, and so on. In this property, when the light is transmitted through a medium, polarization of electrons takes place which in turn reduces the speed of light, thus changing the direction of light.
  • Total Internal Reflection (TIR) - When a beam of light strikes the water, a part of the light is reflected, and some part of the light is refracted. This phenomenon is called as Total internal reflection.
  • Dispersion - It is a property of light, where the white light splits into its constituent colours. Dispersion can be observed in the form of a prism.
  • The other properties of light include diffraction and interference. So, what do you observe when you look out at the beautiful scenario? Whether the light gets reflected, dispersed, refracted, internally reflected, or diffracted.

Applications of Optics

  • The properties of optics are applied in various fields of Physics-
  • The refraction phenomenon is applied in the case of lenses (Convex and concave) for the purpose of forming an image of the object.
  • Geometrical optics is used in studies of how the images form in an optical system.
  • In medical applications, it is used in the optical diagnosis of the mysteries of the human body.
  • It is used in the therapeutical and surgeries of the human tissues.

 Types Of Lenses

The classification of a lens depends on how the light rays bend when they pass through the lens. The two main types of lenses are:

  • Convex Lens (Converging) – Convex lenses are thick in the middle and thinner at the edges. A concave lens is flat in the middle and thicker at the edges. A convex lens is also known as the converging lens as the light rays bend inwards and converge at a point which is known as focal length.
  • Concave Lens (Diverging) – The concave lens is also known as a diverging lens because it bends the parallel light rays outward and diverges them at the focal point.


Light from a mirror is reflected up through the specimen, or object to be viewed, into the powerful objective lens, which produces the first magnification. The image produced by the objective lens is then magnified again by the eyepiece lens, which acts as a simple magnifying glass.

Working Principle of Compound Microscope 

A compound microscope is considered to be one of the standard microscopes that can be used for general purposes. The arrangement of the lens is such that it magnifies the objects from the complex system.

There are two types of lenses that are used in the compound microscope:

  • The objective lens is placed close to the object that needs to be examined.
  • The eyepiece allows the image to be viewed. The eyepiece is also known as the ocular lens.

The light is made to pass through the thin transparent object. A magnified image of the object is obtained by the objective lens. This image is known as the real image. The eyepiece or the ocular lens then magnifies the real image more and is viewed as the virtual image. The compound microscope is also known as the bright-field microscope because the light passes directly through the light source to the eye through the two lenses. This mechanism makes the field of vision brightly illuminated


A telescope is an instrument that is used to view distant objects. If you want to look at the planets, you can use a telescope. The higher the magnification on the telescope, the better your view will be.
The lens in the front of the telescope, called the objective lens, produces an upside-down image of the object one is using the telescope to look at. The lens near the eye, called the eye lens, acts as an ordinary magnifying glass to magnify that upside-down image. This is the basic principle of the telescope.
There are two basic types of telescopes, refracting and reflecting. Refracting telescopes use lenses to focus the light, and reflecting telescopes use mirrors.
Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is. Both lenses are in a shape that’s called 'convex'. Convex lenses work by bending light inwards.
Reflecting telescopes, on the other hand, don’t use lenses at all. Instead, they use mirrors to focus the light together. In this case, the type of mirror that they use is a concave mirror. Mirrors of this shape also accomplish the goal of bending light together, except that they do it by reflecting the light instead of bending it as it passes through

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