basic physics concepts important Gk topic for competitive exam Part 3

basic physics concepts important Gk topic

basic physics concepts important Gk topic --- (Part 3)

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LIGHT - basic physics concepts

  • Light is a form of energy which produces the sensation of vision on our eyes.
  • Light is made of discrete packets of energy called photons.
  • Photons carry momentum, have no mass, and travel at the speed of light, i.e. 300,000 km/sec.
    Sun’s light reaches to earth in 8 minutes 19 seconds (i.e. 499 seconds).
  • The light reflected from moon reaches to earth in 1.28 seconds.
  • Ultraviolet radiation is an electromagnetic radiation that has the wavelength of 400 nm to 10 nm, shorter than that of visible light but longer than X-rays. It is used in water purification.
  • Infrared radiation is the emission of energy as electromagnetic waves in the portion of the
  • spectrum just beyond the limit of the red portion of the visible radiation.
  • X-rays are electromagnetic radiation having a shorter wavelength and produced by bombarding a target made of tungsten, with high-speed electrons. Used in medical diagnosis.
  • Microwaves are short, high-frequency waves lying roughly between very high frequency (infrared) waves and conventional radio waves.

Reflection of light

Reflection by Plane Mirror The image formed by the plane mirror is always erect, of the same size and at the same distance as the object is.

Spherical mirror

Spherical mirror are two types

  • Concave mirror
  • Convex mirror

Position & nature of image formed by a spherical mirror

(Position of object --- Position of image ---  Size of image in comparison to object Nature of image)
Concave mirror --

  • At infinity --- At focus --- Highly diminished Real, inverted
  • Between infinity and Centre of curvature --- Between focus and Center, curvature  --- Diminished Real, inverted
  • At Centre of curvature --- At Centre of curvature --- Of some size Real inverted
  • Between focus and Centre of curvature --- Between Centre of curvature and infinity --- Enlarged Virtual, erect
  • At focus --- At infinity --- Highly enlarged Real, inverted
  • Between focus and pole --- Behind the mirror --- enlarged Virtual, erect
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Convex mirror

At infinity --- At Focus --- Highly diminished Virtual, erect
Infront of mirror --- Between pole and focus --- Diminished Virtual, erect


Uses of concave mirror

  • As a shaving mirror.
  • As a reflector for the head lights of a vehicle, searchlight.
  • In ophthalmoscope to examine the eye, ear, nose by doctors.
  • In solar cookers.

Uses of convex mirror

  • As a rear view mirror in the vehicle because it provides the maximum rear field of view and image formed is always erect.
  • In sodium reflector lamp.

Reflection of Light
The bending of the light ray from its path in passing from one medium to the other medium is called the reflection of light.

If the refracted ray bends towards the normal relative to the incident ray, then the second medium is said to be denser than the first medium. But if the refracted ray bends away from the normal, then the second medium is said to be rarer than the first medium.

Some phenomena based on Refraction

  • Twinkling of stars
  • Oval Shape of the sun in the morning and evening.
  • Rivers appear shallow
  • Coins appear raised in a glass filled with water.
  • Pencils appear broken in the beaker filled with water.
  • Sun appears above horizon at sunset and sunrise/
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Total Internal Reflection

When the angle of incidence, for a ray of light passing from a denser medium to a rarer medium, exceeds a particular value (called critical angle for which angle of refraction 90 degrees), the ray reflects back in the same medium from the boundary. This phenomenon is called total internal reflection.

Some phenomena of total internal reflection.

  • Endoscopy using optical fiber.
  • Sparkling of the diamond.
  • Mirage in desert
  • Increase in duration of sun’s visibility.
  • The appearance of air bubbles in a glass paperweight.
  • Shining of air bubbles in water.

Scattering of Light: Sunlight gets scattered by small particles present in the atmosphere. Red colour scatters least and violet most. Some phenomena like-reddish appearance of the sun at sunrise and sunset, blue colour of the sky, white colour of clouds etc. based on scattering of light.

Human Eye - basic physics concepts

The normal range of vision for a healthy human eye is from 25 cm (least distance of distinct vision to infinity (far point).

Defects of Vision & Remedies

Myopia or Near (short) sightedness:

  • A person suffering from Myopia can’t see the far (distant) object clearly but can see the nearby object clearly.

Causes :

  • The eyeball too long (i.e. elongated) so an image is formed on the retina.
  • The lens is too curved for the length of the eyeball.

Remedy: Concave lens is used to diverge the rays at the retina.

Hyperopia or Hypermetropia (long (far) sightedness)

  • A person suffering from it can’t see near object clear but can see the distant object clearly.
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  • The eyeball is too short so an image is formed beyond the retina.
  • Cornea is not curved enough,
  • The eye lens is farther back in the eye.

Remedy: the Convex lens is used to converge the rays at the retina.
Target group: It can affect both children and adults.
Astigmatism: Astigmatism is the most common refractive problem responsible for blurry vision. The cylindrical lens is used to correct astigmatism.


  • It is the clouding of the lens of the eye that prevents a person to see.

Causes: Protein builds up in the eye lens & make it cloudy.

  • It can be corrected with suitable eyeglasses (lenses).
  • Cataract surgery is performed when eyeglass does not suit.

Dispersion of light -- The splitting of while the ray of light into seven constituents colours (VIBGYOR) is called dispersion of light.

  • The band of seven constituents colours is called spectrum.
    Microscope It is used to see the magnified image of tiny objects.
    Telescope It is used to increase the visual angle of the distant object. It is used to see far off objects clearly.

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