Light - 1 | General Awareness for SSC CGL PDF Download

Introduction

Light is a fundamental aspect of our everyday experiences, enabling us to see and perceive the world around us. It is a form of energy that affects our vision by interacting with our eyes. Understanding light involves exploring its wave and particle nature, its behavior as it interacts with different materials, and the phenomena it produces.

Nature of Light

• Light as a Form of Energy: Light is a type of energy that creates visual sensations when it reaches our eyes. It travels in waves, specifically electromagnetic waves that are transverse in nature.
• Speed of Light: In a vacuum, light travels at a speed of 300,000 kilometers per second (3 × 10^8 m/s). However, this speed can vary when light moves through different substances, such as air or water.

Wave Nature of Light

• Transverse Waves: Light waves are transverse, meaning their oscillations are perpendicular to the direction of their travel.
• Properties of Light Waves: Light exhibits various wave-like behaviors, including:
• Interference: When two light waves overlap, they can reinforce or cancel each other out.
• Diffraction: Light bends around obstacles or spreads out after passing through narrow openings.
• Scattering: Light changes direction when it strikes small particles or molecules.
• Polarization: Light waves oscillate in specific directions; polarization filters these directions.

Particle Nature of Light

• Quantum Theory: According to Max Planck's Quantum Theory, light consists of discrete packets of energy called quanta or photons. This theory helps explain phenomena that classical wave theory cannot, such as:
• Photoelectric Effect: The emission of electrons from a material when light shines on it.
• Compton Effect: The change in wavelength of X-rays when they collide with electrons.
• Raman Effect: The change in the wavelength of light due to its interaction with molecular vibrations.

Luminous and Non-Luminous Objects

• Luminous Objects: These objects produce their own light. Examples include:
• The Sun: A natural source of light.
• Stars: Celestial bodies that emit light.
• Oil Lamp: A man-made source of light.
• Non-Luminous Objects: These objects do not emit their own light but become visible due to light reflected from other sources. Examples include:
• The Moon: Reflects sunlight.
• Furniture (e.g., table, chair): Visible because they reflect light.
• Trees: Seen due to reflected light.

Image Formation

• Real Images: Formed when light rays converge at a point after reflection or refraction. Real images are inverted and can be projected onto a screen. For example, the image formed by a projector on a screen is a real image.
• Virtual Images: Occur when light rays appear to converge but do not actually meet. Virtual images are upright and cannot be projected onto a screen. For example, the image seen in a flat mirror is a virtual image.

Properties of Light

• Rectilinear Propagation: Light travels in straight lines. This property explains phenomena such as shadows:
• Umbra: The region of complete darkness created when light is blocked by an opaque object.
• Penumbra: The partially darkened region surrounding the umbra when light comes from an extended source.
• Solar Eclipse: Occurs when the Moon moves between the Earth and the Sun, casting a shadow on Earth and blocking sunlight.
• Lunar Eclipse: Happens when the Earth is between the Sun and the Moon, causing Earth’s shadow to fall on the Moon and making it dark.

Reflection of Light

• Reflection is the process where light bounces back into the same medium after striking a smooth, shiny surface, such as a plane mirror.
• Examples: The reflection of light from a mirror allows us to see our image, while the reflection of light from water lets us see the water’s surface.

Laws of Reflection

• The incident ray, the reflected ray, and the normal line at the point of incidence all lie in the same plane.
• The angle of incidence is always equal to the angle of reflection, meaning ∠i = ∠r.

Refraction of Light

• Refraction is the phenomenon where light rays change their path as they pass from one transparent medium to another.
• When light moves from a denser medium to a rarer medium, it bends away from the normal. Conversely, when it travels from a rarer medium to a denser medium, it bends towards the normal.
• The cause of refraction is the change in the speed of light in different media.
• During refraction, while the speed and wavelength of light change, its frequency remains constant.
• The refractive index of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v). It is given by:
  Refractive index of a medium (µ) = c / v
• Relative Refractive Index The refractive index of second medium with respect to first medium
  

Laws of Refraction

There are two laws of refraction 

• The incident ray, the refracted ray and the normal at the point of incidence all three lie in the same plane. 
• The ratio of sine of angle of incidence to the sine of angle of refraction remains constant for a pair of media, i.e.
  
• This law is called Snell’s law (₁µ₂ is called refractive index of second medium with respect to the first medium.)

Successive Refractions through several Media

• If refraction takes place successively through several media like air, water, glass and again air then product of refractive indices of second medium with respect to the first medium, third medium with respect to the second medium and so on is, unity.
• When light rays travel from a denser medium towards a rarer medium it deviates away from the normal. Therefore, a pond appears shallower. 
• A coin appears at lesser depth in water. Writing on a paper appears lifted when a glass slab is placed over paper.

Critical Angle and Total Internal Reflection (TIR)

The angle of incidence in a denser medium for which the angle of refraction in rarer medium becomes 90°, is called critical angle (C ).
Refractive index of denser medium,

When a light ray travelling from a denser medium towards a rarer medium is incident at the interface at an angle of incidence greater than critical angle, then light rays reflected back into the denser medium. This phenomenon is called total internal reflection (TIR)

Phenomena Related with TIR

• Mirage is an optical illusion of water appears in desert in a hot summer day. In a hot summer day in desert, the layers of air near the earth surface remains hot and their temperature decreases with altitude and becomes denser. When a ray of light coming from the top of a tree or sky, moves towards the earth it deviates away from the normal gradually. 
• When angle of incidence becomes greater than critical angle, total internal reflection takes place. After that light rays bend upward.
• When light rays enter the eyes, inverted image of tree is obtained which produces illusion of water. 
• Sparkling of diamond takes place due to multiple total internal reflection takes place inside the diamond.

Important Definitions Related to Spherical Mirrors

• The centre of the hollow glass sphere of which the mirror is a part, is called centre of curvature (C ). 
• The radius of the hollow sphere of which the mirror is a part, is called radius of curvature (R).
• The mid-point of a spherical mirror, is called its pole (P). 
• When a parallel beam of light rays is incident on a spherical mirror then after reflection it meets or appears to meet at a point on principal axis, called focus (F ) of the spherical mirror.
  
• The linear distance between the pole and the focus, is called focal length (f ). Focal length of a spherical mirror (f )
  

Mirror Formula

The mirror formula is given by 1/f = 1/v + 1/u
where, u = distance of object from the mirror
v = distance of image from the mirror
f = focal length of the mirror.

Linear Magnification

The ratio of height of image (I) formed by a mirror to the height of the object (O), is called linear magnification (m).
m = I/O = -v/u
Image Formed by a Concave Mirror

Use of Spherical Mirrors

•  Concave mirrors are used in torches, search-light and vehicle head lights to get powerful parallel beams of light. It is also used as a shaving mirror and dentists use it to see large images of the teeth of patients etc. 
• Convex mirrors are used as rear- view mirror in vehicles. It is also used in reflection lamps installed near roads.

Lenses

A lens is a uniform refracting medium bounded by two spherical surfaces or one spherical and one plane surface. Lenses are mainly of two types 

• Convex lens 
• Concave lens

Important Definitions Related to Lenses

• A line joining the centres of curvature of the two bounding surfaces of a lens, is called its principal axis. 
• A point on the principal axis through which light rays pass undeviated , is called optical centre of the lens.

• Focus of the lens is a point on the either side of principal axis. The light rays coming or appear to come from it become parallel to the principal axis after refraction through the lens.
  
• The linear distance between the optical centre and focus of the lens, is called its focal length.

Lens Formula

Lens formula is given by 1/f = 1/v - 1/u
where, f = focal length of the lens
u = distance of the object from the lens
v = distance of the image from the lens.

Lens Maker’s Formula

It is used while making a lens and is given by
where, µ = refractive index of the material of the lens R₁ and R₂ = radii of curvature of the surfaces of the lens

• If lens is splits in two equal parts perpendicular to the principal axis, then focal length of each new lens is double that of original lens. 
• When a lens is used in a medium of refractive index lesser than that of the material of the lens, then its focal length increases but nature remains unchanged. 
• When a lens is used in a medium of refractive index greater than that of the material of the lens, then the nature of the lens changes. 
• When a lens is used in a medium of refractive index equal to that of the material of the lens, then it will work as a plane glass plate.

Power and Magnification for a Lens

• Power of a lens is the reciprocal of its focal length, when it is measured in metre.
  
• Its unit is diopter (D). 
• Total power of the Combination of lenses is given by algebraic sum of powers of the lenses, which form the combination.
  
  or
  P = P₁ + P₂ ...
• Linear magnification for a lens :
  

Prism

Prism is a uniform transparent refracting medium bounded by plane surfaces inclined at some angles forming a triangular shape.

Angle of Deviation

The angle subtended between the incident ray and emergent ray, is called angle of deviation (δ)

Dispersion of Light

• When white light is incident on a glass prism, it splits into its seven colour components in the sequence of VIBGYOR. This phenomenon is called dispersion of white light. 
• The refractive index of glass is maximum for violet colour and minimum for red colour of light. Therefore, violet colour of light deviated maximum and red colour of light deviated least.

Rainbow

• When the sun shines just after a shower of rain, a rainbow is seen in the sky opposite to the sun. It is in the form of circular arcs of seven colours.
• Rainbow is formed due to dispersion of the sunlight by water droplets suspended in air. In each drop dispersion of the sunlight and then internal reflection take place.

Colour of Objects

• When light is incident on an object, it reflects only a part of it. The reflected light gives the objects of their colour. 
• A rose appears red when white light is incident on it because it reflects only red colour of light and absorbs other all colours of light. When same rose is viewed in green light, it appears black because it absorbs green light and reflects no colour of light. 
• Red, green and blue are primary colours. 
• The colours which are obtained by the mixing of two primary colours, are called secondary colours. Yellow, magenta and cyan are secondary colours.
  Red + Green = Yellow
  Red + Blue = Magenta
  Green + Blue = Cyan
• Those primary and secondary colours which on mixing produce white colour, are called complementary colours.
  Red + Cyan = White
  Red + Magenta = White
  Green + Magenta = White
  Blue + Yellow = White

Mixed Coloured Pigments

The pigments in common use are of impure colours. Therefore, on mixing paints of different colours the resultant colour of paint is not obtained as given by colour triangle. When blue and yellow paints are mixed together, they produce green paint in spite of white paint.

Human Eye

• The human eye functions as an optical device that produces a real image of objects on the retina. The retina is made up of numerous cone and rod cells; cones are sensitive to color, while rods detect light intensity. 
• The eye’s ciliary muscles adjust the focal length of the lens, a process known as the eye’s power of accommodation. The eye lens, which is a convex lens composed of a jelly-like substance, helps focus light. 
• The iris, a colored diaphragm, regulates the size of the pupil and controls the amount of light entering the eye. The closest point at which an object can be seen clearly is known as the near point, and the distance from the eye to this point is called the least distance of distinct vision, which is typically 25 cm for a healthy eye. 
• The farthest point at which an object can be seen clearly is known as the far point.

Defects of Vision

• Myopia (Short-Sightedness): This condition causes a person to see nearby objects clearly but struggle with distant objects. In myopia, the far point of vision shifts from infinity to a closer range. It can be corrected with a concave lens of the correct power.
• Hypermetropia (Long-Sightedness): This defect allows a person to see distant objects clearly but makes it difficult to see nearby objects. In hypermetropia, the near point shifts farther away from the eye. A convex lens of appropriate power can correct this condition.
• Astigmatism: In astigmatism, a person has difficulty focusing on horizontal and vertical lines at the same distance simultaneously. This defect can be corrected using suitable cylindrical lenses.
• Color Blindness: This condition prevents a person from distinguishing between certain colors due to the absence of cone cells sensitive to those colors. Color blindness cannot be treated.
• Cataract: Cataracts involve the development of an opaque, white membrane on the cornea, leading to partial or complete loss of vision. This condition can be treated by surgically removing the membrane.