Maarg Manthan · Topic 8.3

Light and Optics

Science & Technology › Physics – Mechanics, Waves and Thermodynamics · Topic 8.3

Light is a form of electromagnetic radiation that the eye can see, and it travels at about 3 lakh kilometres a second in a vacuum. Optics is the study of how light behaves. Light shows reflection, in which it bounces from a surface with the angle of incidence equal to the angle of reflection, and refraction, in which it bends when it passes from one medium to another because its speed changes. The refractive index of a material is the speed of light in a vacuum divided by its speed in the material. Total internal reflection occurs when light inside a denser medium meets the boundary at more than the critical angle, and it makes optical fibres and diamonds work. White light splits into colours by dispersion, as Isaac Newton showed with a prism in 1666. Scattering explains the blue sky and the red sunset.

5 min read · UPSC Prelims · Science & Technology · Physics – Mechanics, Waves and Thermodynamics

At a Glance

Speed of light About 3 x 10 to the power 8 m/s
Refraction Bending at a boundary
Optical fibre Total internal reflection
Blue sky Scattering of light
Raman effect 1928; National Science Day
Exam link UPSC Prelims, Science & Technology: Physics – Mechanics, Waves and Thermodynamics

Where Does It Fit?

This is the third topic of the macro on physics. After energy, it looks at light, the visible part of the spectrum described in the next topic. Previous topic: Forms of Energy. Next topic: The Electromagnetic Spectrum.

What Are the Key Phenomena of Light?

Phenomenon	Explanation and example
Reflection	Light bounces from a surface. Plane mirrors make an upright, same-size, virtual image. Concave mirrors are used in torches, shaving mirrors and telescopes, and convex mirrors in vehicle rear views
Refraction	Bending of light on entering another medium, as given by Snell’s law. A pencil in water appears bent, and a pool appears shallower
Total internal reflection	Occurs when light goes from a denser to a rarer medium at more than the critical angle. Used in optical fibres, prisms in binoculars, mirages and the sparkle of diamonds
Dispersion	Splitting of white light into the colours violet to red because each colour bends differently. Rainbows form by refraction, internal reflection and dispersion in raindrops
Scattering	Rayleigh scattering is stronger for blue light, so the sky is blue. At sunrise and sunset the light travels farther through air, so red light remains. The Tyndall effect is scattering by colloids
Diffraction and interference	Bending around obstacles and the overlap of waves, which show the wave nature of light, as in Young’s double slit experiment

How Do Lenses and the Eye Work?

Convex lens: converges light and is used in magnifying glasses, cameras and spectacles for hypermetropia.
Concave lens: diverges light and is used in spectacles for myopia.
Eye: the cornea and lens focus light on the retina, which has rods for dim light and cones for colour. The iris controls the pupil.
Myopia: short-sightedness, in which distant objects are blurred, corrected by a concave lens.
Hypermetropia: far-sightedness, corrected by a convex lens.
Presbyopia and astigmatism: loss of near focus with age, corrected by bifocal lenses, and uneven curvature of the cornea, corrected by cylindrical lenses.
Colour blindness: an inherited problem with cones, as in Non-Chromosomal (Mendelian) Inheritance Disorders.

What Else Should You Remember?

Refractive index: about 1.33 for water, 1.5 for glass and 2.42 for diamond. The higher the index, the more the light bends.
Raman effect: C.V. Raman found in 1928 that scattered light shows changed wavelengths, for which he received the Nobel Prize in 1930. India marks National Science Day on 28 February.
Optical fibre: thin glass fibres carry light signals by total internal reflection, used in internet, telephone and endoscopes.
Instruments: the microscope magnifies small objects, and the telescope brings distant objects closer, as in Space Observatories and Telescopes (Hubble, Chandra X-Ray Observatory, Thirty Metre Telescope, Spitzer Space Telescope).
Polarisation: light waves vibrating in one plane, used in sunglasses and 3D cinema.
Speed of light: measured by Ole Romer in 1676 from moons of Jupiter, and a light year is the distance light travels in a year, about 9.46 trillion kilometres.
Wave and particle nature: light acts as a wave and as a particle, called a photon, as shown by the photoelectric effect of Einstein in 1905.
Safety: bright lasers and ultraviolet light can damage the eye, as in Types and Applications of Lasers.
Light and optics in daily life: light and optics explain mirrors, spectacles, cameras, projectors, rainbows and the working of the eye.

Why Does It Matter for the Exam?

Phenomenon questions: why the sky is blue, why diamonds sparkle and how rainbows form.
Device questions: which mirror or lens is used where and how eye defects are corrected.
History questions: Raman effect, Newton’s prism and the speed of light.
Technology link: optical fibre networks and BharatNet.
Common errors: believing that myopia needs a convex lens or that the sky is blue because of reflection of the sea.

As an analytical point, most optical phenomena follow from the change of speed of light in different media. The same laws design lenses, fibres and telescopes.

Why Is This Topic Important?

Facts Phenomena and instruments asked
Concept Refractive index, TIR
Link Fibre and telescopes

What Came Next?

The next topic covers the electromagnetic spectrum. Next topic: The Electromagnetic Spectrum.

Key People and Terms

Reflection Bouncing of light
Refraction Bending of light
Critical angle Angle for total internal reflection
Dispersion Splitting of white light
Myopia Short-sightedness
Photon Particle of light

Exam Corner

Points to Remember

Myopia: concave lens; hypermetropia: convex lens.
Diamond refractive index: 2.42.
Raman: 1928, Nobel 1930.
Optical fibres use total internal reflection.
Blue sky: scattering.

Do Not Confuse With

Reflection and refraction Bounce versus bend
Myopia and hypermetropia Near-sight versus far-sight
Dispersion and scattering Splitting by a prism versus scattering by particles

Memory Hook

Reflection bounces, refraction bends; TIR in fibre and diamond, dispersion for the rainbow, Raman effect in twenty-eight.

Mains Angle

Questions ask about natural phenomena and optical technology. These points add depth.

Laws Reflection and refraction
Nature Sky and rainbow
Technology Fibre and lenses

A Question You May Face

An original practice question, not a past paper question.

“Explain the phenomena of refraction, total internal reflection and scattering of light with their applications and natural examples.”

How to Answer

Laws Basic.
TIR Fibre, diamond.
Scattering Sky.
Eye Defects.

GS Relevance

Prelims: laws, natural phenomena, lenses and mirrors, eye defects, Raman effect, optical fibres. GS Paper 3: science and technology.

Frequently Asked Questions

What is total internal reflection?

Total internal reflection is the complete reflection of light back into a denser medium when it strikes the boundary with a rarer medium at an angle greater than the critical angle. It makes optical fibres carry signals over long distances, causes mirages, and gives diamonds their sparkle because their critical angle is small.

Why is the sky blue?

The sky is blue because the gases in the atmosphere scatter short wavelengths of sunlight, such as blue, much more strongly than long wavelengths, such as red, in a process called Rayleigh scattering. At sunrise and sunset sunlight passes through more air, so most blue light is scattered away and red light reaches the eye.

How is myopia corrected?

Myopia, or short-sightedness, is a defect in which distant objects look blurred because the image forms in front of the retina. It is corrected with spectacles that have a concave lens, which diverges light slightly before it enters the eye so that the image falls on the retina.

What is the Raman effect?

The Raman effect is the change in wavelength of light when it is scattered by molecules, discovered by C.V. Raman in 1928. It gives information about the structure of molecules, and Raman received the Nobel Prize in Physics in 1930, while India celebrates National Science Day on 28 February to mark the discovery.

What is the refractive index?

The refractive index of a material is the ratio of the speed of light in a vacuum to its speed in that material. It is about 1.33 for water, 1.5 for glass and 2.42 for diamond, and a higher value means that light bends more when entering the material.

PYQ Practice — Statement Analysis

1 Myopia is corrected using a concave lens.

True

For short-sightedness.

2 Optical fibres work on the principle of refraction only.

False

On total internal reflection.

3 The Raman effect was discovered in 1928.