Light Reflection and Refraction: Mirrors, Lenses, TIR, Dispersion, Eye Defects with Tricks

Light is the form of energy that makes things visible to us. When light falls on a surface, it can bounce back, which is called reflection, or it can bend while passing from one medium into another, which is called refraction. These two simple behaviours explain a huge range of everyday sights – your image in a mirror, a straw that looks bent in a glass of water, the sparkle of a diamond, a rainbow after rain, twinkling stars and even mirages on hot roads.

Light is one of the highest-scoring topics in the General Science section of SSC, Railway (RRB NTPC, Group D, ALP), WBCS, Police and other competitive exams. Questions usually test the laws of reflection and refraction, image formation by mirrors and lenses, sign conventions and formulas, power of a lens, total internal reflection, dispersion, scattering and defects of the human eye. This guide explains every concept in simple language, followed by a quick revision box with memory tricks and 20 MCQs with answers.

➤ Speed of light in vacuum: about 3 × 10⁸ m/s
➤ Reflection: Angle of incidence = Angle of reflection
➤ Refraction: Snell's law – sin i / sin r = constant (refractive index)
➤ Mirror formula: 1/v + 1/u = 1/f · Lens formula: 1/v − 1/u = 1/f
➤ Power of lens: P = 1/f (f in metres), unit dioptre (D)
➤ Key phenomena: Total internal reflection, dispersion, scattering, atmospheric refraction

Nature of Light – Basic Facts

• Light is an electromagnetic wave and does not need a medium to travel – it can travel through vacuum
• It shows a dual nature – it behaves as a wave (interference, diffraction) and as particles called photons (photoelectric effect)
• In a uniform medium, light travels in a straight line (rectilinear propagation) – this is why shadows form
• Speed of light: about 3 × 10⁸ m/s in vacuum; it is slower in water and glass
• Sunlight takes about 8 minutes 20 seconds to reach the Earth
• A light year is a unit of distance (not time) – the distance light travels in one year, about 9.46 × 10¹⁵ m
• Visible light ranges roughly from 400 nm (violet) to 700 nm (red) in wavelength

Part A: Reflection of Light

Reflection is the bouncing back of light from a surface. A polished, shiny surface such as a mirror reflects most of the light falling on it.

Laws of Reflection

1. The angle of incidence is equal to the angle of reflection (∠i = ∠r)
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane

These laws apply to all reflecting surfaces – plane or curved.

Types of Reflection

Image Formed by a Plane Mirror

• Virtual (cannot be caught on a screen) and erect
• Same size as the object
• As far behind the mirror as the object is in front
• Laterally inverted – left appears right (that is why "AMBULANCE" is written in reverse on ambulances)
• To see your full image, a plane mirror must be at least half your height
• Number of images between two mirrors inclined at angle θ: n = (360°/θ) − 1, when 360°/θ is even. For mirrors at 90°, 3 images are formed; for parallel mirrors, infinite images
• Uses: looking glass, periscope, kaleidoscope (uses multiple reflection)

Spherical Mirrors

A spherical mirror is part of a hollow sphere. If the reflecting surface curves inward, it is a concave mirror (converging); if it bulges outward, it is a convex mirror (diverging).

Image Formation by a Concave Mirror

Convex mirror: For any position of the object, the image is always virtual, erect and diminished, formed behind the mirror between P and F.

Uses of Spherical Mirrors

Mirror Formula and Magnification

• New Cartesian sign convention: All distances are measured from the pole. Distances in the direction of incident light are positive; those against it are negative. Heights above the principal axis are positive
• Focal length of a concave mirror is negative; of a convex mirror is positive
• Mirror formula: 1/v + 1/u = 1/f
• Magnification: m = h′/h = −v/u. If m is negative, the image is real and inverted; if positive, virtual and erect

Part B: Refraction of Light

Refraction is the bending of light when it passes obliquely from one transparent medium into another. It happens because light travels at different speeds in different media.

• From a rarer to a denser medium (air → glass), light bends towards the normal
• From a denser to a rarer medium (glass → air), light bends away from the normal
• A ray falling along the normal (at 90° to the surface) passes straight without bending
• The frequency of light does not change during refraction; speed and wavelength change

Laws of Refraction

1. The incident ray, the refracted ray and the normal all lie in the same plane
2. Snell's law: For a given pair of media and a given colour of light, sin i / sin r = constant. This constant is the refractive index of the second medium with respect to the first

Refractive Index

Absolute refractive index n = c / v (speed of light in vacuum ÷ speed of light in the medium). A higher refractive index means light travels slower – the medium is optically denser. Optical density is different from mass density: kerosene is optically denser than water but floats on it.

Refraction Through a Glass Slab

When light passes through a rectangular glass slab, the emergent ray is parallel to the incident ray but shifted sideways. This sideways shift is called lateral displacement.

Everyday Examples of Refraction

• A swimming pool or pond looks shallower than it really is
• A pencil or straw in water appears bent at the surface
• Letters under a glass slab appear raised
• A coin in a bowl becomes visible when water is poured in

Atmospheric Refraction

The Earth's atmosphere has layers of different densities, so starlight and sunlight bend as they pass through it.

• Twinkling of stars: Stars are very far away and act as point sources; changing air layers keep shifting their apparent position and brightness. Planets do not twinkle because they are closer and act as extended sources
• Advanced sunrise and delayed sunset: The Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset, making the day about 4 minutes longer
• The Sun appears flattened (oval) near the horizon
• Stars appear higher than their actual position

Total Internal Reflection (TIR)

When light travels from a denser to a rarer medium and the angle of incidence is greater than the critical angle, all the light is reflected back into the denser medium. This is total internal reflection.

• Two conditions: (1) light must go from a denser to a rarer medium, and (2) the angle of incidence must exceed the critical angle
• Critical angle: The angle of incidence for which the angle of refraction is 90°. Diamond has a very small critical angle (about 24°); for glass it is about 42° and for water about 49°

Mirages are common in the hot deserts of Rajasthan and in the flat salt plains of the Rann of Kutch. Read more about these landscapes in our guide to grassland and scrubland ecosystems of India.

Part C: Refraction by Lenses

A lens is a transparent material bounded by two surfaces, at least one of which is curved.

Image Formation by a Convex Lens

Lens Formula, Magnification and Power

• Lens formula: 1/v − 1/u = 1/f
• Magnification: m = h′/h = v/u
• Power of a lens: P = 1/f, where f is in metres. SI unit: dioptre (D), where 1 D = 1 m⁻¹
• Example: A convex lens of focal length 50 cm (0.5 m) has power +2 D; a concave lens of focal length 25 cm has power −4 D
• When lenses are placed in contact, the total power is the sum: P = P₁ + P₂ + …

Part D: Dispersion, Rainbow and Scattering

Dispersion of Light

When white light passes through a glass prism, it splits into seven colours – VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). This is called dispersion, first explained by Isaac Newton. Different colours bend by different amounts because the refractive index of glass is different for each colour.

• Violet bends the most (shortest wavelength, slowest in glass)
• Red bends the least (longest wavelength, fastest in glass)
• A second inverted prism can recombine the colours into white light

Rainbow

A rainbow forms when sunlight enters tiny raindrops. Each drop acts like a small prism: light is refracted and dispersed on entering, internally reflected at the back of the drop, and refracted again on leaving. A rainbow is always seen in the direction opposite to the Sun. In the primary rainbow, red is on the outside and violet on the inside.

Scattering of Light

Raman Effect – A Kolkata Discovery

Sir C.V. Raman discovered that when light passes through a transparent substance, a small part of the scattered light changes its wavelength. This is the Raman Effect. He made the discovery on 28 February 1928 at the Indian Association for the Cultivation of Science (IACS), Kolkata, and won the Nobel Prize in Physics in 1930 – the first Asian to win a Nobel in science. India celebrates National Science Day on 28 February. Today, programmes like the VAIBHAV Fellowship connect Indian-origin scientists abroad with Indian research labs, carrying forward this legacy.

Part E: The Human Eye and Its Defects

• The eye lens is a convex lens that forms a real and inverted image on the retina; the brain makes it appear erect
• Power of accommodation: The eye lens changes its focal length with the help of ciliary muscles to see near and far objects
• Near point of a normal eye: 25 cm; far point: infinity
• The iris controls the size of the pupil, which controls the amount of light entering the eye
• Image on the retina lasts about 1/16th of a second (persistence of vision) – the principle behind movies

Optical Instruments

• Simple microscope: A single convex lens used as a magnifying glass
• Compound microscope: Two convex lenses (objective and eyepiece) for viewing tiny objects
• Telescope: Used to see distant objects; refracting telescopes use lenses, reflecting telescopes use concave mirrors. Planetariums and space galleries, like the new ISRO Space Gallery at Darbhanga Planetarium, help students learn about telescopes and the sky
• Periscope: Uses two plane mirrors (or prisms) at 45° – used in submarines
• Camera: Uses a convex lens to form a real, inverted image on the sensor

Research labs of CSIR, such as the Central Scientific Instruments Organisation (CSIO), work on optical instruments and photonics in India.

20 MCQs on Light – Reflection and Refraction

Q1. The angle of incidence is equal to the angle of reflection. This is true for:
(a) Plane mirrors only
(b) Concave mirrors only
(c) Convex mirrors only
(d) All reflecting surfaces

Answer: (d) – The laws of reflection apply to all reflecting surfaces, plane or curved.

Q2. The image formed by a plane mirror is:
(a) Real and inverted
(b) Virtual, erect and laterally inverted
(c) Real and enlarged
(d) Virtual and diminished

Answer: (b) – It is also the same size as the object.

Q3. How many images are formed when two plane mirrors are placed at 90° to each other?
(a) 2
(b) 3
(c) 4
(d) Infinite

Answer: (b) – n = (360/90) − 1 = 3.

Q4. Which mirror is used as a rear-view mirror in vehicles?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Parabolic mirror

Answer: (c) – A convex mirror gives an erect image and a wider field of view.

Q5. Which mirror is used by dentists and as a shaving mirror?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Cylindrical mirror

Answer: (b) – When the object is close, a concave mirror gives an enlarged, erect image.

Q6. The focal length of a spherical mirror of radius of curvature 30 cm is:
(a) 60 cm
(b) 30 cm
(c) 15 cm
(d) 10 cm

Answer: (c) – f = R/2 = 30/2 = 15 cm.

Q7. Where should an object be placed in front of a concave mirror to get a real image of the same size?
(a) At the focus
(b) At the centre of curvature
(c) Between the pole and focus
(d) At infinity

Answer: (b) – The image is also formed at C.

Q8. Refraction of light occurs because:
(a) Light changes its frequency in different media
(b) Light changes its speed in different media
(c) Light changes its colour
(d) Light is reflected at the surface

Answer: (b) – Frequency stays the same; speed and wavelength change.

Q9. When light goes from air into glass, it:
(a) Bends away from the normal
(b) Bends towards the normal
(c) Does not bend
(d) Is totally reflected

Answer: (b) – Glass is optically denser than air.

Q10. The refractive index of a medium is the ratio of:
(a) Speed of light in the medium to speed in vacuum
(b) Speed of light in vacuum to speed in the medium
(c) Wavelength in vacuum to frequency
(d) Angle of refraction to angle of incidence

Answer: (b) – n = c/v.

Q11. A swimming pool appears shallower than its actual depth because of:
(a) Reflection
(b) Refraction
(c) Dispersion
(d) Scattering

Answer: (b) – Light from the bottom bends away from the normal as it leaves water.

Q12. Stars twinkle but planets do not, because of:
(a) Total internal reflection
(b) Atmospheric refraction
(c) Dispersion
(d) Diffraction

Answer: (b) – Planets are closer and act as extended sources, so the effect averages out.

Q13. The sparkling of a diamond is due to:
(a) Dispersion
(b) Total internal reflection
(c) Scattering
(d) Interference

Answer: (b) – Diamond has a high refractive index (2.42) and a small critical angle (about 24°).

Q14. Optical fibres work on the principle of:
(a) Refraction
(b) Dispersion
(c) Total internal reflection
(d) Scattering

Answer: (c) – Light travels inside the fibre by repeated total internal reflection.

Q15. The power of a convex lens of focal length 50 cm is:
(a) +0.5 D
(b) +2 D
(c) −2 D
(d) +50 D

Answer: (b) – P = 1/f = 1/0.5 m = +2 D.

Q16. In the spectrum of white light produced by a prism, which colour deviates the most?
(a) Red
(b) Yellow
(c) Green
(d) Violet

Answer: (d) – Violet has the shortest wavelength; red deviates the least.

Q17. The blue colour of the clear sky is due to:
(a) Reflection of light by oceans
(b) Refraction of light
(c) Scattering of light
(d) Dispersion of light

Answer: (c) – Air molecules scatter shorter (blue) wavelengths more strongly.

Q18. Which colour is used for danger signals because it is scattered the least?
(a) Blue
(b) Green
(c) Yellow
(d) Red

Answer: (d) – Red has the longest wavelength among visible colours.

Q19. Myopia (short-sightedness) is corrected by using a:
(a) Convex lens
(b) Concave lens
(c) Bifocal lens
(d) Cylindrical lens

Answer: (b) – Hypermetropia is corrected by a convex lens; astigmatism by a cylindrical lens.

Q20. C.V. Raman discovered the Raman Effect in 1928 at which institution?
(a) Indian Institute of Science, Bengaluru
(b) Presidency College, Chennai
(c) Indian Association for the Cultivation of Science, Kolkata
(d) Bose Institute, Kolkata

Answer: (c) – He won the Nobel Prize in Physics in 1930; National Science Day is on 28 February.

Conclusion

Reflection and refraction explain most of what we see around us – from mirrors and lenses to rainbows, blue skies and sparkling diamonds. For exams, remember the laws of reflection and Snell's law, image formation tables for concave mirrors and convex lenses, the sign rules for f, the mirror and lens formulas, power in dioptres, the two conditions of total internal reflection, dispersion and scattering, and the eye defects with their correcting lenses. Revise the memory tricks above once more before your exam.

Reference: NCERT Science Class X, Chapters "Light – Reflection and Refraction" and "The Human Eye and the Colourful World".