India's #1 AI Tutorformula-sheet · Science · Chapter 13

Class 8 Science Chapter 13 Light — Formulas & Key Points

CBSE Class 8 Science Chapter 13 Light introduces fundamental concepts of how light behaves when it strikes surfaces, splits into colours, and forms images in mirrors and the human eye. This formula sheet organizes every law, definition, and key point from the NCERT textbook into quick-reference tables, complete with solved examples and memory aids to help you ace class tests and annual exams.

Your child's private AI tutor — trained on NCERT.

3-day free trial · ₹1 to start · Cancel anytime.

Start 3-day free trial →

Key takeaways

✓The two laws of reflection state that angle of incidence equals angle of reflection, and that incident ray, reflected ray, and normal all lie in the same plane.
✓A plane mirror always forms a virtual, erect image of the same size as the object, with left-right lateral inversion.
✓Multiple reflections in mirrors placed at angles produce symmetrical patterns; kaleidoscopes use mirrors at 60° to create beautiful designs.
✓White light splits into seven colours (VIBGYOR) when passing through a glass prism due to dispersion caused by different refractive indices.
✓The human eye focuses light using the cornea and lens onto the retina; rod cells detect dim light while cone cells perceive colour.
✓Myopia (short-sightedness) is corrected with concave lenses; hypermetropia (long-sightedness) requires convex lenses.
✓The persistence of vision (around 1/16th second) allows us to perceive movies as continuous motion despite individual frames.

Laws and Principles — Core Formulas Table

Chapter 13 focuses on qualitative laws rather than numerical equations. The two laws of reflection are the backbone of this chapter. These laws apply universally to all reflecting surfaces—plane mirrors, spherical mirrors, and even rough surfaces (though diffuse reflection scatters light). Understanding when each law applies helps you answer diagram-based questions in CBSE exams. Remember that angles are always measured from the normal (the perpendicular line at the point of incidence), never from the mirror surface itself. Students often lose marks by measuring angles incorrectly.

Law 1: Angle of incidence = Angle of reflection (∠i = ∠r) for all types of reflection
Law 2: Incident ray, reflected ray, and normal lie in the same plane
These laws hold for regular reflection (smooth surfaces) and diffuse reflection (rough surfaces)
In diffuse reflection, each ray obeys the laws individually, but normals point in different directions

Mirror Properties and Image Characteristics

Plane mirrors form images with specific predictable properties that appear in every CBSE Class 8 Science exam. The image distance behind the mirror equals the object distance in front; if you stand 2 metres from a mirror, your image appears 2 metres behind it. Lateral inversion means left and right swap—when you raise your right hand, your mirror image raises its left. The NCERT textbook emphasizes that the image is virtual (cannot be captured on a screen) and always the same size as the object, never magnified or diminished. Mark these properties clearly in diagram questions to score full marks.

Image distance from mirror = Object distance from mirror
Image is virtual, erect, and laterally inverted
Image size = Object size (magnification = 1)
The image and object are always symmetrical about the mirror plane

Multiple Reflections — Applications and Geometry

When two plane mirrors are placed at an angle θ, the number of images formed depends on that angle. The formula for number of images is (360/θ) - 1 when 360/θ is even, or just 360/θ when it is odd, assuming the object lies on the bisector. For a periscope, two mirrors at 45° allow you to see over obstacles. Kaleidoscopes typically use three mirrors at 60° to each other, producing (360/60) = 6 images of coloured glass pieces, creating symmetrical patterns. Barber shops use two parallel mirrors (θ approaching 0°) to create infinite reflections. CBSE exams often ask you to draw ray diagrams or calculate image count for given angles.

Number of images ≈ (360°/θ) - 1, where θ is the angle between mirrors
At θ = 90°, you see 3 images; at θ = 60°, you see 5 images
Periscope uses two parallel mirrors at 45° for viewing over barriers
Kaleidoscope uses three mirrors at 60° producing beautiful symmetrical patterns

Dispersion of Light — The VIBGYOR Sequence

White sunlight comprises seven colours that travel at slightly different speeds through glass. When white light enters a triangular glass prism, it bends (refracts) at the first surface. Violet bends the most because it has the shortest wavelength and highest refractive index in glass; red bends the least. This splitting of white light into constituent colours is called dispersion. The spectrum always appears in VIBGYOR order: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Sir Isaac Newton first demonstrated this in 1665. A rainbow forms due to dispersion of sunlight by water droplets acting as tiny prisms. The NCERT Class 8 Science textbook emphasizes that you can recombine the seven colours using a second inverted prism to get white light again.

Dispersion: splitting of white light into seven colours due to different refractive indices
VIBGYOR sequence: Violet, Indigo, Blue, Green, Yellow, Orange, Red
Violet deviates most (highest refractive index); Red deviates least (lowest refractive index)
Rainbow is a natural spectrum caused by dispersion in water droplets

Human Eye Structure — Key Terms and Definitions

The human eye works like a camera, focusing light to form images on the retina. The cornea is the transparent front bulge that does most of the bending of light. Behind it, the pupil (the black circular opening) controls how much light enters; the coloured iris muscles adjust pupil size. The eye lens is a flexible, convex lens that fine-tunes focus by changing shape—a process called accommodation. Light-sensitive cells on the retina convert light into electrical signals sent via the optic nerve to the brain. Rod cells work in dim light (black-and-white vision), while cone cells detect colour in bright light. The blind spot, where the optic nerve exits, has no photoreceptors. CBSE diagrams often ask you to label these parts clearly.

Cornea: transparent outer layer; does maximum light refraction
Iris: coloured diaphragm; controls pupil size and light entry
Eye lens: flexible convex lens; changes shape to focus on near or distant objects
Retina: light-sensitive screen; contains rods (dim light) and cones (colour vision)
Optic nerve: transmits electrical signals from retina to brain
Blind spot: region on retina with no photoreceptors where optic nerve attaches

Vision Defects and Correction — Summary Table

Myopia (near-sightedness or short-sightedness) occurs when the eyeball is too long or the lens is too curved, causing distant objects to focus in front of the retina, appearing blurred. A concave (diverging) lens spreads out light rays before they enter the eye, moving the focus back onto the retina. Hypermetropia (far-sightedness or long-sightedness) happens when the eyeball is too short or the lens loses flexibility with age, so nearby objects focus behind the retina. A convex (converging) lens bends light inward, bringing the focus forward. Presbyopia is age-related loss of accommodation; bifocal lenses with both concave and convex sections help. CBSE Class 8 Science solutions stress drawing ray diagrams showing defect and correction for full marks.

Myopia: distant objects blurred; image forms in front of retina; corrected by concave lens
Hypermetropia: near objects blurred; image forms behind retina; corrected by convex lens
Presbyopia: age-related; near-point recedes; bifocals (convex for reading, concave for distance) needed
Cataract: clouding of eye lens; treated by surgical lens replacement

Important Definitions and Key Terms

Definitions form the base of short-answer questions in CBSE Class 8 Science exams. Regular reflection occurs on smooth polished surfaces like mirrors, where parallel incident rays produce parallel reflected rays. Diffuse reflection happens on rough surfaces like paper or walls; each point obeys reflection laws, but different normals scatter light in all directions, making the surface visible from all angles without forming a clear image. Lateral inversion is the left-right reversal seen in plane mirrors—the reason ambulance is written reversed on the front so drivers see it correctly in rear-view mirrors. Persistence of vision is the phenomenon where an image persists on the retina for about one-sixteenth of a second after the object is removed, enabling us to perceive motion pictures as continuous.

Regular reflection: reflection from smooth surfaces producing clear images
Diffuse reflection: reflection from rough surfaces scattering light in all directions
Lateral inversion: left-right reversal in plane mirror images
Persistence of vision: retinal image lasts ~1/16 second, basis of cinema
Spectrum: band of seven colours formed by dispersion of white light
Accommodation: ability of eye lens to change curvature to focus at varying distances

Units, Symbols, and Notation — Common Mistakes

Class 8 Science Chapter 13 Light is largely descriptive, with few numerical calculations, but clarity in notation matters. Always measure angles of incidence and reflection from the normal, not the mirror surface. Mark the normal with a dashed perpendicular line in diagrams. Use degree symbols (°) for angles. When stating image properties, write 'virtual and erect' not just 'virtual'. In dispersion, list colours in correct VIBGYOR order—many students reverse it or skip Indigo. For vision defects, specify lens type clearly: 'concave lens of suitable focal length' or 'convex lens' rather than vague terms like 'corrective lens'. CBSE marking schemes award marks for precise terminology from the NCERT Class 8 Science textbook.

Angles always measured from the normal (⊥), never from the mirror surface
Write ∠i for angle of incidence, ∠r for angle of reflection
VIBGYOR order must be exact: Violet, Indigo, Blue, Green, Yellow, Orange, Red
Specify 'concave lens' or 'convex lens', not just 'lens'
For images: state all properties—virtual/real, erect/inverted, magnified/same size/diminished

Memory Tricks and Mnemonics for Quick Recall

Remembering VIBGYOR is easiest with the sentence 'Very Intelligent Boys Generally Yearn Over Rainbows' or the classic 'Richard Of York Gave Battle In Vain' (reverse order: ROYGBIV). For the two laws of reflection, think 'Equal Angles, Same Plane'—angle in equals angle out, all in one flat plane. To recall that myopia needs concave and hypermetropia needs convex, remember 'M for Myopia, M for Minus (concave), H for Hypermetropia, H for Hump (convex bulge)'. For lateral inversion, picture the word MOM or SWIMS—they look identical in a mirror, helping you visualize the left-right swap. Persistence of vision: think of a burning sparkler twirled in a circle appearing as a continuous ring because each image lingers 1/16 second. These mnemonics save precious minutes during CBSE exams.

VIBGYOR: 'Very Intelligent Boys Generally Yearn Over Rainbows'
Laws of reflection: 'Equal Angles, Same Plane'
Myopia = M = Minus lens (concave); Hypermetropia = H = Hump lens (convex)
Lateral inversion: MOM, SWIMS look the same in mirror
Persistence of vision: sparkler circle illusion

Worked Mini-Examples Applying Key Concepts

Applying formulas and laws to specific scenarios cements understanding. These three worked examples mirror the type of questions in CBSE Class 8 Science Chapter 13 tests. Example 1 uses the law of reflection for a numerical angle problem. Example 2 calculates the number of images in multiple reflections. Example 3 applies knowledge of vision defects to a real-life correction scenario. Practice these until you can solve them in under two minutes each. When CBSETUTOR.ai students upload a photo of a similar problem at 11 PM before an exam, the AI tutor provides instant step-by-step solutions with diagrams—all at a flat ₹999/month across Classes 6-12, with a 3-day free trial to start.

One-Glance Last-Minute Revision Box

This ultra-compact summary lets you revise the entire chapter in under five minutes right before walking into the exam hall. Pin this section on your phone or print it as a flash card. Cover the answer column and test yourself on each point. If you can recall all these facts confidently, you are ready for any Class 8 Science Chapter 13 Light question CBSE throws at you. Regular weekly revision using this box—combined with NCERT Class 8 Science solutions practice—ensures concepts stay fresh. Students using CBSETUTOR.ai typically revise this box the night before the test, then use the AI photo-solver for any last practice doubts, securing top marks consistently across all chapters.

Laws of reflection: ∠i = ∠r; incident ray, reflected ray, normal in same plane
Plane mirror image: virtual, erect, same size, laterally inverted, image distance = object distance
Multiple reflections: number of images ≈ (360/θ) - 1
Dispersion: white light → VIBGYOR through prism; violet bends most, red least
Eye parts: cornea (refracts), iris (controls pupil), lens (focuses), retina (detects), optic nerve (transmits)
Myopia: distant blur, concave lens; Hypermetropia: near blur, convex lens
Persistence of vision: ~1/16 second; enables movies

Frequently asked questions

What are the two laws of reflection in Class 8 Science Chapter 13?+

First law: The angle of incidence equals the angle of reflection (∠i = ∠r). Second law: The incident ray, reflected ray, and normal to the surface at the point of incidence all lie in the same plane. Both laws apply to all reflecting surfaces.

How many images form when two mirrors are at 90 degrees?+

When two plane mirrors are placed at 90° to each other, the number of images formed is (360/90) - 1 = 4 - 1 = 3 images. This is commonly seen in trial rooms and some periscope designs.

Why does white light split into seven colours in a prism?+

White light comprises seven colours with different wavelengths. Each colour travels at a different speed in glass, causing different amounts of bending (refraction). Violet bends most, red bends least. This splitting is called dispersion, producing the VIBGYOR spectrum.

What is lateral inversion and where do we observe it?+

Lateral inversion is the left-right reversal that occurs in plane mirror images. Your left hand appears as the right hand of your image. This is why AMBULANCE is written reversed on emergency vehicles—so drivers see it correctly in rear-view mirrors.

Which lens corrects myopia and which corrects hypermetropia?+

Myopia (short-sightedness) is corrected using a concave (diverging) lens, which spreads light rays so they focus on the retina. Hypermetropia (long-sightedness) is corrected with a convex (converging) lens, which bends rays inward to focus on the retina.

What is the function of rods and cones in the human eye?+

Rod cells on the retina are sensitive to dim light and enable night vision but do not detect colour. Cone cells function in bright light and are responsible for colour vision. Together they convert light into electrical signals sent to the brain via the optic nerve.

How does a kaleidoscope use multiple reflections?+

A kaleidoscope uses three plane mirrors arranged at 60° to each other. Coloured glass pieces placed at one end undergo multiple reflections, creating (360/60) = 6 symmetrical images. When you rotate the tube, the patterns change beautifully.

What is persistence of vision and how does it relate to movies?+

Persistence of vision means an image formed on the retina persists for about one-sixteenth of a second after the object is removed. Movies project 24 frames per second; our brain blends these still images into continuous motion because each frame lingers briefly on the retina.

Why do we see a rainbow after rain?+

Sunlight enters tiny water droplets suspended in the air, undergoes dispersion (splitting into VIBGYOR), reflects inside the droplet, and exits. Each droplet acts like a prism. Millions of droplets together produce the arc of seven colours we call a rainbow, with red on the outer edge and violet inside.

Where can I get step-by-step solutions for CBSE Class 8 Science Chapter 13 numerical problems?+

CBSETUTOR.ai offers a 24×7 AI tutor for Classes 6-12 at ₹999/month (one flat price for all classes). Upload a photo of any Light chapter problem, and get instant step-by-step solutions with diagrams. Start with a 3-day free trial to see how it boosts your scores.

Related resources

CBSE Class 8 Science Chapter 13 Light Worksheet with Answers Important Questions: CBSE Class 8 Science Chapter 13 Light CBSE Class 8 Science Chapter 12 Some Natural Phenomena Worksheet with Answers Important Questions: CBSE Class 8 Science Chapter 12 Some Natural Phenomena AI Tutor for Class 8: The Smart Alternative to Tuition AI Tutor for Class 8 Social Science: Learn Faster with Instant Help Online Class 9 Science Tutor in Mumbai – 24/7 CBSE-Aligned AI Coaching Class 9 Mathematics Chapter 2 Polynomials — Formulas & Key Points

Keep learning — related guides

Class 8Science

Class 8 Science Tutor in Andheri West Mumbai - 24×7 Help

Class 8Science

Important Questions: CBSE Class 8 Science Chapter 13 Light

Class 8Science

CBSE Class 8 Science Chapter 13 Light Worksheet with Answers

Class 8Science

Class 8 Science Chapter 12 Some Natural Phenomena — Formulas & Key Points

Class 8Science

Important Questions: CBSE Class 8 Science Chapter 12 Some Natural Phenomena

Class 8Science

CBSE Class 8 Science Chapter 12 Some Natural Phenomena Worksheet with Answers

Ready to give your Class 8 child the tutor that never sleeps?

CBSETUTOR.ai covers every chapter in the Class 8 NCERT syllabus — Maths, Science, Social Science, English, Hindi and more. 24×7. Patient. Unlimited. 3-day free trial.

Start your child's 3-day free trial →