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Class 7 Science Chapter 11 Light — Formulas & Key Points

Light is one of the most fascinating chapters in CBSE Class 7 Science. Chapter 11 introduces students to reflection, the behaviour of plane mirrors, concave mirrors, convex mirrors, lenses, and the beautiful phenomenon of dispersion that produces rainbows. This formula sheet distils every key point, law, and definition from the NCERT Class 7 Science textbook into tables, memory aids, and solved examples — perfect for last-minute revision or clarifying doubts.

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Key takeaways

  • ✓The Laws of Reflection state that angle of incidence equals angle of reflection, and both rays lie in the same plane.
  • ✓Plane mirrors always produce virtual, erect, laterally inverted images of the same size as the object.
  • ✓Concave mirrors converge light rays; convex mirrors diverge them — remember 'cave-in' for concave.
  • ✓White light splits into seven colours (VIBGYOR) when passed through a prism; this is called dispersion.
  • ✓Image distance equals object distance for a plane mirror, making it ideal for everyday use.
  • ✓Real images can be projected on a screen; virtual images cannot — concave mirrors can form both.
  • ✓Lenses refract light; mirrors reflect it — both obey specific geometric rules covered in later classes.

Laws and Principles

Chapter 11 Light is built on two fundamental Laws of Reflection that govern how light behaves when it strikes a reflective surface. These laws apply to all types of mirrors — plane, concave, and convex — and form the foundation for understanding image formation. The first law states that the angle of incidence (the angle between the incident ray and the normal) is always equal to the angle of reflection (the angle between the reflected ray and the normal). The second law specifies that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. Understanding these laws is critical for solving problems involving mirror placement, image location, and periscope design. NCERT Class 7 Science emphasises drawing neat ray diagrams to visualise these laws in action.
  • First Law of Reflection: ∠i = ∠r (angle of incidence equals angle of reflection)
  • Second Law of Reflection: Incident ray, reflected ray, and normal lie in the same plane
  • These laws hold for all reflecting surfaces — smooth or rough
  • Regular reflection occurs on smooth surfaces like mirrors; diffuse reflection on rough surfaces like paper

Key Terms and Definitions

Mastering the vocabulary of light is essential for Class 7 Science Chapter 11. Each term has a precise meaning and is used consistently in NCERT Class 7 Science solutions and exam papers. The 'incident ray' is the ray of light approaching the mirror. The 'reflected ray' is the ray bouncing off the mirror. The 'normal' is an imaginary line perpendicular to the mirror surface at the point where the incident ray strikes. The 'angle of incidence' is measured between the incident ray and the normal, not the mirror surface. The 'angle of reflection' is measured between the reflected ray and the normal. A 'real image' is one that can be caught on a screen because light rays actually converge at that point. A 'virtual image' is one that cannot be projected because light rays only appear to diverge from that point. Lateral inversion means left appears right and vice versa in a mirror image.
  • Incident Ray: Light ray striking the mirror surface
  • Reflected Ray: Light ray bouncing back from the mirror
  • Normal: Perpendicular line drawn at the point of incidence
  • Angle of Incidence (∠i): Angle between incident ray and normal
  • Angle of Reflection (∠r): Angle between reflected ray and normal
  • Real Image: Can be projected on a screen; formed by actual convergence of rays
  • Virtual Image: Cannot be projected; formed by apparent divergence of rays
  • Lateral Inversion: Left-right reversal in mirror images

Plane Mirror Properties — Quick Reference Table

Plane mirrors are the simplest reflecting surfaces and are used daily in homes, shops, and salons. The image formed by a plane mirror has very specific characteristics that appear repeatedly in CBSE Class 7 Science exams. The image is always virtual (cannot be caught on a screen), erect (upright, not inverted), of the same size as the object, and laterally inverted (left becomes right). The distance of the image behind the mirror is exactly equal to the distance of the object in front of the mirror. If you stand 2 metres in front of a plane mirror, your image appears 2 metres behind it. The total distance between you and your image is therefore 4 metres. These properties make plane mirrors ideal for everyday grooming and for constructing periscopes. NCERT Class 7 Science notes stress that students should be able to draw ray diagrams showing how at least two reflected rays appear to come from the image location.
  • Nature: Virtual and erect
  • Size: Same as the object (magnification = 1)
  • Position: Image distance = Object distance (behind the mirror)
  • Orientation: Laterally inverted
  • Application: Used in periscopes, kaleidoscopes, dressing mirrors

Concave and Convex Mirrors — Comparison Table

Concave and convex mirrors behave very differently because of their curvature. A concave mirror curves inward like the inside of a spoon or a cave — hence the memory trick 'cave-in' for concave. It converges parallel rays of light to a point called the focus. Concave mirrors can form both real images (when the object is beyond the focus) and virtual images (when the object is between the focus and the mirror). They are used in torches, car headlights, shaving mirrors, and dentist mirrors. A convex mirror curves outward like the back of a spoon. It diverges parallel rays of light, making them spread out. The reflected rays appear to come from a point behind the mirror. Convex mirrors always form virtual, erect, and diminished images, regardless of object position. This wide field of view makes them ideal for vehicle rear-view mirrors and shop security mirrors. Class 7 Science Chapter 11 introduces these concepts qualitatively; numerical problems appear in higher classes.
  • Concave Mirror: Curves inward; converges light; can form real or virtual images; used in torches, headlights, makeup mirrors
  • Convex Mirror: Curves outward; diverges light; always forms virtual, erect, diminished images; used in rear-view mirrors, security mirrors
  • Focus (F): Point where parallel rays meet (concave) or appear to diverge from (convex)
  • Pole (P): Centre point of the mirror surface
  • Principal Axis: Horizontal line passing through pole and focus

Lenses — Basic Introduction

Lenses are transparent objects with at least one curved surface that refract (bend) light passing through them. Chapter 11 Light provides a gentle introduction; detailed study appears in Class 10. A convex lens (also called a converging lens) is thicker at the centre than at the edges. It converges parallel rays of light to a focus and can form real or virtual images depending on object position. Convex lenses are used in magnifying glasses, cameras, spectacles for long-sightedness, and the human eye. A concave lens (also called a diverging lens) is thinner at the centre than at the edges. It diverges parallel rays, causing them to spread out. Concave lenses always form virtual, erect, and diminished images and are used in spectacles for short-sightedness and in peepholes. The behaviour of lenses is governed by the laws of refraction, which bend light as it passes from air into glass and back into air.
  • Convex Lens: Thicker at centre; converges light; can form real and virtual images; used in cameras, magnifying glasses
  • Concave Lens: Thinner at centre; diverges light; forms virtual, erect, diminished images; used in spectacles for myopia
  • Refraction: Bending of light as it passes from one medium to another
  • Focus of Lens: Point where parallel rays converge (convex) or appear to diverge from (concave)

Dispersion of Light and the Rainbow

White light is not a single colour but a mixture of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red — remembered by the mnemonic VIBGYOR. When white sunlight passes through a transparent medium like a glass prism or water droplets, it splits into its constituent colours. This phenomenon is called dispersion. Each colour bends by a different amount because each has a different wavelength. Violet bends the most; red bends the least. A rainbow is a natural spectrum caused by dispersion of sunlight by millions of tiny water droplets suspended in the atmosphere after rain. The droplets act as tiny prisms. The sun must be behind you and rain in front for you to see a rainbow. NCERT Class 7 Science encourages students to observe rainbows and understand that dispersion proves white light is composite. Isaac Newton first demonstrated dispersion using a glass prism in the 1660s.
  • Dispersion: Splitting of white light into seven colours (VIBGYOR)
  • Spectrum: Band of seven colours obtained after dispersion
  • Prism: Transparent glass object with triangular cross-section used to disperse light
  • Rainbow: Natural spectrum formed by dispersion of sunlight through water droplets
  • Order of colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR)

Common Mistakes, Units, and Notations

Students often lose marks in Class 7 Science Chapter 11 due to small errors in diagrams, terminology, and concept application. One frequent mistake is measuring the angle of incidence from the mirror surface instead of from the normal — always measure from the perpendicular normal line. Another error is confusing real and virtual images: remember, real images can be caught on a screen, virtual images cannot. When drawing ray diagrams, use a ruler for straight lines and mark arrowheads to show the direction of light travel. Label all rays clearly as incident ray, reflected ray, and normal. In numericals (rare in Class 7 but common in practice worksheets), remember that image distance and object distance for a plane mirror are equal in magnitude. For dispersion, always write the colour sequence as VIBGYOR, not ROYGBIV, because that is the standard order from the prism. Avoid saying 'mirror image is real' for a plane mirror — it is always virtual. These small precision points boost marks significantly in CBSE exams.
  • Always measure angles from the normal, not the mirror surface
  • Real images: formed by actual convergence; can be projected on screen
  • Virtual images: formed by apparent divergence; cannot be projected
  • Use ruler and pencil for ray diagrams; mark arrowheads on rays
  • Plane mirror image distance = object distance (both measured perpendicularly from mirror)
  • Write dispersion colour order as VIBGYOR (Violet to Red)
  • Do not confuse reflection (mirrors) with refraction (lenses) — both are in this chapter but behave differently

Memory Tricks and Mnemonics

Mnemonics make revision faster and more fun, especially when preparing for CBSE Class 7 Science exams under time pressure. For the seven colours of the spectrum, use VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Some students prefer 'Richard Of York Gave Battle In Vain' in reverse. To remember that concave mirrors curve inward, think 'cave-in' — like a cave going inward. For convex, think 'bulging outward' like a dome. To recall the two laws of reflection, use 'Equal Angels in Same Plane' — angle of incidence equals angle of reflection, and all three (incident ray, reflected ray, normal) lie in the same plane. For mirror image properties in a plane mirror, remember 'VEIL': Virtual, Erect, Inverted Laterally, same size. These tricks reduce recall time during exams and help avoid mixing up concave and convex properties. Write them on a flashcard and revise daily for best results.
  • VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red (spectrum colour sequence)
  • Concave = Cave-in (curves inward); Convex = Bulges out
  • Equal Angels in Same Plane: ∠i = ∠r, and incident ray, reflected ray, normal are coplanar
  • VEIL for plane mirror: Virtual, Erect, Inverted Laterally, same size
  • Real images can be 'Reached' on a screen; Virtual images are 'Vanishing' — cannot be caught

Three Solved Mini-Examples

Applying formulas and concepts to numerical or descriptive problems cements understanding. These three mini-examples mirror the style of questions in NCERT Class 7 Science solutions and CBSE question papers. Example 1 tests the laws of reflection with a numerical angle. Example 2 checks understanding of plane mirror image properties. Example 3 applies the concept of dispersion and rainbow formation. Work through each step carefully, and try similar problems from your textbook exercise or worksheets. If you get stuck on any concept — whether it is drawing ray diagrams, understanding why convex mirrors always give virtual images, or predicting the behaviour of a concave mirror — CBSETUTOR.ai is available 24×7. You can upload a photo of your diagram or question and get step-by-step explanations instantly. The platform covers every CBSE class from 6 to 12 at a flat ₹999 per month, with a 3-day free trial so you can try it risk-free before your next test.

One-Glance Last-Minute Revision Box

Use this box the night before your exam or during breakfast on exam day. It captures every formula, law, and key fact from CBSE Class 7 Science Chapter 11 Light in bullet form. Read it aloud twice, visualise a ray diagram for each point, and you will walk into the exam hall confident. Laws of Reflection: ∠i = ∠r; incident ray, reflected ray, and normal are coplanar. Plane Mirror Image: Virtual, erect, same size, laterally inverted; image distance = object distance. Concave Mirror: Curves inward, converges light, can form real or virtual images; used in torches and dentist mirrors. Convex Mirror: Curves outward, diverges light, always virtual erect diminished images; used in rear-view mirrors. Lenses: Convex lens converges light; concave lens diverges light. Dispersion: White light splits into VIBGYOR through a prism. Rainbow: Natural dispersion by water droplets; sun must be behind observer. This box is your safety net — if you remember nothing else, remember these points and you will score well.
  • Laws of Reflection: ∠i = ∠r; rays and normal coplanar
  • Plane Mirror: Virtual, erect, same size, laterally inverted; d(image) = d(object)
  • Concave Mirror: Cave-in, converges, real/virtual images, torches
  • Convex Mirror: Bulges out, diverges, always virtual diminished, rear-view mirrors
  • Convex Lens: Converges light; Concave Lens: Diverges light
  • Dispersion: White → VIBGYOR via prism
  • Rainbow: Dispersion by water droplets, sun behind observer

Frequently asked questions

What are the two laws of reflection in Class 7 Science Chapter 11?+
The first law states that the angle of incidence equals the angle of reflection (∠i = ∠r). The second law states that the incident ray, reflected ray, and normal all lie in the same plane. These laws apply to all mirrors — plane, concave, and convex.
Why is a plane mirror image called virtual?+
A virtual image is one that cannot be caught on a screen because light rays do not actually meet at the image location; they only appear to diverge from behind the mirror. Since no real convergence occurs, the image is virtual.
How do I remember the difference between concave and convex mirrors?+
Use the mnemonic 'cave-in' for concave — it curves inward like a cave. Convex mirrors bulge outward. Concave mirrors converge light; convex mirrors diverge it. Concave can form real or virtual images; convex always forms virtual, diminished images.
What is lateral inversion, and where do we see it?+
Lateral inversion means the left side of an object appears as the right side in the mirror image and vice versa. You see it every day when you raise your right hand and your mirror image raises its left hand. Ambulance vehicles have laterally inverted text so drivers see it correctly in their rear-view mirrors.
What is dispersion of light?+
Dispersion is the splitting of white light into its seven constituent colours (VIBGYOR) when it passes through a prism or water droplets. Each colour bends by a different amount. Violet bends the most; red bends the least. Rainbows are natural examples of dispersion.
Why is VIBGYOR the correct order for the spectrum?+
VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red) is the sequence in which colours emerge from a prism, with violet bending the most and red the least. NCERT Class 7 Science and CBSE exams follow this standard order, so always write it this way.
Can a concave mirror form both real and virtual images?+
Yes. When the object is placed beyond the focus of a concave mirror, the image is real, inverted, and can be projected on a screen. When the object is between the focus and the mirror, the image is virtual, erect, and magnified — like in a shaving mirror.
What is the difference between reflection and refraction in this chapter?+
Reflection is the bouncing back of light from a surface (mirrors). Refraction is the bending of light as it passes from one transparent medium to another (lenses, prisms). Chapter 11 covers both: mirrors demonstrate reflection, lenses and prisms demonstrate refraction.
Where are convex mirrors used in real life?+
Convex mirrors are used as rear-view mirrors in cars, scooters, and buses because they provide a wide field of view. They are also used in shops and hospitals as security mirrors to monitor large areas. The images are always virtual, erect, and smaller than the object.
How does CBSETUTOR.ai help with Class 7 Science Chapter 11 Light?+
CBSETUTOR.ai is an AI tutor available 24×7 for all CBSE classes 6-12 at a flat ₹999/month. You can upload photos of ray diagrams, numerical problems, or any doubt from Chapter 11 and get instant step-by-step solutions. A 3-day free trial lets you test it before committing — perfect for last-minute doubts before exams.

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