Revision Notes -Chapter 15- Light
Table of Contents
Definition of Light
- Light is energy that lets us see things.
- It is a type of radiation visible to human eyes.
- Light illuminates objects, making them visible.
How Light Travels
- Light moves in straight lines.
- Experiment- Viewing a candle through straight and bent pipes. The straight pipe shows the flame; the bent one doesn’t, proving light’s straight-line travel.
Also Check – Chapter 11- A Detailed Guide to the Light Activities for Class 7 Students
Interaction of Light with Materials
- Transparent Objects- Let all light through (e.g., glass).
- Translucent Objects- Partially let light through (e.g., butter paper).
- Opaque Objects- Don’t let light through (e.g., a book).
Also Check – What is Light Reflection? A Simple Guide to Understanding Reflections
Basic Principles of Reflection
- Reflection- Light bounces back from surfaces instead of being absorbed.
- Nature of Reflection- Light changes its path when reflected.
Interaction with Surfaces
- Surface Characteristics- Determine how light is reflected.
- Shiny Surfaces- Like mirrors, reflect light well.
- Types of Mirrors- Plane (flat) and Curved (concave or convex).
Absorption vs. Reflection-
- Absorption- Rough/dark surfaces absorb more light.
- Reflection- Shiny/light-coloured surfaces reflect more light.
Also Check – Class 7 Science -Chapter 15 – Light- Complete Notes
Transmission of Light
- Through Transparent Materials- Light passes through with minimal absorption or reflection.
- Role in Optics- Essential for lenses, windows, etc.
Formation of Images
- Real Images- Formed by actual convergence of light rays.
- Virtual Images- Formed by apparent convergence of diverging rays.
Also Check – What is Light? An Easy-to-Understand Guide
Laws of Reflection
- Incident Ray- Approaches and strikes a surface.
- Reflected Ray- Bounces off the surface.
- Normal- Imaginary line perpendicular to the surface at the point of incidence.
- Angles- Angle of incidence = Angle of reflection (∠i = ∠r).
- Same Plane Principle- Incident ray, reflected ray, and normal all lie in the same plane.
- Surface Texture- Affects the quality of the reflected ray.
Also Check – NCERT Solutions for Class 7 Science Chapter 15 -Light
Image Formation
- An image is created when light rays reflect from a mirror, seeming to originate from a point.
- This point could be actual or apparent, based on the image type.
Types of Images
- Real Image- Formed by actual convergence of light rays; can be captured on a screen.
- Virtual Image- Formed by apparent convergence of diverging rays; cannot be captured on a screen.
Plane Mirrors and Image Formation
- Produces a virtual, erect, same-sized image.
- Image appears behind the mirror at the same distance as the object.
- Full image visible only if the mirror’s height is at least half the observer’s height.
Lateral Inversion Concept
- The phenomenon where the right side of the object appears as the left in the image, and vice versa.
- Explains reversed text like “AMBULANCE” for readability in mirrors.
Spherical Mirrors
Concave Mirrors
- Can form real, inverted, magnified images.
- Erect and virtual when the object is close.
- Image nature varies with object distance.
- Used in medical, optical devices, everyday objects, and automotive lighting.
Convex Mirrors
- Always forms virtual, erect, diminished images.
- Offers a wider field of view.
- Used in vehicle mirrors, security, and architectural design.
Dispersion of Light
- White Light Composition- Made of seven colours.
- Dispersion Through a Prism- Splits white light into a spectrum of colours.
- Natural Phenomena – Rainbows- Formed by light dispersion in raindrops, showing a colour arc from red to violet.
Types of Reflection
Diffused (Irregular) Reflection
- Occurs on rough surfaces.
- Reflected rays scatter in various directions.
- Forms unclear, indistinct images.
- Examples- walls, wooden objects, unpolished metals.
Regular Reflection
- Occurs on smooth, shiny surfaces.
- Reflected rays are parallel and uniform.
- Forms clear, well-defined images.
- Examples- mirrors, polished metals, still water.
Differences between Diffused and Regular Reflection
- Image Formation- Regular reflection forms clear images, diffused reflection doesn’t.
- Surface Texture- Regular on smooth surfaces, diffused on rough surfaces.
- Applications- Regular used in mirrors, diffused in everyday objects.
Spherical Mirrors in Daily Life
Concave Mirrors
- Used in satellite dishes, medical and dental uses, personal care (shaving mirrors), and automotive lighting.
Convex Mirrors
- Used in vehicle mirrors for a wider view, and in security and surveillance.
Uses of Lenses in Various Devices
Convex Lenses (Converging Lenses)
- Curved outward, used in magnifying glasses, cameras, microscopes, telescopes, binoculars.
Also Check – Convex Lenses- Principles, Applications, and Insights
Concave Lenses (Diverging Lenses)
- Curved inward, used in peepholes, and corrective eyewear for nearsightedness.
Also Check – Concave Lenses- Applications, Image Formation, and Principles
Additional Concepts
Visibility Range in Plane Mirrors
- To see the full image, the mirror must be at least half the height of the observer.
- Important in designing mirrors for various uses.
Lateral Inversion
- Right side of the object appears as left in the mirror image.
- Essential in understanding image formation in plane mirrors.
- Applied in reversed text on ambulances, and other practical scenarios.
Practical Demonstration of Lateral Inversion
- Experiment with lifting hands in front of a plane mirror to understand lateral inversion.
Also Check – Difference Between a Convex and Concave Lens
Also Check – NCERT Exemplar Solutions- Class 7 Science- Chapter 15- Light
Also Check – Class 7 Science -Chapter 15- Light- Definition and Explanation of Important Keywords
Also Check- Chapter 15- Light Class 7 science- Question and Answer (Fill in the Blanks)
Also Check – Chapter 15- Light Class 7 science- Question and Answer (True or False)
Also Check – Chapter 15- Light Class 7 science- Extra Question and Answers
