ICSE 7 Physics Light Short Notes

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Table of Contents

    ⚡ Fast Revision: Reflection of Light

    The Phenomenon of Reflection
    • Definition: The bouncing back of light rays into the same medium after striking a polished surface.
    • Regular Reflection: Occurs from smooth surfaces (mirrors); parallel rays stay parallel.
    • Diffused Reflection: Occurs from rough surfaces; rays scatter in different directions.
    Angle Alert

    Angle of Incidence ($i$): Angle between Incident Ray and Normal.

    Angle of Reflection ($r$): Angle between Reflected Ray and Normal.

    Laws of Reflection:

    $\angle i = \angle r$

    Incident ray, Reflected ray, and Normal all lie in the same plane.

    ❌ Common Error:

    Measuring the angle from the mirror surface instead of the Normal. Fix: Angles $i$ and $r$ are always measured from the perpendicular Normal line.

    Key Terms in Reflection

    ⚡ Fast Revision: Plane Mirrors & Image Formation

    Characteristics of Image in Plane Mirror
    • Virtual & Erect: The image cannot be obtained on a screen and is right-side up.
    • Same Size: The size of the image is exactly equal to the size of the object.
    • Distance Rule: Object distance ($u$) from the mirror = Image distance ($v$) behind the mirror.
    The "Side-Swap" Phenomenon

    Lateral Inversion: The left side of the object appears as the right side of the image.

    Example: The word "AMBULANCE" is written laterally inverted on the front of the vehicle.

    Feature Real Image Virtual Image
    Screen Can be formed on a screen. Cannot be formed on a screen.
    Nature Always Inverted. Always Erect.
    Light Rays Rays actually meet. Rays appear to meet.
    ❌ Common Error:

    Thinking the image is smaller if you move far from the mirror. Fix: The image size remains constant; it only appears smaller because of the perspective angle.

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    Lateral Inversion in a Plane Mirror

    ⚡ Fast Revision: Spherical Mirrors - Concave & Convex

    Types of Spherical Mirrors
    • Concave (Converging): Reflecting surface is curved inwards. It can form both real and virtual images.
    • Convex (Diverging): Reflecting surface is curved outwards. It always forms virtual, erect, and diminished images.
    • Principal Focus ($F$): The point where parallel rays meet (Concave) or appear to diverge from (Convex).
    The Focal Length Relationship:

    $R = 2f$

    Radius of Curvature (R) is twice the Focal Length (f).

    Mirror Type Nature of Image Primary Use
    Concave Magnified (when close) Shaving mirror, Dentist's mirror
    Convex Diminished (Wide view) Rear-view mirror in vehicles
    ❌ Common Error:

    Assuming convex mirrors can form real images. Fix: Convex mirrors always form virtual images, regardless of the object's position.

    Concave vs Convex Ray Convergence

    ⚡ Fast Revision: Color & the Spectrum

    Dispersion of Light
    • Definition: The splitting of white light into its seven constituent colors when passed through a glass prism.
    • Spectrum: The band of seven colors formed—VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
    • Cause: Different colors of light travel at different speeds in glass, causing them to bend by different angles.
    Deviation Alert

    Red Light: Bends the least (Maximum speed in glass).

    Violet Light: Bends the most (Minimum speed in glass).

    Color Category Colors Included
    Primary Colors Red, Green, Blue (RGB)
    Secondary Colors Cyan (G+B), Magenta (R+B), Yellow (R+G)
    Newton's Disc Combines all 7 colors to appear White when rotated.
    ❌ Common Error:

    Thinking a red rose absorbs red light. Fix: An object appears a certain color because it reflects that color and absorbs all other colors of the spectrum.

    Dispersion and Spectrum Formation
    End of Light Energy Revision
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