Class 7 | Science
Chapter 11 | Light: Shadows and Reflections
Aligned with CBSE | NCERT | School & Olympiad Standards
Focus on What Matters Most
The67Sprint
Nature & Sources of Light
- Light is a form of energy that travels in a straight line (rectilinear propagation) — proven by viewing a candle flame through a straight vs. bent pipe.
- Luminous objects generate their own light (Sun, stars, fireflies, lightning, LEDs). Non-luminous objects only reflect light (Moon, Mars, Venus, mirrors).
Materials & Light
- Transparent: lets light pass fully (glass, water).
- Translucent: lets some light pass (tracing paper, wax paper).
- Opaque: blocks light completely (wood, cardboard).
Shadows
- A shadow needs exactly three things: a light source, an opaque object, and a screen.
- Shadows are always dark — colour of the object never matters.
Optical Devices
- Pinhole camera: lens-free device; forms an inverted (upside-down) real image because light rays cross at the tiny hole. Tracing paper gives a sharper screen image than butter paper.
- Plane mirror: forms an image that is erect, same size, and laterally inverted (left-right swapped).
- Periscope: two plane mirrors at 45° let us see over or around obstacles.
- Kaleidoscope: multiple mirror reflections create repeating geometric patterns.
Rectilinear Propagation
Lateral Inversion
Quick Recall — Sprint Facts
Light's Path
Always straight, never curved
Shadow Colour
Always dark, regardless of object
Moon's Status
Non-luminous; reflects sunlight
Pinhole Image
Inverted, real, caught on screen
Mirror Image
Erect & laterally inverted
Periscope Mirrors
Two, set at 45°
Key Terms to Know
🔑 Rectilinear Propagation
Light always travels in straight lines.
🔑 Luminous
Objects that generate their own light.
🔑 Opaque
Blocks all light; forms sharp shadows.
🔑 Lateral Inversion
Left-right swap seen in plane mirror images.
🔑 Reflection
Light bouncing back off a shiny surface.
Olympiad & Challenger Edge
Shadow Scaling
Moving the light source closer makes the shadow larger and blurrier; moving it farther makes it smaller and sharper. A high-flying bird's shadow fades before reaching the ground.
Real vs. Virtual Images
A real image (pinhole camera) can be caught on a screen. A virtual image (plane mirror) cannot — it only appears to form behind the surface.
Persistence of Vision
The retina holds an image for about 1/16th of a second — this is why fast-flashing images blend into smooth motion in movies.
Symmetrical Letters
Letters like T and O look identical in a mirror because their left and right halves are already the same — lateral inversion has no visible effect.
Moon is Luminous
Wrong — the Moon is non-luminous; it only reflects sunlight.
Coloured Shadows
Wrong — shadows are always dark/black, never the colour of the object.
Pinhole Cameras Use Lenses
Wrong — pinhole cameras use only a tiny hole, no lens at all.
Tree-Leaf Pinholes
Gaps between leaves cast circular sun images on the ground on sunny days.
Ambulance Lettering
"AMBULANCE" is written reversed so drivers read it correctly via their rear-view mirror.
Periscopes in Submarines
Soldiers and sailors use periscopes to see above their eye level safely.
Missing Arrowheads
Ray diagrams without arrowheads (and drawn without a ruler) lose marks.
Incomplete Shadow Answer
Naming only light source + object loses marks — the screen is mandatory too.
Pinhole Screen Material
Assuming any paper works the same — tracing paper gives a sharper image than butter paper.
Logic Chain
Step 1
A luminous source emits light.
Step 2
Light travels in a perfectly straight line.
Step 3
An opaque object blocks the light rays in its path.
Step 4
A dark shadow forms on the screen behind the object.
🦉
Mentor Byte
Don't just memorise optics — build it. Blacken the inside of a paper-cup pinhole camera and use tracing paper for the sharpest inverted image. A photon only knows one rule: travel straight until you hit something. Every idea in this chapter — shadows, reflections, inversions — follows from that single line.