Rectilinear Propagation of Light

• Light travels in straight lines — called rectilinear propagation.
• How shadows form when an opaque object blocks light.
• To classify materials as transparent, translucent, or opaque (NCERT Activity 11.1).
• To predict shadow size when the light source or object moves.

Level 1 — Straight-line light and shadows

Verbal: A light source emits light. Light moves outward in straight rays until something blocks it.

Visual (torch → object → screen): Torch sends straight rays → opaque ball blocks them → dark shadow on screen.

Material types:

Shadow: Dark region behind opaque object where light cannot reach — not a substance, but absence of light.

Level 2 — Umbra, penumbra, and activities

Point source: Small torch far away → sharp shadow (umbra).

Extended source: Wide bulb → darker centre (umbra) + lighter edge (penumbra).

Size changes: Move object closer to source → larger shadow on screen; closer to screen → smaller shadow.

NCERT activity: Matchstick between torch and paper — observe straight rays and changing shadow as torch moves.

Predict what happens when you hold a book between a bulb and a wall.

Step 1 — Book is opaque → blocks straight-line rays from bulb
Step 2 — Region on wall behind book receives no direct light → shadow
Step 3 — Move book toward bulb → shadow on wall grows larger
Step 4 — Move book toward wall → shadow shrinks

• State one example each of transparent, translucent, and opaque materials.
• Why does a shadow form behind an opaque object?
• What happens to shadow size if the object moves closer to the light source?
• Draw a simple diagram with straight rays from a torch blocked by a pencil.

Open the Practice tab for graded questions on Rectilinear Propagation of Light.