Sources of Light & Rectilinear Propagation

📋 Teacher notes

• •Sources of light are LUMINOUS (produce their own light — the Sun, a candle, a bulb) or NON-LUMINOUS (only visible because they reflect light from elsewhere — the Moon, this page, a person).
• •A shadow forms because light travels in straight lines (rectilinear propagation) and cannot bend around an opaque object.
• •A POINT source produces a shadow with a sharp edge — only an umbra, no penumbra — because every ray from the single point is blocked in exactly the same way.
• •An EXTENDED source produces a shadow with two regions: the UMBRA (completely dark — no part of the source is visible from there) and the PENUMBRA (partially lit — only part of the source is visible from there, so some light still reaches it).
• •Moving the object closer to an extended source makes the penumbra relatively LARGER compared to the umbra; moving it closer to the screen makes the shadow’s edges sharper.