Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Light Travels in Straight Lines
Light always travels in straight lines (rectilinear propagation of light).
Evidence:
- Shadows are formed (opaque objects block straight-moving light).
- A pinhole camera forms an inverted image.
Sources of Light
Luminous Objects
Objects that produce their own light.
Examples: Sun, stars, burning candle, electric bulb, tubelight, firefly, glowing ember.
Non-Luminous Objects
Objects that do not produce their own light — they only reflect it.
Examples: Moon, book, table, chairs, planets.
The Moon does not produce its own light — it reflects sunlight.
Transparent, Translucent, and Opaque Objects
| Type | Light Passes Through? | Examples |
|---|---|---|
| Transparent | Fully | Clear glass, water, air, clear plastic |
| Translucent | Partially (blurred image) | Frosted glass, oiled paper, butter paper, tissue paper |
| Opaque | Not at all | Wood, metal, stone, cardboard, brick |
Shadows
A shadow is a dark area formed when an opaque object blocks light.
Conditions for Shadow Formation
1. A source of light. 2. An opaque object to block light. 3. A screen (wall, floor, paper) on which shadow falls.
Properties of Shadows
- Shadows are always on the opposite side from the light source.
- Shadow is always dark — no colour.
- Shadow changes shape and size depending on the position of the light source.
- When the light source is directly overhead → short shadow.
- When the light source is low → long shadow.
Pinhole Camera
A pinhole camera is a simple device that shows how light travels.
How to Make One
- Take a box open at one end.
- Close the open end with tracing paper (screen).
- Make a small hole on the opposite face.
- Look at a bright object through the tracing paper side.
How It Works
- Light from the top of the object enters the hole and falls on the bottom of the screen.
- Light from the bottom of the object enters the hole and falls on the top of the screen.
- So the image is inverted (upside down).
- Image is also a laterally reversed (left-right flipped).
Reason: Light travels in straight lines and crosses at the pinhole.
Reflection of Light
When light hits a shiny or smooth surface and bounces back, it is called reflection.
- A plane mirror reflects light very well.
- The image in a plane mirror is:
- Erect (upright)
- Laterally inverted (left and right are swapped)
- Same size as the object
- Virtual (appears behind the mirror, cannot be caught on screen)
Interesting Observations
- Shiny surfaces act as mirrors — a calm water surface, polished spoon, foil.
- Kaleidoscope uses multiple mirrors to create beautiful patterns.
- A periscope uses two mirrors to see over walls or around corners — used in submarines.
Key Vocabulary
| Term | Meaning |
|---|---|
| Luminous | Produces own light |
| Non-luminous | Reflects light, does not produce it |
| Opaque | Does not allow light to pass |
| Shadow | Dark region formed when light is blocked |
| Reflection | Bouncing of light off a surface |
| Pinhole camera | Device using a tiny hole to form an inverted image |
| Lateral inversion | Left-right flip seen in mirror image |
Exam Tips
- Three conditions for shadow formation — memorise all three.
- Moon is non-luminous — reflects sunlight — very common 1-mark question.
- Pinhole camera image is inverted — because light travels in straight lines.
- Transparent vs translucent vs opaque — give examples for all three.
- Mirror image is laterally inverted (not simply "reversed") — use the correct term.