Light: Shadows and Reflections

Given: A list of objects — Mars, Moon, Pole Star, Sun, Venus, Mirror.

Concept: Luminous objects are those that emit their own light.

Working:
- Mars — a planet; it only reflects sunlight. Not luminous.
- Moon — a natural satellite; it only reflects sunlight. Not luminous.
- Pole Star — a star; it produces its own light by nuclear fusion. Luminous.
- Sun — a star; it produces its own light. Luminous.
- Venus — a planet; it only reflects sunlight. Not luminous.
- Mirror — reflects light falling on it. Not luminous.

Answer: The luminous objects from the given list are Pole Star and Sun.

Concept: Definitions and properties of each term.

Matching:

| Column A | Column B |
|---|---|
| Pinhole camera | Forms an inverted image |
| Opaque object | Blocks light completely |
| Transparent object | Light passes almost completely through it |
| Shadow | The dark region formed behind the object |

Explanation:
- A pinhole camera uses the straight-line travel of light through a tiny hole to form an inverted image on a screen.
- An opaque object does not allow light to pass through it at all — it blocks light completely.
- A transparent object allows light to pass almost completely through it.
- A shadow is the dark region formed behind an opaque object where light does not reach.

Given: Four students looking through a pipe at a candle flame. (Fig. 11.16 shows pipes in different orientations — straight or bent — for each student.)

Concept: Light travels in a straight line. A person can see through a pipe only if the pipe is straight (not bent), because light cannot travel around a bend.

Working: Only the student whose pipe is perfectly straight (not bent) will have an unobstructed straight-line path from the candle flame to their eye.

Answer: Only the student whose pipe is straight (aligned in a straight line with the candle) can see the flame. Based on the typical representation of such figures in NCERT, Rekha (or whichever student has the straight pipe) can see the flame. *(Note: Since the figure is not fully visible, the answer depends on which student has the straight pipe — the student with the straight, unbent pipe can see the flame.)*

Concept: For a shadow to form correctly:
1. A source of light is needed.
2. An opaque object (the boy) must block the light.
3. The shadow falls on the opposite side of the object from the light source.
4. The shadow is dark, has roughly the shape of the object, and is connected to the base of the object.

Working: The correct image must show:
- The light source on one side.
- The boy (opaque object) in the middle.
- A dark shadow on the ground/screen on the side opposite to the light source.
- The shadow attached to the feet of the boy.

Answer: The correct option is (b) — it accurately shows the shadow falling on the side opposite to the light source, connected to the boy's feet, with the correct shape. *(Note: Since the figures are not fully visible, the correct answer is the option that shows the shadow on the side away from the light source, attached to the base of the boy.)*

Given:
- Scenario (i): Ball is closer to the torch (far from the wall/screen).
- Scenario (ii): Ball is closer to the wall/screen (far from the torch).

Concept: The size of a shadow depends on the position of the object relative to the light source and the screen.
- When the object is closer to the light source (and farther from the screen), the shadow is larger.
- When the object is closer to the screen (and farther from the light source), the shadow is smaller (closer to the actual size of the object).

Working:
- Scenario (i): Ball near torch → larger shadow on the wall.
- Scenario (ii): Ball near wall → smaller shadow on the wall.

Answer: The correct option is (a) — which shows a larger shadow in scenario (i) and a smaller shadow in scenario (ii). *(Note: Based on the standard physics principle described above, the option showing a bigger shadow when the ball is near the torch and a smaller shadow when the ball is near the wall is correct.)*

Concept: Shadow size depends on the distance between the light source and the object. Shadows require an opaque object; without the object, no shadow forms. Two light sources can create two shadows.

Matching:

| Column A | Column B |
|---|---|
| If the torch is close to the ball | The shadow would be larger |
| If the torch is far away | The shadow would be smaller |
| If the ball is removed from the set-up | A bright spot would appear on the screen |
| If two torches are present in the set-up on the left side of the ball | Two shadows would appear on the screen |

Explanation:
- Torch close to ball: Light rays diverge more, making the shadow larger.
- Torch far away: Light rays are more parallel, making the shadow smaller (closer to the actual size).
- Ball removed: Nothing blocks the light, so a bright spot appears on the screen.
- Two torches: Each torch creates its own shadow of the ball, so two shadows appear on the screen.