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.

Given: A tree is viewed through a pinhole camera.

Concept: In a pinhole camera, light from the top of the object passes through the pinhole and falls at the bottom of the screen, and light from the bottom of the object falls at the top of the screen. This is because light travels in straight lines through the pinhole.

Working:
- Light from the top of the tree travels in a straight line through the pinhole and hits the lower part of the screen.
- Light from the bottom of the tree travels through the pinhole and hits the upper part of the screen.
- Similarly, left and right are also reversed.

Answer: The image of the tree formed in the pinhole camera is inverted (upside down) and also laterally reversed compared to the actual tree.

Sketch description: Draw the same tree outline but flipped upside down — the roots/trunk base at the top and the leafy crown at the bottom of the image frame. The image is also left-right reversed.

Given: A name written on paper is held in front of a plane mirror.

Concept: A plane mirror causes lateral inversion — the left side of the object appears as the right side in the image, and vice versa.

Working:
Suppose the name written is RAVI.
- When held in front of the mirror, the image appears as IVER (each letter is also mirror-reversed).
- The first letter 'R' (on the left of the paper) appears on the right side in the mirror image.
- Each individual letter also appears flipped horizontally.

Sketch: Write your name normally → In the mirror, the letters appear reversed left-to-right, and each letter itself is a mirror image.

Difference noticed: The name in the mirror appears laterally inverted — it reads from right to left instead of left to right, and each letter appears as its mirror image.

Reason: This happens due to lateral inversion. A plane mirror reverses the image in the direction perpendicular to the mirror surface (i.e., left becomes right and right becomes left). The left side of the object corresponds to the right side of the image and vice versa.

Given: Shadow lengths measured at 9 AM, 12 PM, and 4 PM.

Observations (typical):

| Time | Length of Shadow |
|---|---|
| 9 AM | Long |
| 12 PM | Shortest |
| 4 PM | Long |

(i) At which time is the shadow the shortest?

Answer: The shadow is the shortest at 12 PM (noon).

(ii) Why does this happen?

Concept: The length of a shadow depends on the angle at which sunlight falls on the object.

Explanation:
- At 12 PM, the Sun is almost directly overhead (at the highest point in the sky). Sunlight falls nearly vertically on objects, so the shadow formed is very short.
- At 9 AM and 4 PM, the Sun is at a lower angle in the sky. Sunlight falls at a slant, causing longer shadows.
- The lower the Sun is in the sky (greater angle from vertical), the longer the shadow.

Conclusion: At noon, the Sun is at its highest position, so sunlight is most nearly vertical, producing the shortest shadow.

Correct Option: (i) Both statements are true

Justification:

- Statement A is true: A plane mirror causes lateral inversion — the left side of the object appears as the right side in the image and vice versa. This is a well-established property of plane mirrors.

- Statement B is true: The alphabets T and O are bilaterally symmetric (symmetric about their vertical axis).
- The letter T looks the same when laterally inverted (its left and right halves are mirror images of each other).
- The letter O is symmetric about both vertical and horizontal axes, so its mirror image looks identical to itself.
- Therefore, their images in a plane mirror appear identical to the original letters.

Answer: Option (i) — Both statements are true.

Given: A Z-shaped (or S-shaped/offset) tube and two plane mirrors.

Concept: A periscope works by using two plane mirrors fixed at 45° to the direction of light. Light enters from one end, hits the first mirror at 45° and is reflected at 90°, travels along the tube, hits the second mirror at 45°, and is again reflected at 90° to exit through the other end, allowing the viewer to see over or around obstacles.

Answer: Yes, this tube can be used to make a periscope.

Where to fix the mirrors:
- Fix the first plane mirror at the upper bend of the tube at an angle of 45° to the horizontal (facing downward toward the tube).
- Fix the second plane mirror at the lower bend of the tube at an angle of 45° to the horizontal (facing upward toward the viewer's eye).

Working: Light from the object enters the top opening → strikes Mirror 1 at 45° → reflects downward (or sideways) through the middle section of the tube → strikes Mirror 2 at 45° → reflects toward the eye of the observer at the other opening.

Conclusion: The two plane mirrors, each placed at 45° at the two bends of the tube, allow the tube to function as a periscope.

Given: A bird flying high in the sky does not cast a visible shadow on the ground, but when it flies close to the ground, its shadow is visible.

Concept: The size and sharpness of a shadow depend on the distance between the object and the screen (ground). Also, sunlight comes from a very large, extended source.

Explanation:

1. When the bird flies high: The bird is very far from the ground (screen). The shadow formed is extremely large and spread out over a very large area of the ground. As a result, the shadow becomes very faint and diffused — it is too large and too faint to be noticed.

2. When the bird flies close to the ground: The bird is very close to the ground (screen). The shadow is small, well-defined, and concentrated in a small area directly below the bird. This makes the shadow clearly visible.

Additional reason: The Sun is an extended (large) source of light. When the bird is high up, light from different parts of the Sun reaches the ground from around the bird, filling in the shadow area (penumbra effect), making the shadow even more diffuse and invisible.

Conclusion: The shadow of a bird flying high is too large, faint, and diffused to be seen, whereas when the bird is close to the ground, the shadow is small, sharp, and clearly visible.