Light: Shadows and Reflections — NCERT Solutions

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):

(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.