This MCQ module is based on: Pinhole Camera and Light Travels in a Straight Line
TOPIC 40 OF 46
Pinhole Camera and Light Travels in a Straight Line
🎓 Class 7
Science
CBSE
Theory
Ch 11 — Light: Shadows and Reflections
⏱ ~14 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_7" science_domain="physics" difficulty="basic"]
A Magical Window on the Classroom Wall
One summer afternoon, Priya's science teacher closed all the curtains of their classroom until it was almost dark. Then he opened one small round hole — about the size of a ten-rupee coin — in the thick curtain. Slowly, a faint upside-down picture of the busy street outside began to appear on the opposite white wall. Priya gasped. A bus driving past on the road was seen upside down on the wall! How was this possible without any mirror, lens or camera?
Think first: The outdoor scene has trees, cars and people — each sending out light that fills the whole air. Yet when this light is forced through a tiny hole, something extraordinary happens on the other side. Can one simple hole really act like a camera? And if so, why is the picture upside-down?
11.8 The Pinhole Camera
The simplest "camera" ever invented was just a dark box with a small hole. We call it a pinhole camera. No glass, no electricity, no lens — just a hole and a screen.
Construction
A pinhole camera has two parts:
- An outer box painted black inside, with a pinhole (tiny hole) on one face.
- An inner sliding box with a translucent screen (tracing paper or butter paper) on the face opposite the pinhole.
The inner box can slide in and out of the outer box so that the distance between the pinhole and the screen can be changed.
How Does It Work?
Because light travels in straight lines, every point of the object sends its rays outward in all directions. But only those rays that pass through the tiny pinhole can reach the screen. A ray from the top of the object must travel down-and-through the hole and hit the bottom of the screen. A ray from the bottom must travel up-and-through and hit the top. Similarly, rays from the left reach the right side and vice-versa. The result is a small, inverted (upside-down and laterally flipped) image.
Key properties of a pinhole-camera image:
- Inverted (top and bottom swapped)
- Laterally flipped (left-right swapped)
- Smaller than the object (usually)
- In the same colours as the object
- Sharper with a smaller hole; brighter with a larger hole
Effect of Changing Distance
If the inner box is pulled out (screen moved farther from the pinhole), the image becomes larger but dimmer. If the inner box is pushed in (screen moved closer to the pinhole), the image becomes smaller but brighter. This is because the rays form a cone on both sides of the pinhole.
Activity 11.2 — Build Your Own Pinhole Camera L6 Create
You need: two rectangular cardboard boxes (one slightly smaller so it slides inside the other), black paint, tracing paper/butter paper, a sharp pin, glue and scissors.
- Paint the inside of both boxes black (this stops stray reflections).
- Cut a clean square window at one end of the smaller box and paste tracing paper over it — this is the screen.
- Cut off the opposite end of the smaller box completely so light can enter from that side.
- On one face of the bigger box, push a pin through the centre to make a tiny round hole.
- Slide the smaller box into the bigger one with the tracing-paper screen facing inward and the open end facing the pinhole.
- Cover your head and the open end with a dark cloth. In bright sunlight, point the pinhole at a tall sunlit tree or bright wall — the image will appear on the tracing paper!
Predict: If you slide the inner box out, will the image grow larger or smaller? Will it become brighter or fainter?
As the inner box is pulled out, the cone of light inside has more distance to spread, so the image becomes larger. However, the same amount of light now covers a bigger area, so each point is dimmer — the image gets fainter. So size and brightness pull in opposite directions with a pinhole camera.
11.9 Why Does Light Travel in Straight Lines?
Several everyday observations prove that light moves in straight lines:
- Laser pointer: the spot on a distant wall is tiny and stable.
- Sunbeam through a dusty room: the beam looks like a straight pencil of light.
- Three cardboard screens experiment: if holes in three cards line up, a candle flame is visible through all three. If one card is nudged sideways by even a centimetre, the flame disappears.
- Pinhole camera inverted image: only possible because rays cross exactly at the pinhole.
Key idea: In a uniform medium (like still air), light always travels along the shortest path between two points — and the shortest path is a straight line.
11.10 Shadows in the Sky — Eclipses
Eclipses are simply shadows on a cosmic scale. The Sun, Moon and Earth all travel in space, and occasionally three of them come in a straight line. When that happens, one body's shadow falls on another.
Solar Eclipse
When the Moon comes between the Sun and the Earth (during a new-moon day) and the three line up exactly, the Moon casts its shadow on the Earth. Observers inside the shadow see the Sun blocked by the Moon — this is a solar eclipse.
Lunar Eclipse
When the Earth comes between the Sun and the Moon (during a full-moon night) and the three line up exactly, the Earth's large shadow falls on the Moon. We see the Moon go dark or deep red — this is a lunar eclipse.
Safety warning: Never look at a solar eclipse with the naked eye or through sunglasses or X-ray films. The concentrated sunlight can damage the eye permanently. Use certified solar filters or watch the eclipse on a pinhole-camera projection.
Competency-Based Questions
Priya builds a pinhole camera using two shoeboxes. She points it at a lit candle kept 2 metres away. She slides the inner box in and out while watching the image on the tracing-paper screen.
1. The image formed on the screen of a pinhole camera is: L1
(b) Inverted and may be smaller — top and bottom as well as left and right are swapped. The colours remain the same as the object.
2. Why is the image formed by a pinhole camera inverted? L2
Light travels in straight lines. A ray from the top of the candle passes down through the pinhole and lands on the lower part of the screen; a ray from the base of the candle travels up through the pinhole and lands on the upper part. So the top of the object becomes the bottom of the image.
3. Fill in the blank: When the Moon comes between the Sun and the Earth, a ______ eclipse is seen. L1
solar eclipse — the Moon's shadow falls on the Earth.
4. Priya pulls the inner box further out. How will the image of the candle change? L3
The image will become larger but dimmer. The rays coming through the pinhole continue to spread, so a greater screen distance gives a bigger picture, but the same amount of light is now spread over a bigger area, reducing brightness.
5. True or False: A pinhole camera needs a glass lens to work. L1
False. A pinhole camera has no lens at all. The tiny hole itself selects a narrow pencil of rays from each point of the object — that is why only a hole is enough.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R does not explain A. (C) A true, R false. (D) A false, R true.
A: A pinhole camera produces an inverted image.
R: Light travels in a straight line, so rays from the top of the object cross through the pinhole and reach the bottom of the screen.
(A) — both statements are true and R is the correct reason for the image being inverted.
A: A solar eclipse is visible only on a new-moon day.
R: Only on a new-moon day can the Moon come directly between the Sun and the Earth.
(A) — both are true and R explains A. On a new-moon day, the Moon is on the Sun's side of the Earth, so alignment is possible.
A: Light can bend on its own to reach around a corner.
R: Light travels in a straight line in a single uniform medium.
(D) — A is false (light cannot curve on its own); R is true. This is why we need mirrors or other tricks (such as a periscope) to look around corners.
Frequently Asked Questions — Pinhole Camera and Light Travels in a Straight Line
What does the topic 'Pinhole Camera and Light Travels in a Straight Line' cover in Class 7 Science?
The topic 'Pinhole Camera and Light Travels in a Straight Line' is part of NCERT Class 7 Science Chapter 11 — Light: Shadows and Reflections. It covers the key ideas of pinhole camera, rectilinear propagation, straight line, inverted image, rays of light, explained through everyday examples, labelled diagrams and hands-on activities drawn from the NCERT Curiosity textbook. Students learn not just definitions but also the reasoning behind each concept so they can answer competency-based questions and assertion–reason items. The lesson helps Class 7 students build a strong base for higher classes by linking each idea to real observations at home, school and in nature, and by preparing them for CBSE school assessments and Olympiads.
Why is 'Pinhole Camera and Light Travels in a Straight Line' important for Class 7 NCERT Science?
'Pinhole Camera and Light Travels in a Straight Line' is important because it builds core scientific thinking that Class 7 students will use throughout middle and secondary school. NCERT Chapter 11 — Light: Shadows and Reflections — introduces pinhole camera and related ideas that appear again in Class 8, 9 and 10 Science. Mastering this subtopic helps students read labels and safety signs, understand news about science and technology, and perform better in CBSE school exams. The chapter also encourages curiosity and evidence-based thinking — skills that support the National Education Policy (NEP) 2020 focus on conceptual understanding and competency-based learning.
What are the key concepts students should remember from Pinhole Camera and Light Travels in a Straight Line?
The key concepts in 'Pinhole Camera and Light Travels in a Straight Line' for Class 7 Science are: pinhole camera, rectilinear propagation, straight line, inverted image, rays of light. Students should be able to define each term in their own words, give at least one everyday example, and explain how the concept connects to other chapters in NCERT Class 7 Science. For example, linking the idea to daily life — in the kitchen, classroom or outdoors — makes revision easier. Writing short notes, drawing labelled diagrams and solving the NCERT in-text and exercise questions for Chapter 11 will help students retain these concepts for unit tests and the annual CBSE examination.
How is Pinhole Camera and Light Travels in a Straight Line taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Pinhole Camera and Light Travels in a Straight Line' using an inquiry-based approach with Predict–Observe–Explain activities. Students are asked to make a guess first, then perform a simple experiment with safe, easily available materials, and finally explain what they observed. This matches the NEP 2020 focus on learning by doing. For Chapter 11 — Light: Shadows and Reflections — the textbook includes hands-on tasks, labelled diagrams and questions that build Bloom's Taxonomy skills from Remember (L1) to Create (L6). Teachers use these activities, along with competency-based questions (CBQs) and assertion–reason items, to check real understanding rather than rote memorisation.
What real-life examples of pinhole camera can Class 7 students observe at home?
Class 7 students can observe pinhole camera at home in many simple ways linked to 'Pinhole Camera and Light Travels in a Straight Line'. Kitchens, school bags, playgrounds and the night sky are full of examples that connect to NCERT Chapter 11 — Light: Shadows and Reflections. For instance, students can check labels on food and cleaning products, watch changes while cooking, or observe the Sun and Moon across a week. Keeping a small science diary — noting the date, what was observed and a quick sketch — turns everyday life into a science lab. These real-life connections make concepts stick and prepare students well for competency-based questions in CBSE Class 7 Science.
How does 'Pinhole Camera and Light Travels in a Straight Line' connect to other chapters of Class 7 Science?
'Pinhole Camera and Light Travels in a Straight Line' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of pinhole camera appear again when students study related topics like heat, light, changes, life processes and Earth-Sun-Moon. For example, understanding this subtopic helps in building mental models for later chapters and for Class 8, 9 and 10 Science. Teachers often use cross-chapter questions in CBSE examinations to test whether students can apply what they learned in Chapter 11 — Light: Shadows and Reflections — to new situations. This integrated approach matches the NEP 2020 and NCF 2023 focus on holistic, competency-based learning.
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