This MCQ module is based on: Reflection, Plane Mirrors and the Periscope
TOPIC 41 OF 46
Reflection, Plane Mirrors and the Periscope
🎓 Class 7
Science
CBSE
Theory
Ch 11 — Light: Shadows and Reflections
⏱ ~14 min
🌐 Language: [gtranslate]
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The Mirror That Writes Backwards
One morning, Kabir stood brushing his teeth and looked at his T-shirt in the bathroom mirror. His shirt had the word "AMBULANCE" printed on it for a school drama — and in the mirror it read backwards! Yet the mirror did not flip his face upside-down; just left and right seemed swapped. Curious, Kabir wondered why the top of his head still appeared at the top of the mirror, but the word on his chest was reversed. Something special was happening with the way mirrors form images.
Think first: When you raise your right hand in front of a mirror, the image raises its left hand. When you smile, the image smiles back at the same moment. But the image is not a solid thing — you cannot touch it or catch it on a screen. So what exactly is a mirror image?
11.11 Reflection of Light
When light travelling in a straight line meets a smooth, shiny surface, it bounces back into the same medium. This bouncing is called reflection. A mirror is an object specifically designed to reflect light cleanly — usually a sheet of glass with a thin layer of silver or aluminium on the back.
Rough surfaces like paper and wood also reflect light, but the bouncing rays go in many directions at once (scattered reflection). That is why we can see a wall from any angle but it does not form a clear image.
11.12 Laws of Reflection
Reflection follows two simple rules. Before stating them we name the parts of the setup:
- Incident ray — the ray going towards the mirror.
- Reflected ray — the ray going away from the mirror after bouncing.
- Normal — an imaginary line drawn perpendicular (at 90°) to the mirror at the point where the ray strikes.
- Angle of incidence (i) — angle between the incident ray and the normal.
- Angle of reflection (r) — angle between the reflected ray and the normal.
Laws of reflection:
- The angle of incidence is equal to the angle of reflection \((i = r)\).
- The incident ray, the normal and the reflected ray all lie in the same plane.
11.13 Image in a Plane Mirror
A flat, ordinary mirror is called a plane mirror — the kind on your bathroom wall or on a dressing table. Light from every point of the object reflects off the mirror according to the laws of reflection and reaches the eye. The brain traces the rays back in a straight line as if they were coming from a point behind the mirror. That apparent point is the image.
Properties of the Image in a Plane Mirror
| Property | Description |
|---|---|
| Virtual | Image cannot be caught on a screen — it only seems to be behind the mirror |
| Erect | The image is the same way up as the object (not upside-down) |
| Same size | Image is exactly as tall and wide as the object |
| Equal distance | Image appears as far behind the mirror as the object stands in front |
| Laterally inverted | Left of the object becomes right of the image and vice-versa |
Lateral Inversion — Why Letters Look Mirror-Flipped
Lateral inversion is the reason the word AMBULANCE on Kabir's shirt looked flipped. A real ambulance van has the word painted as \(\text{ƎƆИAᴸUᙠMA}\) on its bonnet — so that the driver in front sees it the right way round through his rear-view mirror.
Activity 11.3 — Verify the Laws of Reflection L4 Analyse
You need: a plane mirror, a torch/ray-box (or laser pointer), white chart paper, a protractor, a pencil and a ruler.
- Fix the chart paper on a table. Stand the plane mirror vertically along a drawn line MM′.
- Mark a point O on MM′ and draw the normal (a dashed line perpendicular to MM′ at O).
- Shine a narrow beam of light from the torch towards O at an angle. Mark two points on the incident ray and two on the reflected ray.
- Remove the mirror. Join the incident and reflected ray points through O using a ruler.
- Measure the angle of incidence (i) and angle of reflection (r) with the protractor.
- Repeat with three different angles of incidence — 20°, 40°, 60°.
Predict: Do you think the angle of reflection will always be equal to the angle of incidence, or will it depend on how bright the torch is?
For every incident angle you try, the measured angle of reflection will come out equal to the incident angle — within a small error of ±1° due to ruler thickness and the width of the torch beam. Brightness does not change the angles; it only changes how much light is reflected. This experiment confirms the first law of reflection: i = r.
11.14 The Periscope — Seeing Around Walls
Because light travels in straight lines, you cannot peek over a high wall directly. But by cleverly arranging two mirrors, you can bend the path of the light twice and view things that are hidden from sight. A device that does this is called a periscope.
How a Periscope Works
Inside a long vertical tube, two plane mirrors are fixed parallel to each other, each tilted at 45° to the length of the tube. Light from the distant object enters the top mirror, bounces down through the tube (obeying \(i = r\), so a 45° incidence gives a 90° turn), hits the lower mirror, bounces again and leaves horizontally into the observer's eye. The observer now sees the object as if his eye were at the top.
11.15 The Kaleidoscope — A World of Colours
A kaleidoscope is a tube containing three long strips of mirror placed as a triangular prism, with loose coloured bangle-pieces at one end and a peep-hole at the other. Light entering the tube reflects repeatedly between the three mirrors and you see a beautiful symmetric pattern that keeps changing every time you rotate the tube.
11.16 Useful Mirrors around Us
| Mirror / Device | Where used | Why useful |
|---|---|---|
| Plane mirror | Dressing table, bathroom | Same-size clear image to check how you look |
| Rear-view (convex) mirror | Cars, trucks, two-wheelers | Wide field of view behind the vehicle |
| Dental / shaving (concave) mirror | Dentist's chair, shaving mirror | Enlarges close-by objects for clearer viewing |
| Concave reflector | Torch, car headlight, solar cooker | Focuses a weak source of light or heat into a strong beam/spot |
| Periscope | Submarines, army trenches | See over obstacles without exposing oneself |
11.17 The Rainbow — Nature's Colour Show
After a light shower when the Sun is low, you may spot a rainbow arching across the sky. Ordinary sunlight looks white, but it is really a mix of seven colours. Each tiny raindrop acts like a tiny prism: sunlight enters the drop, gets bent (refracted), bounces once off the inner back surface, and comes out bent again — and the different colours bend by different amounts, so they separate into a band.
The order of colours in a rainbow (from outside to inside) is: Red, Orange, Yellow, Green, Blue, Indigo, Violet — remembered by the handy word VIBGYOR (read from violet upward).
Competency-Based Questions
Kabir writes the word "BOOK" on a sheet of paper in large capital letters and holds it up in front of a plane mirror. He observes its image carefully, especially the shape of each letter.
1. In reflection from a plane mirror, the angle of incidence is 35°. What is the angle of reflection? L2
(b) 35° — by the first law of reflection, \(i = r\).
2. Give two properties of the image formed by a plane mirror. L1
Any two of: (i) virtual — cannot be caught on a screen; (ii) erect — the same way up as the object; (iii) same size as the object; (iv) appears at the same distance behind the mirror as the object is in front; (v) laterally inverted (left–right swapped).
3. Fill in the blank: In a periscope, the two plane mirrors are placed at an angle of ______ to the long tube. L1
45° — this turns the light through a full 90° at each mirror.
4. Kabir notices that the mirror image of "BOOK" looks identical to the original word, but the image of "LION" looks strange. Explain. L4
The letters B, O, O, K in "BOOK" — especially the two O's — have perfect left-right symmetry, so lateral inversion does not change their shape. The letters L, I, N in "LION" are not symmetric left-to-right, so the mirror image looks reversed. This is a direct consequence of lateral inversion in a plane mirror.
5. True or False: The image formed in a plane mirror can be caught on a cinema screen. L1
False. The image in a plane mirror is virtual. The rays only appear to come from behind the mirror; they do not actually meet there, so no screen can catch the image.
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: The word AMBULANCE is written in mirror form on the front of an ambulance.
R: A plane mirror produces a laterally inverted image.
(A) — both true and R explains A. The reversed writing appears correct to drivers in front when they look in their rear-view mirror.
A: A periscope works only in bright sunlight.
R: A periscope uses two plane mirrors inclined at 45° to bend the path of light twice.
(D) — A is false (a periscope works with any light, even dim daylight or artificial light). R is true — that is indeed how it works.
A: A rainbow shows seven colours.
R: White sunlight is actually a mixture of seven colours which separate when light passes through raindrops.
(A) — both are true and R explains A. Each raindrop bends the different colours by slightly different amounts, spreading white light into the VIBGYOR band.
Frequently Asked Questions — Reflection, Plane Mirrors and the Periscope
What does the topic 'Reflection, Plane Mirrors and the Periscope' cover in Class 7 Science?
The topic 'Reflection, Plane Mirrors and the Periscope' is part of NCERT Class 7 Science Chapter 11 — Light: Shadows and Reflections. It covers the key ideas of reflection, plane mirror, laws of reflection, lateral inversion, periscope, image, 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 'Reflection, Plane Mirrors and the Periscope' important for Class 7 NCERT Science?
'Reflection, Plane Mirrors and the Periscope' 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 reflection 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 Reflection, Plane Mirrors and the Periscope?
The key concepts in 'Reflection, Plane Mirrors and the Periscope' for Class 7 Science are: reflection, plane mirror, laws of reflection, lateral inversion, periscope, image. 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 Reflection, Plane Mirrors and the Periscope taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Reflection, Plane Mirrors and the Periscope' 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 reflection can Class 7 students observe at home?
Class 7 students can observe reflection at home in many simple ways linked to 'Reflection, Plane Mirrors and the Periscope'. 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 'Reflection, Plane Mirrors and the Periscope' connect to other chapters of Class 7 Science?
'Reflection, Plane Mirrors and the Periscope' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of reflection 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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