Light: Shadows and Reflections – Long Answer Type Questions
CBSE Class 7 — Science
Chapter 11: Light: Shadows and Reflections — 30 Long Answer Type Questions & Model Answers
Class 7
Science
NCERT
CBSE
Long Answers
Content Bank — Chapter 11 Overview
This set of 30 long-answer questions covers key topics: nature of light, rectilinear propagation, shadows (umbra & penumbra), laws of reflection, plane mirrors, image formation, periscopes and other optical applications, and useful classroom experiments. Answers are concise yet detailed, suitable for CBSE Class 7 descriptive answers (4–8 marks).
30 Long Answer Questions — Topic-wise
Topic A — Fundamentals of Light (Questions 1–6)
1.
Explain what is meant by the rectilinear propagation of light and describe one classroom activity to demonstrate it.
Rectilinear propagation means light travels in straight lines in a uniform medium. A simple classroom activity is the pinhole experiment: make a small hole in a card and hold it so sunlight (or a torch) passes through the hole onto a screen/wall; a clear circular spot of light appears showing the straight path. Alternatively, arrange three cards with aligned holes to show that if one hole is blocked the spot disappears—demonstrating that light needs a straight path. Mention observation, reasoning and conclusion in answer.
2.
Describe what is meant by a light ray and how ray diagrams help in understanding light behaviour.
A light ray is an idealised straight line indicating the direction of light travel; it helps simplify complex wave behaviour into geometric paths. Ray diagrams allow us to visualise incidence, reflection and refraction by drawing incident and reflected rays with normals and measuring angles. They are especially useful to predict image positions in mirrors and lenses and to explain shadow formation. In exams, include labelled diagrams showing an incident ray, normal and reflected ray to illustrate the point.
3.
Explain the terms ‘incident ray’, ‘reflected ray’ and ‘normal’ with a neat labelled diagram.
The incident ray is the incoming ray that strikes a reflecting surface. The reflected ray is the ray that leaves the surface after bouncing. The normal is an imaginary line perpendicular to the surface at the point of incidence. A correct answer should include a labelled diagram: draw a flat surface, the normal at the point, an incident ray approaching making angle i with the normal and the reflected ray making angle r on the other side. State the relationship between these rays (laws of reflection) and conclude.
4.
Describe two observable everyday examples that demonstrate straight-line propagation of light.
Examples: (1) Sharp shadow of a small object under sunlight—light travels straight from the sun and an object blocks it producing a defined shadow. (2) A beam of sunlight entering a dusty room—dust particles illuminate the straight path of light as a visible ray. For each example, describe the observation and link it to the ray model of light and the conclusion that light travels in straight lines.
5.
Explain why the edges of a shadow are sometimes fuzzy. Distinguish between umbra and penumbra in your answer.
Fuzzy edges occur when the light source is not a perfect point but extended; parts of the source are blocked while others illuminate partially, creating a penumbra. The umbra is the innermost dark region where the entire source is blocked; the penumbra surrounds the umbra and receives light from some parts of the source, appearing lighter. Use diagrams showing extended source, object and screen to explain formation of umbra and penumbra and mention how source size and distances affect sharpness.
6.
What are multiple shadows? Give an example and explain the physics behind them.
Multiple shadows occur when there are two or more light sources; each source casts its own shadow. Example: at night under street lights mixed with car headlights, objects cast overlapping shadows in different directions. Physically, each source produces its own umbra/penumbra, and where shadows overlap intensity varies. Explain with a diagram showing two sources and resultant overlapping shadows and note practical implications (the number and angle of shadows reveal positions of light sources).
Topic B — Reflection & Laws (Questions 7–14)
7.
State and explain the two laws of reflection. Provide a labelled ray diagram to support your answer.
The two laws are: (1) The incident ray, reflected ray and normal lie in the same plane. (2) The angle of incidence equals the angle of reflection (i = r). Explain each law with reasoning: symmetry and geometry of reflection at a smooth surface ensure equal angles. Include a clear diagram with normal, incident and reflected rays and mark equal angles. Conclude with remarks on how these laws apply to plane mirrors and smooth surfaces.
8.
Differentiate between regular (specular) and diffuse reflection with examples.
Regular reflection occurs on smooth surfaces where reflected rays remain parallel producing clear images (e.g., mirror, calm water). Diffuse reflection occurs on rough surfaces where reflected rays scatter in many directions, producing no clear image though the surface is visible (e.g., paper, wall). Explain physical cause: micro-roughness causes local normals to vary, scattering light. Give examples and discuss implications for seeing objects versus seeing clear reflections.
9.
A ray of light strikes a plane mirror at 30° to the normal. Find angle of reflection and explain how you obtain it.
By the second law of reflection angle of incidence equals angle of reflection. So angle of reflection = 30°. Explain measurement with normal line and marking angles. Optionally show a small diagram. Conclude that symmetry ensures predictable reflection angles for plane surfaces.
10.
Explain the role of the normal in reflection experiments and diagrams.
Normal is the reference line perpendicular to surface at point of incidence; it provides a basis to measure incidence and reflection angles. Without the normal angles can't be defined consistently. In experiments with protractor and mirror, students draw the normal to measure equal incidence and reflection, verifying the second law. Mention use in ray diagrams and practical measurements.
11.
Describe an experiment to verify the laws of reflection and explain expected observations.
Place a plane mirror on a table and draw a normal at a chosen point. Direct a narrow beam (laser pointer or ray box) at various angles and use a protractor to measure incidence and reflection angles. Record and verify i = r each time. Observations: measured angles of reflection match angles of incidence within experimental error, supporting both laws. Outline materials, procedure, observations and conclusion concisely.
12.
How does surface roughness affect the formation of images? Why do some surfaces not form clear images?
Surface roughness means local normals vary across the surface; incident parallel rays reflect at different angles, scattering light (diffuse reflection) and preventing formation of coherent reflected rays needed for a clear image. Thus rough surfaces like cloth reflect light that makes the surface visible but not a distinct reflected image. Explain with simple sketch and consequences (e.g., why mirrors must be polished).
13.
Explain why mirrors are usually made of polished metal or silvered glass.
Mirrors require surfaces that give regular reflection; polished metal/silvered glass have smooth, reflective layers that maintain parallelism of reflected rays, producing clear images. Metals and silver coating reflect a high fraction of incident light. Discuss durability and common manufacturing methods briefly, and conclude on suitability for image formation and applications.
14.
Discuss how the two laws of reflection are applied in designing periscopes and give a simple explanation of a periscope’s working.
Periscopes use plane mirrors at 45° to change the direction of light by 90°. Using the laws, light from an object strikes the top mirror and reflects down the tube to the bottom mirror which reflects it to the eye; the equal-angle law ensures predictable redirection. Explain mirror placement, ray paths and practical uses (submarines, trenches). Include simple ray diagram and note that periscopes allow observation without direct line-of-sight.
Topic C — Plane Mirrors & Image Formation (Questions 15–22)
15.
Describe the characteristics of an image formed by a plane mirror. Explain each characteristic briefly.
Characteristics: (1) Virtual — image cannot be projected; formed by apparent intersection of reflected rays. (2) Erect — not inverted. (3) Laterally inverted — left-right reversal. (4) Same size as the object — no magnification by plane mirror. (5) Distance behind the mirror equals object distance in front. For each, provide short reason or diagram to support the statement and mention exam tip to show all features in a ray diagram.
16.
Explain with a diagram how to locate the image of an object placed in front of a plane mirror using constructive ray tracing.
Select a point on the object (e.g., top) and draw two incident rays: one parallel to mirror (reflects via normal), and one towards the mirror at any angle. Reflect both using equal angles about the normal and extend reflected rays backward—they intersect at the image point. Repeat for another point to complete image. Provide stepwise instructions and diagram, emphasising use of normal and equal angles.
17.
A student claims that a plane mirror can magnify objects if the mirror is large. Critically analyse this statement.
This is incorrect. A plane mirror produces an image of the same size as the object regardless of mirror size; however, a larger mirror can reflect more of the object if it is tall, allowing the viewer to see the whole body. Clarify difference between field of view (depends on mirror size) and magnification (no change for plane mirrors). Use diagram to show why image size equals object size.
18.
Explain lateral inversion: why does our left appear as right in a plane mirror?
Lateral inversion arises because the mirror reverses the front-back axis; when we face mirror, our left side corresponds to the image’s right when compared from our viewpoint. The mirror swaps directions along the axis perpendicular to the mirror; this is perceived as left-right reversal. Use a brief diagram or thought experiment (e.g., raising left hand and observing image) to clarify; emphasise that mirror does not actually swap left-right intrinsically but reverses front-back causing perceived lateral inversion.
19.
How does the distance between an object and a mirror affect the position of its image? Use the mirror law for plane mirrors in your explanation.
For plane mirrors image distance behind mirror equals object distance in front (i = o). So moving object closer moves image closer by same amount. Explain with algebraic statement and simple diagram showing object and image moving symmetrically; mention that image always appears behind the mirror at equal distance regardless of object position.
20.
List three practical applications of plane mirrors and explain how reflection principles are used in each.
Applications: (1) Dressing mirrors — regular reflection forms clear images for viewing. (2) Rear-view mirrors — allow drivers to see traffic behind; often flat mirrors for true distances. (3) Periscopes and kaleidoscopes — use strategic placement of mirrors at angles to redirect rays or create multiple images. For each, briefly explain mirror orientation and reliance on laws of reflection.
21.
A plane mirror forms a virtual image of an object 1 m in front of it. Where is the image located? Explain.
The image is located 1 m behind the mirror, at the same distance as the object in front. Explain using property i = o in plane mirrors and illustrate with quick diagram showing symmetry about the mirror plane.
22.
Explain how a kaleidoscope produces beautiful patterns using the laws of reflection.
A kaleidoscope contains multiple mirrors arranged at angles; successive reflections produce repeated images of objects inside, creating symmetric patterns. Each reflection obeys the laws, and the multiple reflections produce apparent infinite repetitions and rotational symmetry determined by mirror angles. Describe construction (two or three mirrors) and how angles control pattern repetition.
Topic D — Shadows, Experiments & Applications (Questions 23–30)
23.
Design an experiment to study how shadow size changes with distance of object from a light source. Describe procedure, observations and conclusion.
Procedure: Place a lamp (point-like) and an object between lamp and screen. Vary object-lamp distance keeping screen fixed; measure shadow size each time. Observations: as object moves closer to light, shadow grows larger; moving object away reduces shadow. Conclusion: shadow size increases as the object approaches the source because rays diverge more. Mention controlling variables and recording a table of distances vs shadow sizes for clarity.
24.
Explain how the pinhole camera principle is related to rectilinear propagation of light and shadow formation.
A pinhole camera forms an inverted image because light from each point on an object passes straight through a small aperture and strikes specific points on a screen; rays from the top of object go to bottom of screen and vice versa. This relies on straight-line travel and blocking of other rays by the aperture, making a clear image. Discuss construction, inversion, and practical demonstration linking to ray paths and shadow concepts.
25.
A student notices that shadow edges become fuzzy when using a broad torch but sharp when using a laser pointer. Explain why.
A broad torch acts as an extended light source producing rays from different parts, creating penumbra and fuzzy edges. A laser pointer approximates a narrow, near-point source producing nearly parallel rays and sharp shadows. Discuss role of source size and coherence of light; note safety with laser use in classroom and recommend low-power lasers if used.
26.
How do optical devices such as periscopes and kaleidoscopes rely on controlled reflection for their functioning? Provide concise explanations for both.
Periscopes: use two mirrors at 45° to reflect the light path allowing viewing from hidden positions—laws ensure predictable redirection. Kaleidoscopes: use multiple mirrors at set angles to produce repeated symmetrical patterns by successive reflections. Discuss mirror placement and resulting visual output briefly for each device.
27.
Explain why polished water can act like a mirror and what determines the clarity of the reflected image.
A calm water surface is smooth relative to visible wavelengths, producing regular reflection; if the surface is undisturbed, reflected rays remain ordered giving clear images. Disturbances (ripples) cause scattering and blur. Clarity depends on surface smoothness and angle of incidence; mention grazing angles often produce clearer reflections on water surfaces.
28.
Discuss safety and ethical considerations when performing light experiments (e.g., laser pointers, sunlight). Suggest safe alternatives.
Safety: avoid shining lasers into eyes; use low-power (<1 mW) pointers and never point at faces. Protect eyes from concentrated sunlight; avoid focusing sunlight with lenses on flammable materials. Ethical considerations: ensure experiments do not endanger others and obtain permission for outdoor demos. Safe alternatives: use LED torches instead of lasers, use pinhole and bulbs, or ray boxes designed for classrooms.
29.
Explain how understanding shadows and reflections is useful in real-life professions (two examples).
Examples: (1) Photographers use knowledge of light direction and shadows to compose well-lit images and control contrast. (2) Architects and urban planners consider shadows cast by buildings to ensure daylighting and minimise unwanted shading. Explain briefly how understanding light paths and reflection guides decisions in these fields.
30.
Summarise the key points students should remember from Chapter 11 for exam preparation.
Key points: light travels in straight lines; know umbra and penumbra formation; memorise two laws of reflection and practice ray diagrams; plane mirrors form virtual, erect, laterally inverted images at equal distance; understand simple devices (periscope, kaleidoscope) and common experiments (pinhole, shadow size variation). Emphasise neat labelled diagrams in answers and stepwise experimental descriptions for practical questions.
Exam Tips:
- Always start long answers with a short definition, include a labelled diagram where useful, explain steps logically and end with a clear conclusion.
- Practice drawing normals and angles accurately; diagrams often fetch marks—keep them neat and labelled.
- When writing experiments, list materials, procedure, observations and conclusion separately for clarity.