Light: Shadows and Reflections – Case-based Questions with Answers

CBSE Class 7 — Chapter 11: Light: Shadows and Reflections — 20 Case-Based Questions

Content Bank — Focus Areas

Key concepts covered: rectilinear propagation, pinhole experiments, shadows (umbra & penumbra), influence of source size and distances, laws of reflection, plane mirrors and image properties, periscopes and other mirror-based devices, practical demonstrations and safety.

20 Case-Based Questions (Read the case carefully and answer the questions)

Case 1 — Classroom Pinhole

A teacher makes a pinhole in a cardboard and projects sunlight on the classroom wall. Students notice a sharp circular bright spot.

What conclusion about light does this experiment support?
Why is the spot circular?

Answers: It demonstrates rectilinear propagation — light travels in straight lines through the small aperture. The spot is circular because the hole is circular and rays from the sun passing through it form a circular patch on the wall.

Case 2 — Shadow at Different Distances

A student places a toy between a lamp and a screen. When the toy is moved closer to the lamp, its shadow on the screen grows larger.

Explain why the shadow size increases.
Predict what happens if the toy is moved closer to the screen instead.

Answers: Moving the toy closer to the lamp increases the angle subtended by the object at the screen, so the shadow size increases. If the toy moves closer to the screen (away from lamp), the shadow becomes smaller.

Case 3 — Fuzzy Shadow

Using a broad table lamp, the student finds the toy's shadow edge is fuzzy, but with a small LED torch the edge becomes sharp.

Explain the cause of fuzzy and sharp shadow edges.
Define umbra and penumbra in this context.

Answers: A broad lamp is an extended source producing rays from different parts; this creates a penumbra causing fuzzy edges. A small torch acts closer to a point source producing a sharper shadow. Umbra is the fully dark central part; penumbra is the lighter, partially lit surrounding region.

Case 4 — Multiple Light Sources

On stage, spotlights from different angles cast several overlapping shadows of the same actor.

Explain why multiple shadows appear.
How does this help stage lighting designers?

Answers: Each spotlight acts as a separate source and produces its own shadow; overlapping yields multiple shadows. Designers use multiple shadows to create depth and dramatic effects, controlling angles to soften or emphasise outlines.

Case 5 — Mirror Image Distance

A student stands 2 m in front of a plane mirror and asks where his image appears.

State the distance of the image from the mirror.
Is the image real or virtual? Explain.

Answers: The image appears 2 m behind the mirror (same distance as object). The image is virtual because reflected rays only appear to diverge from the image point and cannot be captured on a screen.

Case 6 — Lateral Inversion Confusion

A student claims that a mirror swaps left and right but not top and bottom.

Explain whether this claim is correct and why we perceive lateral inversion.
Provide a simple demonstration to clarify.

Answers: The mirror reverses front-back direction; perceived left-right swap is due to how we face the mirror. Top-bottom remains because we don't rotate ourselves vertically. Demonstration: raise left hand and observe that the image’s corresponding hand is on the opposite side — illustrating perceived lateral inversion.

Case 7 — Periscope in Submarine

A submarine periscope uses two mirrors at 45° to let the captain see above water while submerged.

Explain, using laws of reflection, how the periscope changes direction of view.
Why are mirrors angled at 45°?

Answers: The incident rays strike the top mirror and reflect downwards; the bottom mirror reflects them horizontally into the eye. By law, equal angles ensure correct redirection. At 45°, the ray is deflected by 90°, allowing vertical-to-horizontal direction change in a compact tube.

Case 8 — Diffuse Reflection

A student notices they cannot see their face on white paper but can on a polished surface.

Explain why white paper does not form a clear image.
What type of reflection occurs on paper?

Answers: Paper has a rough surface causing diffuse reflection; reflected rays scatter in many directions so no coherent image forms. Polished surfaces give regular reflection and clear images.

Case 9 — Guttation vs Condensation (Trick)

A student sees droplets on leaves in morning and suggests leaves produce water by transpiration.

Is the student correct? Explain the phenomenon.
How is this different from condensation from transpired water?

Answers: Morning droplets are guttation (exudation) through hydathodes due to root pressure, not transpiration. Condensation can occur if transpired vapour cools, forming droplets on nearby surfaces, but guttation is liquid exudation from plant vessels.

Case 10 — Shadow on a Sunny & Cloudy Day

On a sunny day shadows are sharp; on a cloudy day they are faint or invisible.

Explain why cloud cover affects shadow visibility.
What does this tell about the sun as a light source?

Answers: Clouds scatter sunlight, turning the sun into an extended, diffused source reducing contrast between umbra and penumbra; shadows become faint. This shows sun behaves like a point source on clear days (sharp shadows) and more like an extended/diffuse source under clouds.

Case 11 — Storing Mirrors in Schools

A school stores mirrors with their reflective side touching other objects and finds scratches on them later.

Explain how scratches affect mirror performance.
Suggest safe storage practices.

Answers: Scratches disrupt smoothness causing scattered reflection and loss of image clarity. Store mirrors in padded covers, reflective face away from abrasive contact, or use separators to avoid scratching.

Case 12 — Shadow Measurement Activity

Students measure shadow length of a stick at different times of day and notice variation.

Why does shadow length change during the day?
How could this be used to estimate time?

Answers: Shadow length changes as sun's angle changes—long in morning/evening, short at noon. Historically sundials use this predictable change: marking shadow positions estimates time of day.

Case 13 — Brightness vs Image Visibility

A shiny metal spoon shows distorted reflections while a flat mirror shows clear ones.

Why is reflection in a curved spoon distorted?
What determines image clarity?

Answers: Curved surfaces change angle of incidence across surface producing magnified or reduced and distorted images (like convex/concave effects). Image clarity depends on smoothness and curvature — flat smooth surfaces produce clear undistorted images.

Case 14 — Safety with Lasers

A student aims a laser pointer at a classmate as a joke and is reprimanded.

Explain why pointing lasers at eyes is dangerous.
Recommend safer classroom alternatives for ray demonstrations.

Answers: Lasers concentrate light and can damage the retina, causing permanent eye injury. Safer alternatives: low-power LED torches, ray boxes designed for education, or shining light through small apertures for demos.

Case 15 — Periscope Design Error

A student builds a periscope but places mirrors incorrectly and gets no clear view.

Identify likely mistakes in mirror placement.
Suggest corrections.

Answers: Mistakes: mirrors not at 45°, misaligned so reflected rays miss second mirror, or mirrors tilted wrong direction. Correct by ensuring both mirrors are parallel, each at 45°, and aligned so incoming rays reflect down then into the eye—test and adjust angles.

Case 16 — Image in a Funhouse Mirror

A distorted tall image appears in a wavy amusement-park mirror.

Explain using reflection why the image is distorted.
How would you describe the difference between concave/convex distortions?

Answers: Wavy surfaces have varying local normals causing different reflection angles and varying magnification across the image, producing distortion. Concave mirrors can magnify and invert depending on distance; convex mirrors reduce and provide wide-angle views.

Case 17 — Using Mirrors for Safety

A shop installs a convex mirror above the doorway to watch for shoplifting.

Explain why a convex mirror is chosen.
What are the advantages and limitations?

Answers: Convex mirrors provide a wider field of view and form reduced virtual images allowing surveillance of a large area. Advantage: wide coverage; limitation: objects appear smaller and distance estimation is poor.

Case 18 — Observing Sunlight Through Leaves

Sunlight through a tree canopy shows dappled light and moving bright spots on the ground.

Explain the pattern using concepts of light and shadow.
Why do spots move with wind?

Answers: Leaves partially block sunlight creating a patchwork of umbra/penumbra where gaps act like multiple small sources forming bright spots. Wind moves leaves changing light paths and shadow positions, so spots shift accordingly.

Case 19 — Mirror Placement in Clinics

A dentist places mirrors to see inside a patient’s mouth. The assistant uses a small angled mirror to reflect light into hidden areas.

Explain how reflection assists in medical examinations.
Why are small angled mirrors useful?

Answers: Mirrors redirect light into otherwise hidden cavities, improving visibility without moving patient. Small angled mirrors let clinicians see around corners and reflect light precisely; laws of reflection ensure predictable orientation of view.

Case 20 — Classroom Assessment Task

Students are asked to explain why a shiny black car surface reflects light differently when wet vs dry.

Explain changes in reflection when the car is wet.
What does this tell about surface smoothness and reflection?

Answers: Water smooths microscopic roughness, creating a more continuous reflective layer—wet surface can act more like a mirror increasing regular reflection and clearer highlights. Dry roughness scatters light producing diffuse reflection. Thus smoother surfaces give stronger regular reflections.