By Subhash Kumar | Science Educator | UREducator
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Light
Complete Teaching Resource for CBSE Class 7 Science Chapter 11
Class: VII
Subject: Science
Chapter: 11
Duration: 4 Hours
What Students Will Achieve
Knowledge
- Explain that light travels in straight lines
- Describe reflection of light from plane surfaces
- Differentiate between concave and convex mirrors
- Identify characteristics of images formed by mirrors and lenses
Understanding
- Compare real and virtual images
- Analyze why ambulance lettering is reversed
- Understand how white light splits into colors
- Explain practical uses of different mirrors and lenses
Skills
- Conduct experiments with mirrors and lenses
- Trace light paths in reflection experiments
- Create rainbow effects with prisms
- Demonstrate image formation principles
Prior Knowledge Assessment
Starter Questions:
- Why can't we see around corners?
- Why is the word "AMBULANCE" written backwards on vehicles?
- What causes a rainbow to form in the sky?
How Light Moves Through Space
Key Properties of Light
Rectilinear Propagation
Light travels in straight lines
- Evident in light beams and shadows
- Explains why we can't see around corners
- Basis for pinhole camera operation
Speed of Light
Extremely fast but finite
- ~300,000 km/s in vacuum
- Slows in different media (water, glass)
- Explains time delay in astronomical observations
Dual Nature
Wave-particle duality
- Exhibits both wave and particle properties
- Explains reflection, refraction, and color
- Basis for modern optics
Demonstrating Straight-Line Travel
| Experiment | Setup | Observation |
|---|---|---|
| Straight Pipe Test | View candle through straight pipe | Flame visible - light travels straight |
| Bent Pipe Test | View candle through bent pipe | Flame not visible - light can't bend |
| Pinhole Camera | Light through small hole forms image | Inverted image proves straight path |
Experiment: Making a Pinhole Camera
Objective: Demonstrate light travels in straight lines
- Take a cardboard box and paint black inside
- Make a small pinhole on one side
- Cover opposite side with tracing paper
- Point at bright object in dim room
- Observe inverted image on tracing paper
- Explain why image is inverted
Bouncing Back of Light
Laws of Reflection
First Law
Incident ray, reflected ray and normal all lie in same plane
- Demonstrated with laser and mirror
- Explains why reflections appear flat
Second Law
Angle of incidence = Angle of reflection
- Measured from normal line
- Explains mirror symmetry
- Basis for periscope design
Mirror Images
Characteristics of plane mirror images
- Virtual (can't be projected)
- Same size as object
- Laterally inverted (left-right reversed)
- Same distance behind mirror as object in front
Applications of Reflection
| Application | How Reflection Helps | Example |
|---|---|---|
| Safety | Reversed lettering becomes readable in mirror | Ambulance lettering |
| Navigation | See around obstacles or over walls | Periscopes in submarines |
| Security | View multiple angles simultaneously | Convex mirrors at store corners |
| Decoration | Creates illusion of space and light | Mirror walls in interior design |
Activity: Tracing Light Paths
Objective: Verify laws of reflection experimentally
- Fix plane mirror vertically on paper
- Shine torch with slit at mirror at angle
- Mark incident and reflected rays
- Draw normal line perpendicular to mirror
- Measure angles with protractor
- Verify angle of incidence = angle of reflection
Curved Mirrors and Their Uses
Types of Spherical Mirrors
Concave Mirror
Curved inward (like a cave)
- Converges light rays
- Can form real or virtual images
- Used in telescopes, shaving mirrors
- Focal point where rays meet
Convex Mirror
Curved outward (bulging)
- Diverges light rays
- Always forms virtual, diminished images
- Used in vehicle side mirrors
- Provides wider field of view
Image Formation Comparison
| Property | Concave Mirror | Convex Mirror |
|---|---|---|
| Image Type | Real or virtual depending on position | Always virtual |
| Image Size | Can be enlarged, same size, or diminished | Always diminished |
| Orientation | Inverted (real) or erect (virtual) | Always erect |
| Uses | Searchlights, dental mirrors, telescopes | Vehicle side mirrors, security mirrors |
Experiment: Spoon Mirror Images
Objective: Explore concave and convex reflections
- Use back (convex) side of spoon as mirror
- Observe image characteristics (size, orientation)
- Repeat with front (concave) side of spoon
- Move spoon closer/farther from face
- Compare with plane mirror image
- Record observations in table
Light Bending Through Transparent Materials
Types of Lenses
Convex Lens
Thicker in middle, converges light
- Can form real or virtual images
- Used in magnifying glasses, cameras
- Corrects farsightedness
- Focal point where rays meet
Concave Lens
Thinner in middle, diverges light
- Always forms virtual, diminished images
- Used in peepholes, some eyeglasses
- Corrects nearsightedness
- Virtual focal point where rays appear to diverge from
Practical Applications
| Device | Lens Type | Function |
|---|---|---|
| Magnifying Glass | Convex | Enlarges small objects when close |
| Camera | Convex | Focuses light to form real image on film/sensor |
| Eyeglasses | Convex or Concave | Corrects vision problems (farsighted/nearsighted) |
| Peephole | Concave | Provides wide-angle view of outside |
Experiment: Focusing Sunlight
Objective: Demonstrate lens focusing ability
- Take convex lens outside on sunny day
- Hold lens and paper to find focus point
- Observe bright spot (real image of Sun)
- Note paper may smoke/burn at focus
- Repeat with concave lens - no focus point
- Explain difference in light behavior
Dispersion of White Light
Color Spectrum
White Light Composition
Sunlight contains all colors
- Visible spectrum: ROYGBIV
- Red has longest wavelength
- Violet has shortest wavelength
- Colors bend differently in prism
Rainbow Formation
Natural prism effect
- Sunlight refracts in water droplets
- Internal reflection separates colors
- Appears opposite the Sun
- Double rainbows possible
Color Perception
How we see colors
- Objects reflect certain wavelengths
- Others are absorbed
- Mixing colors creates new colors
- Primary colors of light (RGB) differ from pigments
Color Mixing Experiments
| Experiment | Method | Result |
|---|---|---|
| Newton's Disc | Spin disc with rainbow colors | Colors blend to appear white |
| Color Top | Spin top with colored segments | Demonstrates additive color mixing |
| CD Reflection | Shine light on CD surface | Diffraction creates rainbow patterns |
Project: Make a Rainbow
Objective: Create and observe rainbow effects
- Use garden hose to make fine water spray
- Stand with back to Sun (morning/evening best)
- Observe rainbow in water droplets
- Alternatively use prism near window
- Compare natural and artificial rainbows
- Identify all seven colors
Evaluation Strategies
Formative Assessments
- Mirror image labeling exercises
- Experiment report evaluation
- Think-pair-share discussions
- Quick quizzes on light properties
Summative Assessments
- Chapter test (MCQs + short answer)
- Practical exam with mirrors/lenses
- Project on rainbow formation
- Case study on mirror applications
Extended Learning: Visit an Optician
Objective: Observe practical applications of lenses
- Interview optician about different lens types
- Learn how eyeglasses correct vision
- Understand lens manufacturing process
- Explore optical instruments
- Prepare a report with diagrams
Frequently Asked Questions
Why is the image in a plane mirror laterally inverted?
Lateral inversion occurs because:
- Light rays reflect straight back from mirror surface
- The "left" side of object reflects to "right" side of image
- It's a result of the front-to-back reversal in reflection
- Not actual left-right swap but depth perception effect
- Demonstrated when writing appears reversed in mirror
How do convex mirrors provide wider field of view than plane mirrors?
Convex mirrors diverge light rays which allows:
- Light from wider angles to reach the eye
- More objects to be visible in same mirror size
- Trade-off is smaller image size of each object
- Essential for vehicle side mirrors to reduce blind spots
- Common in store security and road safety mirrors
