💡 1. Sources of Light
We can see objects only when light enters our eyes. Light is a form of energy. Objects that
themselves emit (produce) light are called luminous objects.
Objects that do not produce light but reflect it are called non-luminous objects.
| Type |
What it is |
Examples |
| Luminous (Natural) |
Natural objects that produce their own light |
Sun, stars, fireflies (jugnu), glowworm, bioluminescent deep-sea fish |
| Luminous (Man-made) |
Artificial objects that produce light |
Electric bulb, torch, fire/candle, tube light, LED |
| Non-luminous |
Objects visible only because they reflect light from a luminous source |
Moon, planets, table, book, human face, mirror |
NO! The Moon is non-luminous — it has no light of its own. We see it
because it reflects sunlight. That's why we can't see the Moon when the Sun is on the opposite side (new
moon)!
🌑 2. Shadows — How Are They Formed?
A shadow is formed when an opaque object blocks light from a
source. The area behind the object (where light doesn't reach) forms the shadow.
Conditions for Shadow Formation:
- A source of light (torch, bulb, sunlight)
- An opaque object to block the light (cannot be transparent)
- A screen or surface to receive the shadow (wall, paper, ground)
Properties of Shadows:
- A shadow is always on the opposite side of the light source
- Shadow is always darker than its surroundings (less/no light inside)
- A shadow does NOT show the colours or features of the object — it is always
dark (usually black)
- The size of the shadow depends on the distance:
- Object closer to light source → larger shadow
- Object farther from light source → smaller shadow
- Object closer to screen → smaller, sharper shadow
- The shadow changes length throughout the day — shortest at noon (when sun is
overhead), longest at sunrise and sunset (when sun is at an angle)
📸 Image Prompt
A scientific diagram of shadow formation: A bright light bulb (yellow glow) on the
far left. A solid red opaque ball in the middle. A white screen on the right. Light rays (yellow lines)
shown going from bulb around the ball, and a dark shadow region (dark gray/black) shown behind the ball
on the screen. The three conditions labeled: "Source of Light (Bulb)", "Opaque Object (Ball)", "Screen
(Wall)". The shadow area labeled "Shadow (No light reaches here)". Clean educational diagram on white
background with labeled arrows. A smaller inset shows the shadow getting larger as the ball moves closer
to the bulb.
Fig. 11.1 — Formation of a shadow
Umbra and Penumbra (Bonus for curious students):
- Umbra = the dark central region of the shadow where NO light reaches
- Penumbra = the partial shadow region around the umbra where some light reaches
This concept explains why solar and lunar eclipses happen!
🪟 3. Transparent, Translucent, and Opaque Objects
Based on how light passes through them, objects are classified into three types (already studied in Ch. 4,
but very important for this chapter too):
| Type |
Behaviour |
Shadow formed? |
Examples |
| Transparent |
All light passes through; objects seen clearly |
No shadow (or very faint) |
Clear glass, clean water, air |
| Translucent |
Some light passes through; objects seen blurrily |
Faint/partial shadow |
Frosted glass, butter paper, thin cloth, oiled paper |
| Opaque |
No light passes through |
Dark, distinct shadow |
Wood, metal, stone, thick cardboard |
🪞 4. Reflection of Light
Reflection means the bouncing back of light when it hits a
surface. Smooth, shiny surfaces like mirrors reflect light very well.
How we see ourselves in a mirror:
- Light from a source (bulb/sunlight) falls on our face
- Our face reflects the light in all directions
- Some of the reflected light falls on the mirror
- The mirror reflects (bounces back) that light into our eyes
- We see our image in the mirror!
Laws of Reflection (Class 6 Level):
- The angle of incidence (angle at which light hits the mirror) always equals the
angle of reflection (angle at which it bounces back)
- Angle of incidence = Angle of reflection
- Both angles are measured from the normal (an imaginary line perpendicular to the
mirror surface at the point of incidence)
📸 Image Prompt
A clean physics diagram showing reflection of light: A flat mirror shown
horizontally (or as a flat surface). A single light ray (yellow arrow) striking the mirror at an angle,
labeled "Incident Ray". A dashed vertical line perpendicular to the mirror at the point of contact,
labeled "Normal". The reflected light ray (orange arrow) bouncing off at the same angle, labeled
"Reflected Ray". The angle between incident ray and normal labeled "Angle of Incidence (i)", and the
angle between reflected ray and normal labeled "Angle of Reflection (r)". Both angles shown with arc
symbols and clearly labeled "i = r". White background, clean educational physics diagram style.
Fig. 11.2 — Law of reflection: angle of incidence = angle of reflection
Regular vs Diffuse Reflection:
- Regular (Specular) Reflection: When light reflects off a very smooth surface (like
a mirror or still water). Parallel rays remain parallel after reflection → clear image formed.
- Diffuse (Irregular) Reflection: When light reflects off a rough surface (like
paper, wood, cloth). Light scatters in all directions → no clear image. This is how we see most
objects in daily life!
🌈 5. Light Travels in a Straight Line
Light travels in straight lines (this is called rectilinear propagation of light). This is
why:
- We cannot see around a corner (light can't bend around the wall)
- Shadows form — light cannot bend around opaque objects
- A pinhole camera forms an image — light from different parts of an object travels in straight lines
through the pinhole
- Laser beams travel in perfectly straight lines
🔦 Pinhole Camera — Understanding Straight-line Travel
How a Pinhole Camera (Camera Obscura) works:
Take a box with a tiny pinhole on one side and a screen on the other. When a bright object (candle) is
placed outside in front of the pinhole:
- Light from the TOP of the candle travels in a straight line through the hole → hits the BOTTOM of
the screen
- Light from the BOTTOM of the candle travels in a straight line through the hole → hits the TOP of
the screen
- Result: An inverted (upside-down) image is formed on the screen
This proves light travels in straight lines!
📸 Image Prompt
A cutaway side-view diagram of a pinhole camera: Left side shows a lit candle
(flame pointing up). Center shows a box with a small pinhole in its left wall. Right side shows the
inner screen of the box with an inverted/upside-down image of the candle. Two dotted light rays drawn:
one from the TOP of the candle flame going DOWN through the pinhole and hitting the BOTTOM of the
screen, another from the BOTTOM of candle going UP through the pinhole hitting the TOP of the screen.
Both rays labeled with arrows. Text "Inverted image formed" on the screen inside the box. Clean
educational physics diagram style, white background with blue/yellow lines.
Fig. 11.3 — Pinhole camera: inverted image proves light travels straight
📝 6. Quick Revision
- Luminous = produces own light (Sun, bulb). Non-luminous = reflects
light (Moon, mirror)
- Shadow formed when opaque object blocks light → needs (1) light source, (2) opaque object, (3)
screen
- Shadow is always dark — never shows colour or details of the object
- Closer to light source → bigger shadow. Closer to screen → smaller, sharper
shadow
- Transparent → no shadow; Translucent → faint shadow;
Opaque → distinct dark shadow
- Reflection = light bouncing off a surface. Angle of incidence = Angle of
reflection
- Regular reflection (smooth mirror) → clear image. Diffuse
reflection (rough surface) → scattered light
- Light travels in straight lines (rectilinear propagation)
- Pinhole camera forms an inverted image — proof that light travels
straight
- Moon is non-luminous — it reflects sunlight
