11. Light, Shadows and Reflections
Introduction: We see so many objects around us. In the dark, we cannot see anything. We need light to see things. The torch bulb is an object that gives out light of its own. The Sun is another familiar object that gives its own light.
Luminous and Non-Luminous Objects
- Luminous Objects: Objects that give out or emit light of their own. (e.g., Sun, Torch bulb, Candle, Firefly).
- Non-Luminous Objects: Objects that do not give out their own light. We see them when light from a luminous object falls on them and then travels to our eyes. (e.g., Moon, Chair, Shoe, Book).
Transparent, Opaque and Translucent:
- Opaque: Does not allow light to pass. Casts a dark shadow. (Wood).
- Transparent: Allows light to pass fully. (Glass).
- Translucent: Allows light to pass partially. (Butter paper).
Shadows
A shadow is formed when an opaque object comes in the path of light. For a shadow to form, we need:
- A source of light.
- An opaque object.
- A screen (like a wall or ground) behind the object.
Characteristics: Shadows are always black. They give us an idea about the shape of the object, but sometimes can be misleading (like making hand shadows of animals).
ACTIVITY 3: Shadow Play
Observation: Try to hide your shadow only using a torch? You can't unless you remove the screen or the object! Shadows change size if distance between source, object or screen changes.
Pinhole Camera
A simple camera that can be made with two boxes sliding into each other. One box has a small pinhole, the other has a translucent screen.
Images: The image in a pinhole camera is upside down (inverted) because light travels in straight lines. We can see images of the Sun or bright objects.
ACTIVITY 5: Natural Pinhole Camera
Observation: When you pass under a tree covered with leaves, you see small circular patches of sunlight on the ground. These are pinhole images of the Sun! The gaps between leaves act as pinholes.
Mirrors and Reflections
Light travels in a straight line. We can see this by looking at a candle through a straight pipe vs a bent pipe.
Reflection: A mirror changes the direction of light that falls on it. This is called reflection. We see our face in the mirror due to reflection.
ACTIVITY 7: Mirror Reflection
Setup: Shine a torch onto a mirror in a dark room. You will see a patch of light reflected on another wall. This shows mirror changes direction of light.
SOLVED IN-TEXT QUESTION: Boojho
Q: Boojho has a question. If we are in a completely dark room, can we see our reflection in the mirror?
Ans: No! We need light to be reflected from our face to the mirror and back. In a dark room, there is no light to reflect.
Chapter Summary
- Opaque objects do not allow light to pass through them.
- Transparent objects allow light to pass through them and we can see through these objects clearly.
- Translucent objects allow light to pass through them partially.
- Shadows are formed when an opaque object comes in the path of light.
- Pinhole camera can be made with simple materials and can be used to image the Sun and brightly lit objects.
- Light travels in straight line.
- Mirror changes the direction of light that falls on it.
Exercise Q1
Q: Rearrange the boxes given below to make a sentence that helps us understand opaque objects.
Exercise Q2: Classify
Q: Classify as Opaque, Transparent, Translucent, Luminous, or Non-luminous.
- Transparent: Air, Water, Sheet of plane glass.
- Translucent: Fog, Sheet of polythene, Smoke.
- Opaque: Piece of rock, Sheet of aluminum, Wooden board, Wall, Mirror.
- Luminous: Sun, Firefly, Lighted torch, Flame of gas burner.
- Non-Luminous: Moon, Earth, Pen, Shoe.
Exercise Q3
Q: Can you create a shape that gives a circular shadow if held in one way and a rectangular shadow if held in another way?
- If light falls on its circular face, the shadow is circular.
- If light falls on its curved side, the shadow is rectangular.
Exercise Q4
Q: In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror?
Exercise Q5
Q: Why can we not see through a bent pipe?
Key Facts and Definitions
- Luminous Objects: Objects that give out or emit light of their own. (e.g. Sun, Torch, Candle).
- Non-Luminous Objects: Objects that do not give out their own light. They become visible when light falls on them. (e.g. Moon, Chair, Shoe).
- Reflection: Process of sending back light rays which fall on the surface of an object.
- Shadow: Dark patch formed by an opaque object obstructing light.
- Light travels in a straight line. This property is called rectilinear propagation of light.
- Light is a form of energy.
- Speed of light is very fast (300,000 km per second).
| Property | Shadow | Image |
|---|---|---|
| Color | Black | Colorful |
| Formation | Blocking of Light | Reflection of Light |