Light, Shadows and Reflections
Overview
A ray of sunlight — entering through a hole in a room — travels in a straight line. Light travels in straight lines, which is why we get curtains, mirrors, and shadows. Let us understand the wonderful world of light.
Typed Notes
Where does light come from — luminous vs non-luminous
An object that produces its own light is called luminous: the Sun, stars, a glowing bulb, a candle, a firefly (which produces light through a chemical reaction inside its body). An object that does not produce its own light is called non-luminous: a book, a chair, a tree, the Moon. The Moon appears to shine at night, but it only reflects the Sun's light. We humans are also non-luminous — we are visible because light from a bulb falls on us.
Light travels in a straight line
Light travels in a straight line — this is called the straight-line propagation of light. Proof: (1) a beam of torchlight appears as a straight shaft in dusty air; (2) rays of sunlight entering through a small hole in a room travel in a straight line; (3) pinhole camera — light from an object passes through a tiny hole and forms an inverted image on the screen (the top part appears at the bottom and the bottom part appears at the top). This is also why we cannot see behind a mountain — light does not bend around it.
Shadow — the opposite of light
When an opaque object comes in the path of light — light cannot pass through — a dark region forms behind the object: this is called a shadow. A shadow always forms on the side opposite to the light source. As the direction of the Sun changes, the shadow changes: in the morning the Sun is low on the horizon — shadows are long; at noon the Sun is directly overhead — shadows are short; in the evening they become long again. A transparent object (clear glass) does not form a shadow — light passes straight through it; a translucent object forms a faint shadow; an opaque object forms a dark shadow.
Mirror and reflection
A mirror sends light back — this is called reflection. The surface of a mirror is very smooth — that is why we can see our face in it. If the mirror has water or dust on it — the surface is no longer smooth — the reflection becomes blurry. Why can we see our face in a mirror? Because we are non-luminous — light from a bulb or the Sun falls on us, bounces off the mirror, and travels back to our eyes — that is how we see our own face. A periscope — made using two mirrors — is a device that lets us see over or around a wall, and it works on the same principle.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Luminous objects: produce their own light — the Sun, fireflies, a glowing bulb, a candle. Non-luminous: do not produce their own light — a book, a chair, the Moon.
- 02Light travels in a straight line — this is called straight-line propagation. A pinhole camera produces an inverted image (proof of this).
- 03Shadow: an opaque object blocks light — a dark region forms behind it. Shadows are long in the morning and short at noon.
- 04Transparent (clear glass) allows light to pass through completely; translucent (butter paper) allows only some light; opaque allows none.
- 05Mirror: bends light back — this is called reflection. A smooth surface gives a clear reflection.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Is the Moon luminous or non-luminous? Why?
When and how is a shadow formed? Why is there a difference between a morning shadow and a noon shadow?
Why does a pinhole camera produce an inverted image?
Transparent, translucent, opaque — which one forms a shadow and why?
What kind of object is a firefly?
Why does a mirror show us our own face?
Light travels in a straight line (straight-line propagation). Luminous objects produce their own light (the Sun), while non-luminous objects reflect light (the Moon). An opaque object forms a shadow on the side opposite to the light source. A mirror reflects light; a pinhole camera produces an inverted image.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Through which type of material can light NOT pass?
Q2.What is a shadow like at noon?
Q3.What kind of object is a firefly?
Q4.What kind of image does a pinhole camera form?
Mark karo — dashboard par progress dikhega.